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Noun

predators
  1. Plural of predator

Extensive Definition

In ecology, predation describes a biological interaction where a predator organism feeds on another living organism or organisms known as prey. Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the (ecologically significant) death of the prey. The other main category of consumption is detritivory, the consumption of dead organic material (detritus). It can at times be difficult to separate the two feeding behaviors

Degree of specialization

Among predators there is a large degree of specialization. Many predators specialize in hunting only one species of prey. Others are more opportunistic and will kill and eat almost anything (examples: humans, leopards, and dogs). The specialists are usually particularly well suited to capturing their preferred prey. The prey in turn, are often equally suited to escape that predator. This is called an evolutionary arms race and tends to keep the populations of both species in equilibrium. Some predators specialize in certain classes of prey, not just single species. Almost all will switch to other prey (with varying degrees of success) when the preferred target is extremely scarce, and they may also resort to scavenging or a herbivorous diet if possible.

Trophic level

Predators are often another organism's prey, and likewise prey are often predators. Though blue jays prey on insects, they may in turn be prey for snakes, which may themselves be the prey of hawks. One way of classifying predators is by trophic level. Organisms which feed on autotrophs, the producers of the trophic pyramid, are known as herbivores or primary consumers; those that feed on heterotrophs such as animals are known as secondary consumers. Secondary consumers are a type of carnivore, but there are also tertiary consumers eating these carnivores, quartary consumers eating them, and so forth. Because only a fraction of energy is passed on to the next level, this hierarchy of predation must end somewhere, and very seldom goes higher than five or six levels. A predator at the top of any food chain (that is, one that is preyed upon by no organism) is called an apex predator; examples include the orca, tiger, and crocodile and even omnivorous humans. An apex predator in one environment may not retain this position if introduced to another habitat, such as dogs among crocodilians.
The problem with this system of classification is that many organisms eat from multiple levels of the food chain. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons. Organisms showing both carnivory and herbivory are known as omnivores. Even supposedly strict herbivores may supplement their diet with meat. Carnivorous plants would be very difficult to fit into this classification, producing their own food but also digesting anything that they may trap. Organisms which eat detritivores would also be difficult to classify by such a scheme.

Predation as competition

An alternative view offered by Richard Dawkins is of predation as a form of competition: the genes of both the predator and prey are competing for the body (or 'survival machine') of the prey organism. This is best understood in the context of the gene centered view of evolution.

Ecological role

Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are known as keystone species, may have a profound influence on the balance of organisms in a particular ecosystem. Introduction or removal of this predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over.

Adaptations and behavior

The act of predation can be broken down into a maximum of four stages: Detection of prey, attack, capture and finally consumption. The relationship between predator and prey is one which is typically beneficial to the predator, and detrimental to the prey species. Sometimes, however, predation has indirect benefits to the prey species, though the individuals preyed upon themselves do not benefit. This means that, at each applicable stage, predator and prey species are in an evolutionary arms race maximize their respective abilities to obtain food or avoid being eaten. This interaction has resulted in a vast array of adaptations in both groups.

General

One adaptation helping both predators and prey avoid detection is camouflage, a form of crypsis where species have an appearance which helps them blend into the background. Camouflage consists of not only color, but also shape and pattern. The background upon which the organism is seen can be both its environment (e.g. the praying mantis to the right resembling dead leaves) other organisms (e.g. zebras' stripes blend in with each other in a herd, making it difficult for lions to focus on a single target). The more convincing camouflage is, the more likely it is that the organism will go unseen.
While successful predation results in a gain of energy, hunting invariably involves energetic costs as well. When hunger is not an issue, most predators will generally not seek to attack prey since the costs outweigh the benefits. For instance, a large predatory fish like a shark that is well fed in an aquarium will typically ignore the smaller fish swimming around it (while the prey fish take advantage of the fact that the apex predator is apparently uninterested). Surplus killing represents a deviation from this type of behaviour. The treatment of consumption in terms of cost-benefit analysis is known as optimal foraging theory, and has been quite successful in the study of animal behavior. Costs and benefits are generally considered in energy gain per unit time, though other factors are also important, such as essential nutrients that have no caloric value but are necessary for survival and health.
Size-selective predation involves predators preferring prey of a certain size. Large prey may prove troublesome for a predator, while small prey might prove hard to find and in any case provide less of a reward. This has led to a correlation between the size of predators and their prey. Size may also act as a refuge for large prey, for example adult elephants are generally safe from predation by lions, but juveniles are vulnerable.
Mobbing can be an interspecies activity: it is common for birds to respond to mobbing calls of a different species. Many birds will show up at the sight of mobbing and watch and call, but not participate. It should also be noted that some species can be on both ends of a mobbing attack. Crows are frequently mobbed by smaller songbirds as they prey on eggs and young from these birds' nests, but these same crows will cooperate with smaller birds to drive away hawks or larger mammalian predators. On occasion, birds will mob animals that pose no threat.
Black-headed Gulls are one species which aggressively engages intruding predators, such as Carrion Crows. Experiments on this species by Hans Kruuk involved placing hen eggs at intervals from a nesting colony, and recording the percentage of successful predation events as well as the probability of the crow being subjected to mobbing. The results showed decreasing mobbing with increased distance from the nest, which was correlated with increased predation success. Mobbing may function by reducing the predator's ability to locate nests, as predators cannot focus on locating eggs while they are under direct attack.

