Freedman defines a pollutant as "the occurrence of toxic substances or energy in a larger
quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
Paul Cordova
L. Lehr
December 11, 1995
"An Ecosystem's Disturbance by a Pollutant
Freedman defines a pollutant as "the occurrence of toxic substances or energy in a
larger quality then the ecological communities or particular species can tolerate without
suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an
organism vary depending on the dose and duration (how long administered). The impact can
be one of sublethality to lethality, all dependent upon the factors involved. These
factors need to be looked at when determining an ecosystem's disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur
dioxide, elements such as mercury and arsenic, and even pollution by nutrients which is
referred to as eutrophication. Each of these pollutants pose a different effect on the
ecosystem at different doses. This varied effect is what is referred to as dose and
duration. The amount of the pollutant administered over what period of time greatly
affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population
level can be either non-target or target. Target effects are those that can kill off the
entire population. Non-target effects are those that effects a significant number of
individuals and spreads over to other individuals, such is the case when crop dusters
spread herbicides, insecticides. Next we look at population damage by a pollutant, which
in turn has a detrimental effect on the ecosystem in several ways. First, by the killing
of an entire population by a pollutant, it offsets the food chain and potentially kills
off other species that depended on that organism for food. Such is the case when a
keystone species is killed. If predators were the dominant species high on the food
chain, the organisms that the predator keep to a minimum could massively over produce
creating a disturbance in the delicate balance of carrying capacity in the ecosystem.
Along with this imbalance another potential problem in an ecosystem is the possibility of
the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way
through the food chain it increases with the increasing body mass of the organism. These
potential problems are referred to as bioconcentration and biomagnificaiton,
respectively. Both of these problems being a great concern of humans because of their
location on the food chain. These are only a few of the impacts that a pollutant can have
on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a
pollutant on an ecosystem. A carrying capacity curve describes the number of individuals
that a specific ecosystem can sustain. Factors involved include available resources
(food, water, etc.), other members of the species of reproductive age and abiotic factors
such as climate, terrain are all determinants of carrying capacity. This curve is drawn
below:
# of individuals
Years
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity
curve of several organisms (Chiras, 127). This effect on the curve is caused by the
killing off of the intolerant and allowing more room for both the resistant strain and
new organisms. In some cases the pollutant will create unsuitable habitats causing
migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic)
equation: The actual growth rate is equal to the potential growth rate multiplied by the
carrying capacity level. Three major characteristics exist for this equation. First, that
the rate of growth is density dependent, the larger the population, the slower it will
grow. Secondly, the population growth is not limited and will reach a stable maximum.
Lastly, the speed at which a population approaches its maximum value is solely determined
by the rate of increase (r). In a population with a stable age structure this would be
the birth rate minus the death rate, but this is almost impossible. If any of the
variables in this equation are affected by a pollutant then the growth rate of an
organism can be seriously affected which can in turn affect the entire ecosystem
(Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of
chemicals on individual animals resulting in lethal or sublethal effects. Effects on
individuals may range from rapid death (lethal) through sublethal effects to no effects
at all. The most obvious effect of exposure to a pollutant is rapid death and it is
common practice to assess this type of toxicity by the LD50 (the lethal dose for 50% of
test animals) values, scientist can judge the relative toxicity of two chemicals. For
example, a chemical with an LD50 of 200 milligrams per kilogram of body weight is half as
toxic as one with an LD50 the more toxic a chemical. Death is rarely instantaneous, and
even cyanide takes at least some tens of seconds to kill a human being. Death is alwaBAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often
ill defined, with one type of exposure, and with no indication of the influence of other
environmental variables.
Perkins (1979) suggests that a sublethal exposure kills at most only a small proportion
of a population, but the possibility that s sublethal exposure could cause a small
proportion of individuals to die from acute toxicity seems self contradictory (Freedman,
126). For both the sake of this assignment and for practical purposes, it would be
incautious to suppose that a sublethal exposure that affects individual organisms
adversely is not close to that which will affect the population. There is no good reason
to suppose that there is a constant relationship for different pollutants or different
species, between the dose needed to kill and that needed to impair an organism.
