The relationship between predators and their prey is an intricate and complicated
relationship; covering a great area of scientific knowledge. This paper will examine the
different relationships between predator and prey; focusing on the symbiotic relations
between organisms, the wide range of defense mechanisms that are utilized by various
examples of prey, and the influence between predators and prey concerning evolution and
population structure.
Symbiosis is the interaction between organisms forming a long term relationship with
each other. Many organisms become dependent on others and they need one another or one
needs the other to survive. Symbiotic interactions include forms of parasitism,
mutualism, and commensalism.
The first topic of discussion in symbiosis is parasitism. Parasitism is when the
relationship between two animal populations becomes intimate and the individuals of one
population use the other population as a source of food and can be located in or on the
host animal or animal of the other population(Boughey 1973). No known organism escapes
being a victim of parasitism(Brum 1989).
Parasitism is similar to preditation in the sense that the parasite derives nourishment
from the host on which it feeds and the predator derives nourishment from the prey on
which it feeds(Nitecki 1983). Parasitism is different from most normal predator prey
situations because many different parasites can feed off of just one host but very few
predators can feed on the same prey(1973). In parasite-host relationships most commonly
the parasite is smaller than the host. This would explain why many parasites can feed
off of one single host. Another difference in parasite-host relationships is that
normally the parasite or group of parasites do not kill the host from feeding, whereas a
predator will kill it's prey(1983). Efficient parasites will not kill their host at
least until their own life cycle has been completed(1973). The ideal situation for a
parasite is one in which the host animal can live for a long enough time for the parasite
to reproduce several times(Arms 1987).
Parasites fall under two different categories according to where on the host they live.
Endoparasites are usually the smaller parasites and tend to live inside of the
host(1973). These internal parasites have certain physiological and anatomical
adaptations to make their life easier(1987). An example of this is the roundworm, which
has protective coating around it's body to ensure that it will not be digested. Many
internal parasites must have more than one host in order to carry out reproduction(1989).
A parasite may lay eggs inside the host it is living in, and the eggs are excreted with
the host's feces. Another animal may pick up the eggs of the parasite through eating
something that has come into contact with the feces.
The larger parasites tend to live on the outside of the host and are called
ectoparasites(1973). The ectoparasites usually attach to the host with special organs or
appendages, clinging to areas with the least amount of contact or friction(1973). Both
endo and ectoparasites have the capability of carrying and passing diseases from
themselves to hosts and then possibly to predators of the host(1973). One example of
this is the deer tick which can carry lyme disease and pass it on to humans or wildlife
animals. The worst outbreaks of disease from parasites usually occur when a certain
parasite first comes into contact with a specific population of hosts(1975). An example
of these ramifications would be the onset of the plague.
Many parasites are unsuccessful and have a difficult time finding food because
appropriate hosts for certain parasites may be hard to find(1987). To compensate for low
survival rates due to difficulty in finding a host, many parasites will lay thousands or
millions of eggs to ensure that at least some of them can find a host and keep the
species alive(1987). The majority of young parasites do not find a host and tend to
starve to death. Parasites are also unsuccessful if they cause too much damage to their
host animal(1987). Parasites are what is called host specific, this means that their
anatomy, metabolism, and life-style is adapted to that of their host(1973).
Some parasites react to the behavior of their hosts, an interaction called social
parasitism(1989). More simply put a parasite might take advantage of the tendencies of a
particular species for the benefit of it's own. An example of this is the European
Cuckoo. In this case the grown cuckoo destroys one of the host birds eggs and replaces
it with one of it's own(1991). The host bird then raises the cuckoo nestling even when
the cuckoo is almost too large for the nest and much bigger than the host bird(1991).
This is a case where the parasite uses the host to perform a function and making life and
reproduction easier on itself.
Parasite and host relationships hold an important part of homeostasis in nature.(1975).
Parasitism is an intricate component in the regulation of population of different
species in nature.
Mutualism is another topic at hand in discussing predator-prey relationships.
