Although wild dogs, like wolves, are apex predators, they can be killed in territory disputes with wild animals.[139] Furthermore, in areas where both dogs and other large predators live, dogs can be a major food source for big cats or canines. Reports from Croatia indicate wolves kill dogs more frequently than they kill sheep. Wolves in Russia apparently limit feral dog populations. In Wisconsin, more compensation has been paid for dog losses than livestock.[139] Some wolf pairs have been reported to prey on dogs by having one wolf lure the dog out into heavy brush where the second animal waits in ambush.[140] In some instances, wolves have displayed an uncharacteristic fearlessness of humans and buildings when attacking dogs, to the extent that they[which?] have to be beaten off or killed.[141] Coyotes and big cats have also been known to attack dogs. Leopards in particular are known to have a predilection for dogs, and have been recorded to kill and consume them regardless of the dog's size or ferocity.[142] Tigers in Manchuria, Indochina, Indonesia, and Malaysia are reputed to kill dogs with the same vigor as leopards.[143] Striped Hyenas are major predators of village dogs in Turkmenistan, India, and the Caucasus.[144] Reptiles such as alligators and pythons have been known to kill and eat dogs. 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.[1] Zoologists define predation as the killing and consumption of another organism (which generally excludes parasites and most bacteria).[2] 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.[3] 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 do not need to be hypercarnivores. For example, grizzly bears and humans[4] are each apex predators and are omnivores. [edit]Ecological role ee 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.[5] 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.[6] 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.[7][8] 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.[9] 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.[10][11] They can also eat wolf kills in autumn to prepare for hibernation.[12] As grizzly bears give birth during hibernation, a greater food supply may improve the motherís nutrition and increase the number of cubs.[13] Dozens of other species, including eagles, ravens, magpies, coyotes, and black bears, have been documented scavenging from wolf kills.[14] 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.