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 populations (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 its 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 its 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 hosts 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 its 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 its 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 barnacles 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 ones 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 its 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 its background only if it chooses the right one. When the right one is chosen, the organism will position itself so that its 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 1960s 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 dont 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 didnt 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 its defense mechanisms to prolong its 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 its host alive. In some cases, the parasite will act so efficiently that it will lead to the death of its host, but most parasites can achieve a balance with their hosts. Even though parasites might not lead directly to the death of its 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 its 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 its 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 hosts evolution. This helps explain why the cuckoo continues to successfully lay its 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 cuckoos 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 its 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 its 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 its 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 its 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 Dunnocks 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 its sibling (1990). Drommond et al. observed the older eaglet deal 1569 pecks to its 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.