Therefore, given the difficulties of studying an ecosystem, the most effective way to
predict biological effects is likely to be by discerning the least exposure that produces
a deleterious response in individual organisms (Moriarty, 1960) and then examining the
extent to which different environmental conditions alter this minimum exposure.
Further adding to the complexity several additional factors come into play with the
effect and response of an organism from a pollutant. One such factor is age. Although we
think of youngsters of all species as resilient creatures, young, growing organisms are
generally more susceptible to toxic chemicals than adults (Chiras, 127). Health Status is
determined by many factors, among them one's nutrition, level of stress, and personal
habits such as smoking. As a rule, the poorer one's health, the more susceptible he or
she is to a toxin (Freeman, 214). Toxins may also interact with each other producing
several different responses. Some chemical substances for example, team up to produce an
additive response that is, an effect that is simply the sum of the individual responses.
Others may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur dioxide gas
and particulates (minute airborne particles) inhaled together can reduce air flow through
the lungs' tiny passages. The combined response is much greater than the sum of the
individual responses.
Plants have three strategies in response to a disturbance - this was suggested by
Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.
Plant response to a disturbance was suggested by Connell and Slatyer (1977) using
models. Model I (the "facilitation" model assumes that only certain species that come
early in the succession are capable of colonizing the site. In contrast the other two
models both assume that any individual of any species that happens to arrive at the site
is capable of colonizing it, although all models accept that certain species will tend to
appear first because of their colonizing abilities. All models also suppose that the
first colonist will so modify the site that it becomes unsuitable for those species that
normally occur early in the succession. The three hypotheses then suggest three different
ways in which other species will appear. Model I suggests that early occupants modify the
environment so that it becomes more suitable for species that come later in the
succession. Model II (the "tolerance" model) suggests that the sequence in which species
appear depends solely on their speeds of dispersal and growth. Model III (inhibition) -
the species already present makes the environment less suitable for subsequent
recruitment of later species. All these hypothesis do not rely on the idea of a community
as a sugra-organism but on succession as a sugra-organism but on succession as a process
that relies on two factors: 1) the probabilities that propagules of different species
will be present and
2) the ability of these propagules to survive, develop and reproduce.
Now to look at the whole picture, we ask ourselves: "How do we predict the response of a
community from a pollutant?" Should we look at one population at a time, or in some
holistic approach. Moriarty suggests that some of the currently favored approaches rest
ont he assumption, often implicit rather than explicit, that communities are
sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when
dealing with the idea of community response: 1) indicator species
2) biological or environmental health may be misleading.
The term indicator species, which is used in the classification of communities (p. 62)
is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea
that knowledge of one species within a community will indicate the well-being or
biological health of the whole community. Moriarty suggests that this seems a reasonable
proposition if one accepts the traditional view of community as sugra-organism, but
suggests that it is in fact misleading. He (Moriarty) adds that there is no fundamental
reason from community structure to suppose that any particular species within the
community will give a better measure of impact from pollutants than will another.
Pollutants will affect populations of particular species, and which species are first
affected will depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species than of the
community. An indicator species can only be used to assess the impact of pollution on a
community if quite a lot is known about both the pollution and the community (Moriarty,
69). Concerning the idea of the concept of biological or environmental health being
misleading: one may properly refer to the health of a community. A community can change
"markedly" if affected by a pollutant, but it will just become a different community that
is neither more nor less "healthy" just different (Moriarty, 69). It may be a less
desirable community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a retrogression - a
decrease in diversity, productivity, biomass and structural complexity. Moriarty argues
that while there may be the appearance of a retrogression process it should not be taken
as a generality. In conclusion, on the effect and response of an organism from a
pollutant, the most appropriate emphasis is on populations. The effect of pollutants on
populations within a community can be complex and apart from reduction or elimination of
populations - resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.
Another very important characteristic of populations that we cannot overlook is their
emetic composition. Much of the variation between individuals is inherited from their
parents. It is common knowledge that relatively few offspring of any species survive to
reproduce. Charles Darwin (biologist, 1859) formed the idea of natural selection: the
idea that some individuals will have a higher probability of survival than others, and on
average such individuals will then leave more descendants than other less well adapted
individuals. We will use Darwin's, Mendel's and Watson and Crick's and other information
to investigate our concern - the role of pollutants in natural selection. It has been
shown many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how natural
selection acts on populations.
