Predation
Predation
Predation is the interaction in which the predator attacks live prey and consumes it. The interaction can be between two or more individuals, and is to the benefit of the predator and at the expense of the prey. The study of predator-prey interactions is broad and includes behaviors of the predator (such as searching, handling, and consuming prey), adaptations of the prey (survival strategies), and phenomena of their coexistence, such as stabilization factors that allow both groups to persist. It should be noted that there are four types of predators: true predators (including cannibals), grazers, parasitoids, and parasites. This entry will focus on true predators.
If a predator were fully efficient, all of its prey would be eaten. The prey would go extinct, and so would the predator. But the predator-prey interactions seen in nature allow both to sustain themselves. The first researchers to model how these interactions operated were A. J. Lotka and V. Volterra, in 1925 and 1926 respectively.
The Lotka-Volterra model assumes that predator reproduction is a function of the amount of prey consumed, so that when predators eat more prey, the predators increase in number through increased reproduction and immigration. There is a circular pattern of predator-prey interactions in this model: (1) when the predator population increases, the prey population decreases; (2) when the prey decrease in number, the predators decrease in number; (3) when the predator population decreases, the prey population increases; and (4) when the prey increase in number, the predator population again increases and the cycle begins anew.
When populations are plotted out over time, a pattern of coupled oscillation can be seen in which the apex, or peak, of one population coincides with the low point of the other. The numeral values of the two populations then cross and reverse positions.
A famous example of coupled oscillation between predator and prey populations occurs with the snowshoe hare and the lynx. Population peaks, as determined by numbers of pelts lodged with the Hudson Bay Company, were alternately spaced in time, with that of the lynx closely following that of the showshoe hare. The Lotka-Volterra model easily explains this pattern of predator-prey population sizes.
Although this model is not incorrect, it does oversimplify the scope of predator-prey interactions given that the major assumption, that when predators eat more prey the predator population increases, is often not exactly what is seen in nature. In actuality, when prey increases, a predator can have a numerical response, where predators do in fact increase in number through reproduction or immigration, or a functional response, where each predator eats more prey items.
Three types of functional responses are recognized, each of them showing a different relationship between prey density and amount of prey consumed. The Type I functional response is a direct relationship in which the predator eats all of the prey available up to a certain saturation point, when the predator can eat no more. After the predator reaches that saturation point, the prey density can continue to increase with no effect on how many prey items are being eaten. Some insects employ the strategy of having thousands of offspring that all hatch at once. This suddenly floods the food supply, ensuring that a significant portion will remain after predators eat their fill.
The Type II functional response is more commonly seen because it is more realistic, since it incorporates a factor called handling time. Handling time is the amount of time a predator must devote to each prey item it consumes. It is the time needed for pursuing, subduing, and consuming the prey, and then preparing for further search. In this type of response the relationship between prey density and consumption is not linear because it changes over time. At first, the consumption rate increases, but as prey density continues to increase, there is a decline in the rate at which consumption increases until a maximum level is reached. This gradual deceleration of consumption reflects the factor of handling time.
Lastly, the Type III functional response is the most complex. It is similar to Type II at high prey densities, but includes the additional factor that there is very little or no prey consumption when prey is at low densities. This means that the predator does not eat any of the prey until there is a certain amount available.
One reason for this is that when there are very few prey animals, they can all find ideal hiding places and easily keep themselves out of the reach of predators. When there are more prey, however, some are forced into less ideal refuges, or into foraging places that are out in the open, where they are more visible to predators.
Another reason that prey are often not consumed when they are at low densities relates to search images. A predator gets accustomed to looking in certain types of habitat for certain shapes, colors, or movement patterns in order to hunt at maximum efficiently. Using a search image for the prey items that are most abundant pays off, because the predator will have the most success in hunting that prey. Searching for something that is very rare, on the other hand, only wastes time and likely results in less food obtained in a given amount of time.
Related to the idea of search image is the phenomenon of switching. Even though a predator may have a preference for one type of prey, at times when that prey is at low densities and other prey is at high densities, the predator will switch to an alternate prey that is at a high density.
All three of these factors—the ability of prey to hide, a search image for the predator, and prey switching by the predator—combine to result in little or no prey taken when prey densities are particularly low. This allows prey populations to recover. Then, the predators increase their consumption until handling time again becomes a limiting factor. When this happens, the rate of consumption increase slows down and consumption evens out at a maximum.
