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Apex predators (also known as alpha, super-, or top-level predators) are predators that have virtually no predators of their own, residing at the top of their food chain. Apex predator species are often at the end of long food chains, where they have a crucial role in maintaining the health of ecosystems.

Definition

Predation in its zoological sense is the killing and consumption of another organism, which excludes bacteria and parasites from the apex predator concept. Predation has been used in this context since 1932. The apex predator concept is commonly applied in wildlife management and conservation, as well as eco-tourism. In these contexts it has been defined in terms of trophic levels. Trophic levels are "hierarchical strata of a food web characterized by organisms which are the same number of steps removed from the primary producers." Primary, secondary, tertiary, and higher level consumers occupy successive trophic levels. One study of marine food webs defined apex predators as greater than trophic level four.

Food chains are often far shorter on land, with the top of the food chain limited to the third trophic level, as where such predators as the big cats, crocodilians, hyenas, wolves, or giant constrictor snakes prey upon large herbivores. Apex predators need not be hypercarnivores. For example, grizzly bear and humans are each apex predators, and yet they are omnivores that eat considerable vegetable material as well as much meat. Some animals may be superpredators in some environments but not others, like dogs and domestic cats, both of which can ravage ecosystems.

Ecological role

See also Mesopredator release hypothesis.
Apex predators affect prey species' population dynamics. Where two competing species are in an ecologically unstable relationship, apex predators tend to create stability if they prey upon both. Inter-predator relationships are also affected by apex status. Non-native fish, for example, have been known to devastate formerly dominant predators. One lake manipulation study found that when the non-native smallmouth bass was removed, lake trout, the suppressed native apex predator, diversified its prey selection and increased its trophic level.

Effects on wider ecosystem characteristics, such as plant ecology, have been debated, but there is evidence of a significant impact by apex predators: introduced arctic foxes, for example, have been shown to turn subarctic islands from grassland into tundra through predation on seabirds. Such wide-ranging effects on lower levels of an ecosystem are termed trophic cascades. The removal of top-level predators—often, recently, through human agency—can radically cause (or disrupt) trophic cascades. A commonly cited example of apex predators affecting an ecosystem is Yellowstone National Parkmarker. After the reintroduction of the gray wolf in 1995, researchers noticed drastic changes occurring in the Greater Yellowstone Ecosystem. Elk, the primary prey of the gray wolf, became less abundant and changed their behavior, freeing riparian zones from constant grazing. The respite allowed willows and aspens to grow, creating habitat for beaver, moose, and scores of other species. In addition to the effects on prey species, the gray wolf's presence also affected the park's grizzly bear population. The bears, emerging from hibernation, chose to scavenge off wolf kills to gain needed energy and fatten up after fasting for months. Dozens of other species have been documented scavenging from wolf kills. Keystone species, as first defined by Robert Paine for Seastars are apex predators within functional groups [141174].

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