<P> Apex predators affect prey species' population dynamics and populations of other predators, both in aquatic and in terrestrial ecosystems . Non-native predatory fish, for instance, have sometimes devastated formerly dominant predators . A 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 . As a terrestrial example, the badger, an apex predator, predates on and also competes with the hedgehog, a mesopredator, for food such as insects, small mammals, reptiles, amphibians and ground - nesting bird's eggs . Removal of badgers (in a trial investigating bovine tuberculosis) caused hedgehog densities to more than double . Predators that exert a top - down control on organisms in their community are often considered keystone species . The presence of any top predator (bear, big cat, wolf, dog, or especially human) can greatly alter the behavior of such a smaller predator as a badger . </P> <P> Apex predators can have profound effects on ecosystems, as the consequences of both controlling prey density and restricting smaller predators, and may be capable of self - regulation . They are central to the functioning of ecosystems, the regulation of disease, and the maintenance of biodiversity . 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 cause or disrupt trophic cascades . For example, reduction in the population of sperm whales, apex predators with a fractional trophic level of 4.7, by hunting has caused an increase in the population of large squid, trophic level over 4 (carnivores that eat other carnivores). This effect, called mesopredator release, occurs in terrestrial and marine ecosystems; for instance, in North America, the ranges of all apex carnivores have contracted whereas those of 60% of mesopredators have grown in the past two centuries . </P> <P> Because apex predators have powerful effects on other predators, on herbivores, and on plants, they can be important in nature conservation . Humans have hunted many apex predators close to extinction, but in some parts of the world these predators are now returning . They are increasingly threatened by climate change . For example, the polar bear requires extensive areas of sea ice to hunt its prey, typically seals, but climate change is shrinking the sea ice of the Arctic, forcing polar bears to fast on land for increasingly long periods . </P> <P> Dramatic changes in the Greater Yellowstone Ecosystem were recorded after the gray wolf, both an apex predator and a keystone species (one with a large effect on its ecosystem), was reintroduced to Yellowstone National Park in 1995 as a conservation measure . 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, especially during the autumn as they prepared to hibernate once again . 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 . </P>

Is a polar bear a predator or prey
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