Ecological+Biology

= **Predation** = In ecology, **predation** describes a biological interaction where a **predator** (an organism that is hunting) feeds on its **prey** (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption. Other categories of consumption are herbivory (eating parts of plants) and detritivory, the consumption of dead organic material (detritus). All these consumption categories fall under the rubric of consumer-resource systems. It can often be difficult to separate our various types of feeding behaviors. For example, parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on it while it continues to live or on its decaying corpse after it has died. The key characteristic of predation however is the predator's direct impact on the prey population. On the other hand, detritivores simply eat dead organic material arising from the decay of dead individuals and have no direct impact on the "donor" organism(s). Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations. The 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 the predator's interaction with prey.

A predator is usually a carnivore that hunts, kills and eats other animals. For example a snake eating a mouse, the snake is considered the predator because it is consuming the mouse. In another example, a striped marlin is a predator. It lives in the Pacific Ocean and preys on sardines, also a Pacific animal. Similarly, various birds eat earthworms. However, a predator can become the prey of another larger predator. For instance, a snake may become a meal for a hawk. "In ecology, predation is a mechanism of population control. Thus, when the number of predators is scarce, the number of preys should rise. When this happens, the predators would be able to reproduce more and possibly change their hunting habits. As the number of predators rise, the number of preys decline. This results in food scarcity for predators that can eventually lead to the death of many predators." (www.Biology-Online.org) Because of this, predation is called a "positive-negative" relationship. (Campbell). media type="youtube" key="GFTBFDPfMh8" height="315" width="420"

Functional classification
Classification 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. Additionally, whether or not the prey are directly killed by the predator is considered, with true predation and parasitoidism involving certain death.

True predation


A true predator can commonly be known as one which kills and eats another organism. Whereas other types of predator all harm their prey in some way, this form certainly kills them. 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 or a human; others may eat their (usually much smaller) prey whole, as does a bottlenose dolphin swallowing a fish, or a snake, duck or stork swallowing a frog. Some animals that kill both large and small prey for their size (domestic cats and dogs are prime examples) may do either depending upon the circumstances; either would devour a large insect whole but dismember a rabbit. Some predation entails venom which subdues a prey creature before the predator ingests the prey by killing, which the box jellyfish does, or disabling it, found in the behavior of the cone shell. In some cases the venom, as in rattlesnakes and some spiders, contributes to the digestion of the prey item even before the predator begins eating. In other 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 and egg predation are other forms of true predation, as seeds and eggs represent potential organisms. Predators of this classification need not eat prey entirely, for example some predators cannot digest bones, while others can. Some may eat only part of an organism, as in grazing (see below), but still consistently cause its direct death.

Grazing
//Grazing// 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.

Parasitism
//Parasites// 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 (butterfly and moth) 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.

Parasitoidism
//Parasitoids// 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, Diptera and Coleoptera parasitoids make up as much as 10% of all insect species.

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. Some 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 cats and snakes, which, in the latter's case, 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, and may go only as high as three trophic levels (for example, a lion that preys upon large herbivores such as wildebeest which in turn eat grasses). 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, sperm whale, anaconda, Komodo dragon, tiger, lion, Tiger shark, Nile crocodile, and most eagles -- and even omnivorous humans and grizzly bears. An apex predator in one environment may not retain this position as a top predator if introduced to another habitat, such as a dog among alligators or a snapping turtle among jaguars; a predatory species introduced into an area where it faces no predators, such as a domestic cat or a dog in some insular environments, can become an apex predator by default. Many organisms (of which humans are prime examples) eat from multiple levels of the food chain and thus make this classification problematic. 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 herbivores such as the giant panda may supplement their diet with meat. Scavenging of carrion provides a significant part of the diet of some of the most fearsome predators. 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 or parasites would also be difficult to classify by such a scheme.

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**Competition**
According to the competitive exclusion principle, species less suited to compete for resources should either adapt or die out. According to evolutionary theory, this competition within and between species for resources plays a critical role in natural selection, however, competition may play less of a role than expansion among larger groups such as families. media type="youtube" key="D1aRSeT-mQE" height="315" width="560"
 * Competition** is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource (such as food, water, and territory) used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology . Competition is one of many interacting biotic and abiotic factors that affect community structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. Competition is not always straightforward, and can occur in both a direct and indirect fashion.

Intraspecific competition
Intraspecific competition is a particular form of competition in which members of the //same// species vie for the same resource in an ecosystem (e.g. food, light, nutrients, space). This can be contrasted with interspecific competition, in which different species compete. For example, two trees of the same species growing close together will compete for light, water and nutrients in the soil. Getting less resources, they will perform more poorly than if they grew by themselves (for example lowered growth rates and fewer seed output). Trees have therefore adapted to grow taller or develop larger root systems through natural selection. Grasshoppers provide an animal example. By eating grass, individual grasshoppers deprive their fellow conspecifics of food. This is an example of exploitation competition, which means that the grasshoppers do not interact directly with each other, but rather have a negative effect on others' growth and reproduction by their effect on a resource (in this case, grass). In other cases, intraspecific competition may be a case of interference competition, in which the animals interact directly. This is the case, most notably, in territorial animals: some individuals actively prevent others from exploiting a given resource, usually food or space. Intraspecific competition is a major factor affecting the carrying capacity of a population (maximum population level supported by the environment). The levelling of population growth at high densities (known as density dependent inhibition ) can be seen as an effect of intraspecific competition. Indeed, whereas at low densities organisms do not compete for resources, at higher densities resources become limiting, and the population size can no longer increase. In terms of population growth rate, this produces a sigmoidal curve, which is a familiar sight for ecologists.

