Everything about Predation totally explained
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's 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's 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 don't 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 (for example the
praying mantis to the right resembling dead leaves) other organisms (for example
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's 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 can't 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 isn't 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's 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's 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 can't focus on locating eggs while they're under direct attack.
Advertising unprofitability
Once a predator has detected its prey, one would expect it to pursue it. However, it isn't 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's some way the prey species can convey the information that it's 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're 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 (for example 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 isn't 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're 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.
Further Information
Get more info on 'Predation'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://predation.totallyexplained.com">Predation Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