Mutualism is a symbiotic relationship in which both members of the association
benefit(1989). Mutualistic interaction is essential to the survival or reproduction of
both participants involved(1989). The best way to describe the relationships of
mutualism is through examples. We will give examples of mutualism from different
environments.
Bacteria that lives inside mammals and in their intestinal tract receive food but also
provide the mammals with vitamins that can be synthesized(1975). Likewise termites whose
primary source of food is the wood that they devour, would not be able to digest the food
if it was not for the protozoans that are present in their intestinal tract(Mader 1993).
The protozoans digest the cellulose that the termites cannot handle. Mycorrhizae which
are fungal roots have a mutualistic symbiotic relationship with the roots of
plants(1989). The mycorrhizae protect the plants roots and improve the uptake of
nutrients for the plant, in exchange the mycorrhizae receives carbohydrates from the
plant.
Mutualistic partners have obtained many adaptations through coevolution. Coevolution
has led to a synchronized life cycle between many organisms and through mutualism many
organisms have been able to coincide together as a working unit rather than individuals.
Commensalism is a relationship in which one species benefits from another species that
is unaffected(1975). For instance several small organisms may live in the burrows of
other larger organisms at no risk or harm to the larger organisms. The smaller organisms
receive shelter and eat from the larger organisms excess food supply.
An example of commensalism is a barnacle's relationship with a whale. The barnacles
attach themselves to the whale and they are provided with both a home and transportation.
Another example are the Remoras which are fish that attach themselves to the bellies of
sharks by a suction cup dorsal fin. The Remora fish gets a free ride and can eat the
remains of a sharks meals. Clownfish are protected from predators by seeking refuge in
the tentacles of sea anemones. Most other fish stay away because the anemones have
poison that does not affect the clownfish, therefore the clownfish is safe.
Commensalism consists of dominant predators and opportunistic organisms that feed off
of the good fortune of the larger predators. Another topic concerning predator prey
relationships is the defense mechanisms that are necessary for prey to outwit their
predators.
In order for an animal to sustain life, it must be able to survive among the fittest of
organisms. An animals anti-predatory behavior determines how long it can survive in an
environment without becoming some other animals prey. Some key antipredator adaptations
will be described and examined .
Perhaps the most common survival strategy is hiding from one's enemies(Alcock,1975).
Predators are extremely sensitive to movement and locate their prey by visual cues. By
getting rid of these key signals, enemies(predators) are forced to invest more time and
energy looking for them. This may increase the time a prey has to live and
reproduce(1975).
Hiding is generally achieved through cryptic coloration and behavior(1975). How
effective an organisms camouflage is depends on how long an organism can remain immobile
for a long amount of time. Animals can resemble a blade of grass, a piece of bark, a
leaf, a clump of dirt, and sand or gravel. In less than 8 seconds, a tropical flounder
can transform it's markings to match unusual patterns on the bottom of their tanks in the
laboratory(Adler,1996). When swimming over sand, the flounder looks like sand, and if
the tank has polka dots, the flounder develops a coat of dots(1996). Without any serious
changes, the flounder can blend surprisingly well with a wide variety of backgrounds
(Ramachandran, 1996). Behavioral aspects of camouflage in organisms include more than
just remaining motionless. An organism will blend into it's background only if it
chooses the right one. When the right one is chosen, the organism will position itself
so that it's camouflage will match or line-up with the background. Despite the fact that
an organism may be beautifully concealed, it may still be discovered at some point by a
potential consumer(Alcock,1975).
Detecting a predator is another antipredator adaptation that is very useful. Some prey
species have an advantage over other prey species by being able to detect a predator
before it spots them or before it gets to close to them. In order to detect enemies in
good time to take appropriate action, prey species are usually alert and vigilant
whenever they are at all vulnerable(Alcock,1975). A test was conducted in the early
1960's at Tufts University dealing with ultrasonic sound wave that bats give off, and the
way moths can detect these soundwaves(May,1991). In most cases bats are blind, so they
rely only on their sense of hearing to help them maneuver and hunt while flying in the
dark. Also flying in the dark/nighttime, are insects, moths in this case. In a
laboratory, bats and moths were observed, and every time a moth would come close to a bat
giving off an ultrasonic signal, the moth would turn and go the opposite way(1991). When
the moth would become too close to the bat, it would perform a number of acrobatic
maneuvers such as rapid turns, power dives, looping dives, and spirals(1991).