For the purpose of this assignment I will outline/review all the general findings of
important works that proved significant in understanding the concepts of genetics. A good
place to start would be with an outline of some of Mendel's results obtained when
breeding peas (Pisum sativum). "A" indicates the dominate gene for yellow seed, "a" the
recessive gene for green seed.
However, genes do not always fall into this simple dominant/recessive pattern. Some may
be incompletely dominant in the heterozygote, showing a transition stage between the
phenotypes of the homozygous dominant and recessive conditions. Later workers also found
that there are often more than two alternative forms alleles) of a gene. One such worker
was Avery (1944) who showed that the genetic material in a bacterium consists of the
nucleic acid DNA (deoxyribonucleic acid), and in 1953 Watson and Crick first suggested
the three-dimensional structure of DNA from which has developed all the subsequent work
on the genetic code. The essential feature of this code is that: genes are arranged along
chromosomes, which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten nucleotides in
both chains for each complete turn of the helix (Watson, 1965).
Diagram to illustrate the double helix of DNA with the two polynucleotide chains linked
by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine (G) with Cytosive
(C). Replication occurs when the two strands separate and both act as templates on which
new complementary strands are formed (Moriarty, 62).
Occasionally, something goes wrong with the replication process and one or more genes
may be altered, lost or gained. These changes, or mutations are usually less favorable to
the organism than the original gene, and are often sufficiently unfavorable to be lethal.
Nevertheless, mutations in the reproductive cells are of crucial importance: these are in
favorable, the source of new genetic variation in subsequent generations.
This knowledge about gene structure and function modifies the Mendelian view of
inheritance.
Now, after the brief introduction and history of genetics it is time to consider the
relevance of ecological genetics to pollution. Most current problems of pollution occur
on a much shorter time-scale than that required for the evolution of new species. The
critical difference between evolutionary change and that wrought by pollution is the
speed: populations can disappear very rapidly from pollution and if unchecked, we would
have a very impoverished fauna and flora (Moriarty, 81).
One very popular example of the effects of pollution on wildlife, and perhaps the most
striking evolutionary change over to be actually witnessed was the occurrence of melanism
in moths. This effect is commonly associated with industrial development. White moths
would rest on white lichen on trees and were well-nigh visible on them. But with
industrial pollution (between 1848 and 1990) lichen turned a black color exposing and
making the white moth (f. typica) prey to birds. Birds posed a selective pressure against
the white moths. Now black moths were favored evolutionary. This is known as the
heterozygous advantage, in which a bank of recessive alleles becomes favored due to a
change in the environment. The biological significance of melanism was a matter for
debate for some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still unclear.
Although several points are worth emphasizing. Pollution in this instance is not having a
direct effect on the moth populations, nor indeed on their predators, but an alteration
to the habitat has altered greatly the relative fitness of different genotypes. Melanism
also illustrates the difficulty of producing adequate proof, or disproof, of cause and
effect when pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of pollutants
can be modified by an organisms genetic constitution, and that pollutants can alter a
population's gene pool (Freeman, 128). The interactions between pollutants and genes can
be relevant both to understanding and to predicting effects and are potentially of great
value for monitoring (Moriarty, 102).
In summary, as stated throughout this school year in my 2375 Pollution class, the
effects of pollutants on populations are mediated via their effects, direct or indirect
on individuals and the likelihood of these effects depends on the dose. Sublethal effects
can be unravelled from knowledge of the mode of action. Alternatively, emphasis in the
study of sublethal effects can be placed on the health of the individual organism. With
both approaches, the effect of other environmental variables needs to be given much more
prominence than heretofore and this could profitably be linked with studies on amounts of
pollutant within organisms (Moriarty, 176). It is from this basis that Moriarty states
that we have to consider how best to predict and to monitor the ecological effects of
potential pollutants.
In my opinion, I feel that (as does Moriarty) one should relate pollution to the wider
aspects of man's impact on his environment. We can, to a considerable extent, control and
mitigate our negative impacts upon this planet because as we have learned from our past
experiences, this planet does have a finite carrying capacity for our own as well as for
all other species.
References
Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.
Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th ed), 1994.
The Benjamin/Cummings Publishing Company.
Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution, disturbances
and other stresses / Bill Freedman (2nd ed.), 1995.
Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.
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