Endemic

Endemic is a biogeographic term referring to a distinct race or species that originated in a local place or region, and that has a geographically restricted distribution. Endemic species tend to occur in certain ecological contexts, being especially frequent in places that are ecologically isolated, and that have not been affected by a regional-scale, catastrophic disturbance for a very long time.

For example, islands situated in remote regions of the oceans are physically isolated from other landmasses. In any cases where such islands have continuously supported ecosystems for a long period of time (at least tens of thousands of years), their biota will mostly be composed of endemic species of plants and animals that are not found elsewhere. This attribute of island biodiversity occurs because, under highly isolated conditions, evolution over long periods of time proceeds towards the development of new species from older, founder species. In itself, this process leads to the evolution of distinctly different, endemic races and species on remote islands.

Moreover, the biotas of remote islands tend to be depauperate in numbers of species, compared with any continental area of similar size and habitat type. As a result, competition among similar species tends to not be very intense in some types of island habitats, and there may be relatively broad niche opportunities available to be exploited by plants and animals. Consequently, on the rare occasions when remote islands are successfully colonized by new species, these founders may be able to evolutionarily radiate, and develop a number of new species that are found nowhere else. Some of the more remarkable examples of evolutionary radiation of endemic species on oceanic islands include: (1) the 13 species of Darwin's finches (subfamily Geospizinae of the finch family, Fringillidae) on the Galápagos Islands of the Pacific Ocean; (2) the 25 species of honeycreepers (Drepanididae) of the Hawaiian Islands; and (3) the approximately 1,250 species of fruit flies (genus Drosophila) on the Hawaiian Islands. All of these diverse groups likely evolved by adaptive radiations into unfilled habitat opportunities of single, founder species.

Therefore, because of the evolutionary influences of isolation and adaptive radiation on islands, these places tend to have many endemic species. For example, almost 900 species of birds, about 90% of all bird species, are endemic to oceanic islands. The native flora of the Hawaiian archipelago is estimated to have originally contained 2,000 species of angiosperm plants, of which 94-98% were endemic. Similarly, 76% of the plants of the Pacific island of New Caledonia are endemic, as are 50% of those of Cuba, and 36% of those of Hispaniola.

Habitat "islands" can also occur on land, in the form of isolated communities located within a dominant matrix of another type of ecosystem. Large numbers of endemic species may evolve over long periods of time in terrestrial habitat islands. This could be part of the basis of the evolution of the extraordinary species richness of tropical rainforests, which may have been contracted into isolated fragments during past episodes of dry climate. It is likely that more than 90% of Earth's species live in tropical forests, and most of these are endemics.

Endemic species are rare in places that have "recently" been subject to some sort of catastrophic, regional-scale disturbance. (Note that in the sense used here, "recent" on the evolutionary time scale means within the most recent, several tens of thousands of years.) For example, much of North America was covered by glacial ice up until about 10,000-12,000 years ago. Although most of that region has since supported vegetation for about ten thousand years, this has not been enough time to allow many endemic species to develop. Consequently, countries like Canada, which were entirely glaciated, have virtually no endemic species.

Because endemic species have geographically restricted distributions, and they have often evolved in isolation from many types of diseases, predators, and competitors, they tend to be highly vulnerable to extinction as a result of the activities of humans. About three-quarters of the species of plants and animals that are known to have become extinct during the past few centuries were endemics that lived on islands. For example, of the 108 species of birds that have become extinct during that period, 90% lived on islands. About 97% of the endemic plants of Lord Howe Island are extinct or endangered, as are 96% of those of Rodrigues and Norfolk Islands, 91% of those of Ascension Island, and 81% of those of Juan Fernandez and the Seychelles Islands. Tremendous numbers of endemic species also occur in the tropical rainforests of continental regions. Most of these species are now endangered by the conversion of their habitat to agriculture, or by disturbances associated with forestry and other human activities.