Advertising unprofitability

Once a predator has detected its prey, one would expect it to pursue it. However, it is not always profitable for the predator to do so. Consider the example of a Thomson's Gazelle being spotted by a predator. Giving chase to prey requires a sacrifice in energy. If, however, there is some way the prey species can convey the information that it is unprofitable, energy will be saved by both organisms. Thomson's Gazelles are hunted by species such as lions and cheetahs. When they see the predator approach, they may start to run away, but then slow down and stot. Stotting describes a behavior involving jumping into the air with the legs kept straight and stiff, and the white rear fully visible. Obviously this behavior is maladaptive if they hope to outrun the predator, so it must serve some other purpose. Although other hypotheses have been put forward, evidence supports the proposition that they stot to signal an unprofitable chase. For example, cheetahs abandon more hunts when the gazelle stots, and in the event they do give chase, they are far less likely to make a kill.
Aposematism, where organisms are brightly colored as a warning to predators, is the antithesis of camouflage. Some organisms pose a threat to their predators - for example they may be poisonous, or able to harm them physically. Aposematic coloring involves bright, easily recognizable and unique colors and patterns. Upon being harmed (e.g. stung) by their prey, the appearance of such an organism will be remembered as something to avoid.

Population dynamics

It is fairly clear that predators tend to lower the survival and fecundity of their prey, but on a higher level of organization, populations of predator and prey species also interact. It is obvious that predators depend on prey for survival, and this is reflected in predator populations being affected by changes in prey populations. It is not so obvious, however, that predators affect prey populations. Eating a prey organism may simply make room for another if the prey population is approaching its carrying capacity.
The population dynamics of predator-prey interactions can be modelled using the Lotka–Volterra equations. These provide a mathematical model for the cycling of predator and prey populations.

Humans and predation

In conservation

Predators are an important consideration in matters relating to conservation. Introduced predators may prove too much for populations which have not coevolved with them, leading to possible extinction. This will depend largely on how well the prey species can adapt to the new species, and whether or not the predator can turn to alternative food sources when prey populations fall to minimal levels. If a predator can use an alternative prey instead, it may shift its diet towards that species in a behavior known as functional response, while still eating the last remaining prey organisms. On the other hand the prey species may be able to survive if the predator has no alternative prey - in this case its population will necessarily crash following the decline in prey, allowing some small proportion of prey to survive. Introduction of an alternative prey may well lead to the extinction of prey, as this constraint is removed.
Predators are often the species endangered themselves. Competition for prey from other species could prove the end of a predator - if their ecological niche overlaps completely with that of another the competitive exclusion principle requires only one can survive. Loss of prey species may lead to coextinction of their predator. In addition, because predators are found in higher trophic levels, they are less abundant and much more vulnerable to extinction.

Biological pest control

Predators may be put to use in conservation efforts to control introduced species. Although the aim in this situation is to remove the introduced species entirely, keeping its abundance down is often the only possibility. Predators from its natural range may be introduced to control populations, though in some cases this has little effect, and may even cause unforeseen problems. Besides their use in conservation biology, predators are also important for controlling pests in agriculture. Natural predators are an environmentally friendly and sustainable way of reducing damage to crops, and are one alternative to the use of chemical agents such as pesticides.

See also

References

External links

Further reading

  • Barbosa, P. and I. Castellanos (eds) 2004. Ecology of predator-prey interactions New York : Oxford University Press. 394 p. ISBN 0195171209
  • Curio, E. 1976. The ethology of predation Berlin ; New York : Springer-Verlag. 250 p. ISBN 0387077200
predators in Bulgarian: Хищник
predators in Catalan: Depredació
predators in Czech: Predátor
predators in Danish: Prædation
predators in German: Prädator
predators in Spanish: Depredación
predators in French: Prédateur
predators in Ido: Predato
predators in Indonesian: Pemangsa
predators in Italian: Predazione
predators in Lithuanian: Plėšrūnas
predators in Hungarian: Ragadozó életmód
predators in Dutch: Predator
predators in Japanese: 捕食
predators in Norwegian: Predator
predators in Norwegian Nynorsk: Predasjon
predators in Polish: Drapieżnictwo
predators in Portuguese: Predação
predators in Simple English: Predator
predators in Finnish: Saalistus
predators in Swedish: Predation
predators in Vietnamese: Săn mồi
predators in Turkish: Avcı
predators in Chinese: 掠食
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