Interspecific competition
Interspecific competition, in ecology, is a form of competition in which individuals of //different// species compete for the same resource in an ecosystem (e.g. food or living space). The other form of competition is intraspecific competition, which involves organisms of the same species. If a tree species in a dense forest grows taller than surrounding tree species, it is able to absorb more of the incoming sunlight. However, less sunlight is then available for the trees that are shaded by the taller tree, thus interspecific competition. Cheetahs and lions can also be in interspecific competition, since both species feed on the same prey, and can be negatively impacted by the presence of the other because they will have less food. Competition is only one of many interacting biotic and abiotic factors that affect community structure. Moreover, competition is not always a straightforward, direct, interaction. Interspecific competition may occur when individuals of two separate species share a limiting resource in the same area. If the resource cannot support both populations, then lowered fecundity, growth, or survival may result in at least one species. Interspecific competition has the potential to alter populations, communities and the evolution of interacting species. On an individual organism level, competition can occur as interference or exploitative competition. Direct competition has been observed between individuals, populations and species, but there is little evidence that competition has been the driving force in the evolution of large groups. For example, between amphibians, reptiles and mammals.

Competitive exclusion principle
In ecology, the competitive exclusion principle, sometimes referred to as Gause's law of competitive exclusion or just Gause's law, is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant. When one species has even the slightest advantage or edge over another, then the one with the advantage will dominate in the long term. One of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into the maxim "complete competitors cannot coexist.

Mutualism
Mutualism is the way two organisms of different species biologically interact in a relationship in which each individual derives a fitness benefit (//i.e.//, increased or improved reproductive output). Similar interactions within a species are known as co-operation. Mutualism can be contrasted with interspecific competition, in which each species experiences //reduced// fitness, and exploitation, or parasitism, in which one species benefits at the //expense// of the other. Mutualism is a type of symbiosis. Symbiosis is a broad category, defined to include relationships which are mutualistic, parasitic or commensal. Mutualism is only one //type//. A well known example of mutualism is the relationship between ungulates (such as cows ) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion; the bacteria benefit from having a stable supply of nutrients in the host environment. Mutualism plays a key part in ecology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 48% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms (important for pollination mutualisms) and co-evolution between groups of species. However mutualism has historically received less attention than other interactions such as predation and parasitism. Measuring the exact fitness benefit to the individuals in a mutualistic relationship is not always straightforward, particularly when the individuals can receive benefits from a variety of species, for example most plant-pollinator mutualisms. It is therefore common to categorize mutualisms according to the closeness of the association, using terms such as obligate and facultative. Defining "closeness," however, is also problematic. It can refer to mutual dependency (the species cannot live without one another) or the biological intimacy of the relationship in relation to physical closeness (//e.g.//, one species living within the tissues of the other species).

Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host (e.g. //Taenia solium//). These are now called macroparasites (typically protozoa and helminths ). The word parasite now also refers to microparasites, which are typically smaller, such as viruses and bacteria, and can be directly transmitted between hosts of the same species. Unlike [|predators], parasites are generally much smaller than their host; both are special cases of consumer-resource interactions. Parasites show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the //Plasmodium// species, and fleas. Parasitism is differentiated from the parasitoid relationship, though not sharply, by the fact that parasitoids generally kill or sterilise their hosts. Parasitoidism occurs in much the same variety of organisms that parasitism does. The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology, such as parasitic castration and impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for resources necessary for the parasite's survival, e.g. food, water, heat, habitat, and genetic dispersion. Although the concept of parasitism applies unambiguously to many cases in nature, it is best considered part of a continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived.

Amensalism
Amensalism is the type of symbiotic relationship that exists where one species is inhibited or completely obliterated and one is unaffected. This type of symbiosis is relatively uncommon in rudimentary reference texts, but is omnipresent in the natural world. An example is a sapling growing under the shadow of a mature tree. The mature tree can begin to rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients. Throughout the process the mature tree is unaffected. Indeed, if the sapling dies, the mature tree gains nutrients from the decaying sapling. Note that these nutrients become available because of the sapling's decomposition, rather than from the living sapling, which would be a case of parasitism.

Commensalism
Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal used of human social interaction. The word derives from the medieval Latin word, formed from //com-// and //mensa//, meaning "sharing a table". Commensal relationships may involve one organism using another for transportation ( phoresy ) or for housing ( inquilinism ), or it may also involve one organism using something another created, after its death ( metabiosis ). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies and spiders building their webs on plants.