Detection by groups of animals will usually benefit the whole group formation. By
foraging together several animals may increase the chance that some individual in the
herd, flock, or covey will detect a predator before it is too late(Alcock,1975). Each
individual benefits from the predator detection and alarm behavior of the others, which
will increase the probability that it will be able to get away.
There is always a chance that prey will be chased by a predator. Evading predators is
sometimes necessary for an organism to employ, to make sure they will not be captured
when being pursued. Outrunning an enemy is the most obvious evasion tactic(Alcock,1975).
When a deer or antelope is being chased, they don't just run in one direction to flee,
they alter their flight path. The prey will demonstrate erratic and unpredictable
movements(1975). The deer or antelope may zig and zag across a savanna to make it more
difficult for the predator to capture them.
Repelling predators is a strategy that can either be last chance tactic or the primary
line of defense for an organism. This attack on the predator is used drive it away from
the prey. These adaptations can be classified as (1)mechanical repellents, (2)chemical
repellents, (3)and group defenses(Alcock,1975). An example of a mechanical repellent is
sharp spines or hairs that make organisms undesirable. Some chemical repellents involve
substances that impair the predators ability to move or cause a predator to retreat due
to undesirable odor, bad taste, or poisonous properties. Groups of organisms can also
repel predators. Truly social insects utilize many ingenious group defenses(1975). For
example, soldier ants posses an acidic spray and a sticky glue to douse their enemies
with(1975). They can also chop and stab their enemies with their sharp jaws.
One of the last types of antipredator behaviors/adaptations is mimicry. An organism
that is edible but looks like it is a bad tasting organism is known as a Batesian mimic.
A good example of this mimicry works is how birds at first were more likely to go after
the more conspicuous looking items rather than those that didn't stand out(Adler,1996).
If too many mimics exist, more predators will consume them, and soon they will become a
primary food source. Organisms that share the same style of coloration take part in
Mullerian mimicry. An example of this is the yellow and black stripes on bees and wasps.
The symbiont states that this single look helps bird-brained predators to learn which
organisms to avoid. This warning coloration in turn saves the organisms life as well as
helps the predator to avoid a distasteful, maybe even toxic meal.
Defense mechanisms vary drastically, and change according to different circumstances.
The ability of an organism to survive depends solely on how well it can use it's defense
mechanisms to prolong it's life.
The next topic of discussion is the relationship between predators and their prey.
Predators and prey effect each other from day to day interactions to the evolution of
each other. Predator and prey populations move in cycles, the number of predators will
influence the number of prey and the number of prey available will influence the
population of predators. Predators and their prey also influence the evolution of each
other. Michael Brooke(1991) points out that natural selection should favor traits that
help a species survive. A general example would be the increase in speed of potential
prey. These evolutionary traits are usually followed with an evolution in the predator.
Using the increase of maximum speed as an example, evolution will favor predators that
are fast enough to continue to catch the prey. This will lead to the evolution of a
faster predator. Brooke (1991)compares the evolutionary process to an arms race, for
both sides have to keep advancing in order to stay alive.
While predator/prey populations fluctuate, it is important to note that they operate
within a cycle. In an ideal cycle, the predators and prey will establish stable
populations. Predators play a crucial role in the population of the prey. The
importance of predators can be seen in the Kaibab Plateau in Arizona(Boughey, 1968). At
the beginning of this century, 4,000 deer inhabited 727, 000 acres of land. Over the
next 40 years, 814 mountain lion were removed from the area. At the same time, over
7,000 coyote were removed. When the predators were removed, the population jumped up to
100,000 deer by 1924 (Boughey, 1968). This population crashed in the next two years by
60% due to overpopulation and disease. Without predators, the prey could not establish a
stable population and the land supported a much smaller number than the estimated
carrying capacity of 30,000 (Boughey, 1968).
