Apex predators (also known as alpha-, super-, top- or top-level predators) are predators with no predators of their own, residing at the top of their food chain.[3] Zoologists define predation as the killing and consumption of another organism (which generally excludes parasites and most bacteria).[4] In this context, "apex predator" is usually defined in terms of trophic dynamics. Apex predator species occupy the highest trophic level(s) and have a crucial role in maintaining the health of their ecosystems. One study of marine food webs defined apex predators as greater than trophic level four.[5] The apex predator concept is commonly applied in wildlife management, conservation, and ecotourism.
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 bears and humans[6] are each apex predators and are omnivores. A dog, more carnivorous than either humans or most bears, is usually more of a scavenger than a predator, but as an occasional killer of livestock or wildlife and a participant in some human hunts it qualifies as a superpredator in much of its extensive range; this is especially true of giant molosser breeds that have for their size the strength, power, agility, speed, cunning, intelligence, and aggression characteristic of wolves, bears, big cats, and hyenas but unlike those other predators do not ordinarily prey on humans. In this sense, an apex predator can be defined as being too difficult to kill for them to be a regular source of food for other predators. Some animals may be superpredators in some environments but not others, such as domestic dogs and cats, both of which can ravage ecosystems (see Stephens Island Wren).
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.[7]
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.[8] 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.[9][10] A commonly cited example of apex predators affecting an ecosystem is Yellowstone National Park. 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, aspens, and cottonwoods to grow, creating habitat for beaver, moose, and scores of other species.[11] In addition to the effects on prey species, the gray wolf's presence also affected the park's grizzly bear, a vulnerable species. The bears, emerging from hibernation, chose to scavenge off wolf kills after fasting for months.[12][13] They can also eat wolf kills in autumn to prepare for hibernation.[14] As grizzly bears give birth during hibernation, a greater food supply may improve the mother’s nutrition and increase the number of cubs.[15] Dozens of other species, including eagles, ravens, magpies, coyotes, and black bears, have been documented scavenging from wolf kills.[16] Keystone species are apex predators within functional groups, a concept first described by zoologist Robert Paine to explain the relationship between Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel.[17]
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