A cannibal is a special type of predator. The term "cannibal" is applied to an individual that consumes another individual of the same species. Typically, cannibalism appears when there is simply not enough food available; in dense populations that are stressed by overcrowding (even when food is adequate); when an individual is weakened and vulnerable to attack as a consequence of social rank; and when vulnerable individuals, such as eggs and nestlings, are available. Frequently, the larger individuals do the cannibalizing, which can serve the purposes of obtaining a meal and reducing competition for food, mates, or territory in the future.
see also Food Web; Foraging Strategies; Interspecies Interactions.
Jean K. Krejca
Bibliography
Begon, Michael, John L. Harper, and Colin R. Townsend. Ecology, 2nd ed. Cambridge, MA: Blackwell Scientific Publications, 1990.
Lotka, A. J. Elements of Physical Biology. Baltimore, MD: Williams & Wilkins, 1925.
Ricklefs, Robert E., and Gary L. Miller. Ecology, 4th ed. New York: W. H. Freeman, 2000.
Smith, Robert L. Elements of Ecology, 2nd ed. New York: Harper & Row, 1986.
Many large predators, such as mountain lions, tigers, and wolves, are on the federal list of endangered or threatened species. One of the reasons is that these animals need large areas to hunt; another reason is habitat loss.
Predation
Predation
Predation is the act of one animal hunting, killing, and eating another animal. A predator is an animal that survives by killing and eating other animals. Predation can be important in regulating the size of a prey species (the hunted animal). It is also a mechanism of evolution since it weeds out animals that are poorly adapted to being hunted, or to their environment, thus promoting adaptation by natural selection.
Predation can be coldly described as nature's "kill or be killed" approach to who survives in the wild and who does not. While there are some species of animals that have no natural enemies or are simply too difficult to catch and eat (such as a mature, healthy elephant), virtually all animals are at some point in their lives either predator or prey. All predators are heterotrophs, meaning that they cannot make their own food as plants (autotrophs) do, so they must consume another organism and digest it to obtain its energy. Many animals are herbivores (planteaters) and do not kill other animals to eat. They eat living plants that do not have to be hunted, caught, and killed before they can be consumed. Animals that exclusively eat plants are not considered predators
(although from a living plant's point of view, it could be said that the plant is being preyed upon). Carnivorous animals as well as omnivores are predators, since carnivores exclusively eat other animals, and omnivores eat both plants and animals, usually according to what is available.
Successful predation requires that a catch be made, and a catch can occur in a great variety of ways. Spiders spin webs to trap their prey; cats lie in wait for the proper moment to spring; hyenas stalk in groups and exhaust their prey; and humans use their technology to kill from a distance. Specialized predators usually go after a single species, while generalized predators will feed on a variety of other species. Predation is one of the ways in which nature selects who has done the best job of adapting to its environment and who will have the best chance to survive. In this way, predation can be said to be a mechanism or a means that nature employs to continue the important role of evolution.
Predators almost always select a meal that will give them more energy than they will use up to catch it. When faced with an opportunity to select an individual animal from a group to eat, predators will usually select the easiest to catch, such as the weakest, the slowest, or the youngest or oldest. Those animals that are poorly adapted will probably not survive and therefore not be able to pass on their poorly adapted traits to another generation. In the dynamic relationship between predator and prey, there is a continuing type of improvement that goes on in which the prey that survive pass on traits that make their offspring slightly better at avoiding being caught. On the other hand, predators that survive because they are excellent hunters will also pass on the good traits that made them better at catching prey. So in the long run, the seemingly cruel standards of the natural world use predation to improve the species.
In terms of entire populations in a certain habitat, predator and prey relationships often move along the same lines. Thus, when conditions favor a prey population—such as when field mice thrive during a good growing season and their population increases—the predator population will also be well fed and grow larger as its members grow strong and live longer. The opposite happens when populations drop. Finally, when the only predator of a certain species disappears (sometimes human intervention causes this to happen), prey populations will take off. The systematic killing of timber wolves and gray wolves in the American West has led to an increase in the number of rabbits and rodents that regularly served as their prey.
[See alsoEvolution; Natural Selection; Survival of the Fittest ]