Apex predator

Overview
Apex predators (also alpha predators, superpredators, or top-level predators) are predators that, as adults, are not normally preyed upon in the wild in significant parts of their range. 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
The term 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. In land environments with far shorter food chains, for example, lions preying upon zebras which eat grass illustrate that a superpredator may be an animal at the third trophic level.

Ecological role

 * See also Mesopredator release hypothesis.

That apex predators affect prey species' population dynamics is clear. Research has shown, for instance, that 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 fishes, 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 suppresed 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 through human agency—can radically cause (or disrupt) trophic cascades.