The example can work in reverse; an increased number of predators feeding on a limited
number of prey can lead to the extinction of the predators. This is the case with the
ancient trilobites, these marine anthropods died 200 million years ago in the Permian
age(Carr, 1971). According to Carr, (1971)over 60 families of this animal have been
found in fossil records. This highly successful creature became extinct due to changes
in the prey population. During the Permian period, glaciation took place that changed
the availability of the trilobites food source, algae. One may conclude that the prey
population dwindled and the trilobites could no longer support themselves.
Parasite/prey relations fit under the topic of predator/prey relationships. Parasites
feed off of their prey just as predators do(Ricklefs, 1993), but it is in the interest of
the parasite to keep it's host alive. In some cases, the parasite will act so
efficiently that it will lead to the death of it's host, but most parasites can achieve a
balance with their hosts. Even though parasites might not lead directly to the death of
it's host, it can effect the host in a variety of other ways. A host could become weaker
and not be able to compete for food or reproduce, or the parasite could make it's host
less desirable to mate with, as is the case with Drosophila nigrospiracula(the Sanoran
desert fruit fly).
Michal Polak et al.(1995) conducted a study examining the effects of Macrocheles
subbadius (a Ectoparasitic mite) on the sexual selection of the fruit flies. The mites
feed off of animal dung and rotting plant tissue (Polak et al., 1995) and relies on the
fruit flies for transportation between feeding sites as well as a food source. Polak et
al. found that male flies infested with the mites had less of a chance of mating compared
to males that had never been infested. But Polak et al.(1995) also found that once the
mites were removed from the flies and the male was allowed to recover from any damage
done by the mite, the fruit fly had the same chance of mating than a male which was never
infested. This suggests that females are selective when choosing their mates.
With females choosing not to mate with males that are infected with the mites, the
evolution of the species is being affected. Males that exhibit resistance to mites are
favored, so these characteristics will be passed onto the offspring, leading to the
development of mite resistant Drosophila nigrospiracula. There are several theories on
what basis the mites affect the males. Based on the research compiled by Polak et al.
(1995), males could be overlooked because infested males might not survive to help raise
the offspring, or males do not mate because they are weakened by the parasites and do not
perform well in contests for mates. Whatever the case, parasites have an effect on their
prey.
In a similar scenario, the parasitic relationship between cuckoos and other birds, the
development of resistance to a parasite leads to the evolution of the parasite. This
polymorphism is known as coevolution. Nitecki uses grass as a simple example of this
phenomenon(1983). Grass evolves a resistance to a strain of rust by making a single gene
substitution, and the rust counters this step with it's own single gene
substitution(Nitecki, 1983). He adds that many parasites are host specific, so they are
keyed into their host and can adjust to the appropriate changes when necessary. This is
why parasites are a continual problem, not just an irritant that is rendered extinct by
one simply change in the host's evolution.
This helps explain why the cuckoo continues to successfully lay it's eggs in the nests
of Meadow Pipits, Reed Warblers, Pied Wagtails, and Dunnocks(Brooke, 1991). According to
Brooke(1991), the host birds usually are deceived by the cuckoo's egg and then raise the
cuckoo chick instead of their own. By examining the cuckoo, it is easy to see how
evolution has perfected the parasitic process. According to Brooke (1991), the cuckoo
will watch it's prey as it builds its nest, wait until both parents are away from the
nest, then enter the nest to remove one of the original eggs and lay it's own. Each
species of cuckoo has evolved to specifically target one of the four possible birds.
According to Brooke, (1991) the Great Reed Warbler-Cuckoo will lay an egg that is similar
in size and color to the hosts, and the cuckoo has perfected the intrusion to a science,
spending about 10 seconds in the nest of it's host.
