An apex predator, also known as an alpha predator or top predator, is a predator at the top of a food chain, with no natural predators. [a] Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic levels and serve as keystone species, vital to their ecosystems. One study of marine food webs defined apex predators as those feeding at trophic levels above four. Food chains are often far shorter on land, usually limited to the third trophic level – for example, wolves prey mostly upon large herbivores. The apex predator concept is applied in wildlife management, conservation and ecotourism.
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 sometimes devastated 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.
Apex predators have "profound" effects on ecosystems, as the consequences of both controlling prey density and restricting smaller predators, and may be capable of self-regulation. When introduced to subarctic islands, for example, Arctic foxes' predation of seabirds has been shown to turn grassland into tundra. 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.
Keystone species are often apex predators, though there may be other species at the same trophic level. The first example to be described (by Robert Paine) was the relationship between the starfish Pisaster ochraceus and the mussel Mytilus californianus. Keystone species thus do not have to be at the highest trophic level, but may have their own predators. Thus, sea stars are prey for sharks, rays, and sea anemones, while sea otters are prey for orca.
Dramatic changes in the Greater Yellowstone Ecosystem were recorded after the gray wolf was reintroduced to Yellowstone National Park in 1995. Elk, the wolves' primary prey, became less abundant and changed their behavior, freeing riparian zones from constant grazing and allowing willows, aspens and cottonwoods to flourish, creating habitats for beaver, moose and scores of other species. In addition to their effect on prey species, the wolves' presence also affected one of the park's vulnerable species, the grizzly bear: emerging from hibernation, having fasted for months, the bears chose to scavenge wolf kills,:56 especially during the autumn as they prepared to hibernate once again.:90 The grizzly bear gives birth during hibernation, so the increased food supply is expected to produce an increase in the numbers of cubs observed. Dozens of other species, including eagles, ravens, magpies, coyotes and black bears have also been documented as scavenging from wolf kills within the park.
Ecologists have debated whether Homo sapiens is an apex predator. Sylvain Bonhommeau and colleagues argued in 2013 that across the global food web, a fractional human trophic level (HTL) can usefully be calculated as the mean trophic level of every species in the human diet, weighted by the proportion that species forms in the diet. This analysis gives an average HTL of 2.21, varying between 2.04 and 2.57. These values are comparable to those of non-apex predators like anchovy or pig. Peter D. Roopnarine criticised Bonhommeau's approach, arguing that humans are apex predators, and that the HTL was based on terrestrial farming where indeed humans have a low trophic level, mainly eating producers (crop plants at level 1) or primary consumers (herbivores at level 2), which as expected places humans at a level slightly above 2. Roopnarine instead calculated the position of humans in two marine ecosystems, a Caribbean coral reef and the Benguela system near South Africa. In these systems, humans mainly eat predatory fish and have a fractional trophic level of 4.65 and 4.5 respectively, making them "apex predators in those systems."[b]
Ecotourism sometimes relies on apex predators to attract business. Tour operators may in consequence decide to provide food for these predators to attract them to areas that can conveniently be visited. This in turn could have effects on predator population and therefore on the wider ecosystem. As a result, provisioning of species such as the tiger shark is controversial, but its effects are not well established by empirical evidence.
In densely populated areas like the British Isles, all the large predators like the wolf, bear, wolverine and lynx have become locally extinct, allowing herbivores such as deer to multiply unchecked except by hunting. In 2015, plans were made to reintroduce lynx to the counties of Norfolk, Cumbria, and Northumberland in England, and Aberdeenshire in Scotland as part of the rewilding movement. The reintroduction of large predators is controversial, not least because of concern among farmers for their livestock. Conservationists such as Paul Lister propose instead to allow wolves and bears to hunt their prey in a "managed environment" on large fenced reserves.
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