also be a factor: white nose syndrome in bats, for example, is causing a substantial decline in their populations and may even lead to the extinction of a species. Overhunting also has an impact. Terrestrial mammals, such as the tiger and deer, are mainly hunted for their pelts and in some cases meat, and marine mammals can be hunted for their oil and leather. Specific targeting of one species can be problematic to the ecosystem because the sudden demise of one species can inadvertently lead to the demise of another (coextinction) especially if the targeted species is a keystone species. Sea otters, for example, were hunted in the maritime fur trade, and their drop in population led to the rise in sea urchins—their main food source—which decreased the population of kelp—the sea urchin's and Steller's sea cow's main food source—leading to the extinction of the Steller's sea cow. The hunting of an already limited species can easily lead to its extinction, as with the bluebuck whose range was confined to and which was hunted into extinction soon after discovery by European settlers. Australia Island creatures are usually endemic to only that island, and that limited range and small population can leave them vulnerable to sudden changes. While Australia is a continent and not an island, due to its geographical isolation, its unique fauna has suffered an extreme decline in mammal species, 10% of its 273 terrestrial mammals, since European settlement (a loss of one to two species per decade); in contrast, only one species in North America has become extinct since European settlement. Furthermore, 21% of Australia's mammals are threatened, and unlike in most other continents, the main cause is predation by feral species, such as cats. Extinct species A species is declared extinct after exhaustive surveys of all potential habitats eliminate all reasonable doubt that the last individual of a species, whether in the wild or in captivity, has died. Recently extinct species are defined by the IUCN as becoming extinct after 1500 CE. Extinct subspecies Extinct in the wild A species that is extinct in the wild is one which has been categorized by the International Union for Conservation of Nature (IUCN) as only known by living members kept in captivity or as a naturalized population outside its historic range due to massive habitat loss. A species is declared extinct in the wild after thorough surveys have inspected