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Predation
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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. It can at times be difficult to separate the two feeding behaviors, for example where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. The key characteristic of predation however is the predator's direct impact on the prey population. On the other hand, detritivores simply eat what is available and have no direct impact on the 'donor' organism(s).
Classification of predatorsThe unifying theme in all classifications of predation is the predator lowering the fitness of its prey, or put another way, it reduces its prey's chances of survival, reproduction, or both. Ways of classifying predation surveyed here include grouping by trophic level or diet, by specialization, and by the nature of their interaction with prey.
Functional classificationClassification of predators by the extent to which they feed on and interact with their prey is one way ecologists may wish to categorize the different types of predation. Instead of focusing on what they eat, this system classifies predators by the way in which they eat, and the general nature of the interaction between predator and prey species. Two factors are considered here: How close the predator and prey are physically (in the latter two cases the term prey may be replaced with host). Additionally, whether or not the prey are directly killed by the predator is considered, with the first and last cases involving certain death.
True predation
A true predator is one which kills and eats another organism. Whereas other types of predator all harm their prey in some way, this form results in their instant death. Predators may hunt actively for prey, or sit and wait for prey to approach within striking distance, as in ambush predators. Some predators kill large prey and dismember or chew it prior to eating it, such as a jaguar, while others may eat their (usually much smaller) prey whole, as does a bottlenose dolphin or any snake. In some cases the prey organism may die in the mouth or digestive system of the predator. Baleen whales, for example, eat millions of microscopic plankton at once, the prey being broken down well after entering the whale. Seed predation is another form of true predation, as seeds represent potential organisms. Predators of this classification need not eat prey entirely, for example some predators cannot digest bones, while others can. Some may merely eat only part of an organism, as in grazing (see below), but still consistently cause its direct death.
GrazingGrazing organisms may also kill their prey species, but this is seldom the case. While some herbivores like zooplankton live on unicellular phytoplankton and have no choice but to kill their prey, many only eat a small part of the plant. Grazing livestock may pull some grass out at the roots, but most is simply grazed upon, allowing the plant to regrow once again. Kelp is frequently grazed in subtidal kelp forests, but regrows at the base of the blade continuously to cope with browsing pressure. Animals may also be 'grazed' upon; female mosquitos land on hosts briefly to gain sufficient proteins for the development of their offspring. Starfish may be grazed on, being capable of regenerating lost arms.
ParasitismParasites can at times be difficult to distinguish from grazers. Their feeding behavior is similar in many ways, however they are noted for their close association with their host species. While a grazing species such as an elephant may travel many kilometers in a single day, grazing on many plants in the process, parasites form very close associations with their hosts, usually having only one or at most a few in their lifetime. This close living arrangement may be described by the term symbiosis, 'living together,' but unlike mutualism the association significantly reduces the fitness of the host. Parasitic organisms range from the macroscopic mistletoe, a parasitic plant, to microscopic internal parasites such as cholera. Some species however have more loose associations with their hosts. Lepidoptera larvae may feed parasitically on only a single plant, or they may graze on several nearby plants. It is therefore wise to treat this classification system as a continuum rather than four isolated forms.
ParasitoidismParasitoids are organisms living in or on their host and feeding directly upon it, eventually leading to its death. They are much like parasites in their close symbiotic relationship with their host or hosts. Like the previous two classifications parasitoid predators do not kill their hosts instantly. However, unlike parasites, they are very similar to true predators in that the fate of their prey is quite inevitably death. A well known example of a parasitoids are the ichneumon wasps, solitary insects living a free life as an adult, then laying eggs on or in another species such as a caterpillar. Its larva(e) feed on the growing host causing it little harm at first, but soon devouring the internal organs until finally destroying the nervous system resulting in prey death. By this stage the young wasp(s) are developed sufficiently to move to the next stage in their life cycle. Though limited mainly to the insect order Hymenoptera, parasitoids make up as much as 10% of all insect species.
Degree of specializationAmong 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 competitionAn 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 rolePredators 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 behaviorThe 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 to 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.
Mimicry is a related phenomenon where an organism has a similar appearance to another species. One such example is the drone fly, which looks a lot like a bee, yet is completely harmless as it cannot sting at all. Another example of batesian mimicry is the io moth, (Automeris io), which has markings on its wings which resemble an owl's eyes. When an insectivorous predator disturbs the moth, it reveals its hind wings, temporarily startling the predator and giving it time to escape. Predators may also use mimicry to lure their prey, however. Female fireflies of the genus Photuris, for example, copy the light signals of other species, thereby attracting male fireflies which are then captured and eaten.
Predator
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.
It has been observed that well-fed predator animals in a lax captivity (for instance, pet or farm animals) will usually differentiate between putative prey animals who are familiar co-inhabitants in the same human area from wild ones outside the area. This interaction can range from peaceful coexistence to close companionship; motivation to ignore the predatory instinct may result from mutual advantage or fear of reprisal from human masters who have made clear that harming co-inhabitants will not be tolerated. Pet cats and pet mice, for example, may live together in the same human residence without incident as companions. Pet cats and pet dogs under human mastership often depend on each other for warmth, companionship, and even protection, particularly in rural areas.
Anti-predator adaptationsAntipredator adaptations have evolved in prey populations due to the selective pressures of predation over long periods of time.
Mobbing behaviorMobbing behavior occurs when a species turns the tables on their predator by cooperatively attacking or harassing it. This is most frequently seen in birds, though it is also known to occur in other social animals. For example, nesting gull colonies are widely seen to attack intruders, including humans. Costs of mobbing behavior include the risk of engaging with predators, as well as energy expended in the process; mockingbirds can effectively force a cat or dog to seek something less troublesome. One mockingbird might fly in front of the cat or dog, enticing it to lunge, while another pecks at the cat or dog from behind. While mobbing has evolved independently in many species, it only tends to be present in those whose young are frequently preyed on, especially birds. It may complement cryptic behavior in the offspring themselves, such as camouflage and hiding. Mobbing calls may be made prior to or during engagement in harassment.
Mobbing behavior has functions beyond driving the predator away. Mobbing draws attention to the predator, making stealth attacks impossible. Mobbing also plays a critical role in the identification of predators and inter-generational learning about predator identification. Reintroduction of species is often unsuccessful because the established population lacks this cultural knowledge of how to identify local predators. Scientists are exploring ways to train populations to identify and respond to predators before releasing them into the wild.
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 dynamicsIt 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.
Evolution of predation
Predation appears to have become a major selection pressure shortly before the Cambrian period - around - as evidenced by the almost simultaneous development of calcification in animals and algae, and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least .
Humans and predation
In conservationPredators 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, especially apex predators who are often in competition with humans. 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 controlPredators 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
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
External links - by Eric W. Weisstein, The Wolfram Demonstrations Project.
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