The next step of parasitism comes once the cuckoo has hatched. The process that the
chick goes through is described by Brooke (1991); the chick hatches before the rest of
the clutch due to it's shorter incubation period and then pushes the other eggs out of
the nest. The host family will not abandon the chick, while the exact reason is not
known, there are several theories. According to Brooke (1991), the parents have nothing
to compare the chick with or do not decide that it is too late to raise a new clutch and
will raise their adopted chick.
Brooke describes some of the tests carried out in his research (1991) concerning the
factors that influence the rejection rate of cuckoo eggs. Most birds will not reject
eggs that are similar too their eggs, but larger eggs are have a higher rate of
rejection. But if the host birds see the cuckoo in the nest, then the rate of rejection
is much increased(Brooke, 1991), which explains why cuckoos have evolved such a fast
predatory process.
Brooke shows an example of the evolutionary process at work when he examines the
Dunnock's relationship with the cuckoo(1991). The Dunnock-Cuckoo has not developed an
egg that mimics the Dunnock egg because Dunnocks accept eggs of any size and color.
Brooke (1991) believes that the Dunnock is a new species of bird under parasitism, for
only 2% of the Dunnocks are preyed upon in England. Therefore, Dunnocks have not yet
developed any defenses against the cuckoo, so the cuckoo has no need to develop any
traits to aid in parasitism. Brooke (1991) showed other examples of evolution by testing
isolated species of hosts. These birds were not as discriminating, implying that they
lacked the evolutionary advancements of detecting and rejecting parasitic eggs. The
cuckoo and their hosts are clear examples of how both the predators and they prey affect
the evolution of each other.
In some cases, predator/prey relations take place between members of the same species.
Many animals exhibit group behavior; worker bees serve the queen bee and wolves follow an
established ranking system. But when members of the same species endanger each other for
individual protection, the member of the species that faces death is being used as prey
by the member of the species surviving. Robert Heisohn describes this relationship in
lions when territorial disputes occur. The leader lion will be 50-200 meters ahead of
the laggards when approaching an invading lion(Heinsohn, 1995). The leader will face
severe injury and even death while the laggards reduce their risk by staying
behind(Heinsohn, 1995). Similar behavior has been observed in many species of birds.
The hatchlings commit siblicide in order to maximize their own chances of survival as
described by Hugh Drommond et al. (1990). Drommond et al. observed cases of siblicide in
black eagles; one of the chicks is hatched usually 3 days before the other and therefore
is significantly larger than it's sibling (1990). Drommond et al. observed the older
eaglet deal 1569 pecks to it's younger sibling in 3 days, eventually killing the younger
chick. This phenomena supports several key concepts in evolution. The older sibling is
competing with others for resources(food and nesting space), so killing the weaker member
promotes the survival of the older bird (Drommond et al., 1990). If resources are
limited and both siblings cannot survive, the species will continue to survive due to the
death of the younger sibling. However, Drommond et al.(1990) point out that there are
several evolutionary losses that occur when a sibling dies; reproductive potential is
lost as well as a degree of insurance(in case one of the offspring does not survive to
maturity). Excuse the pun, but putting all of the eggs in one basket is a large risk.
Predators and their prey are part of a cycle; both are necessary components and they
depend on each other for their existence. Any change made in one area will affect the
other.
Overall, predator prey relations are very complex. By breaking the topic into the three
topics of; symbiotic relationships, defense mechanisms, and the influence relationship
between predators and prey. It is important to see how all three of these subjects tie
in together. Parasitism is an example of a symbiotic relationship, parasites are
predators living off of their prey, and parasites also effect the evolution of their
hosts. Natural selection favors species that are resistant to parasites, so these
organisms evolve. The organisms have a range of defense mechanisms available in order to
protect themselves from predators. So, predators now face tougher prey, so they undergo
evolution in order to stay successful. This completes the cycle and leads to a diverse
and interesting world.
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