Endemism

Endemism, term used in ecology to describe the tendency of plants and animals to be naturally confined to a given area; within such an area they are said to be “endemic”.

Endemism can be considered at a wide range of geographical scales: an organism may be endemic to a single mountain-top or lake, to a mountain range or river system, an island, a country, or even a continent. Normally it is applied to species, but may also be used for subspecies, genera, families, or other taxonomic groupings.

Areas with high endemism—that is, with large numbers of endemic species—are of great importance in conservation. Loss of such areas will lead to the extinction of a significant number of species.

Endemism is the result of the combination of evolution and geographical isolation. When a population of plants or animals becomes isolated for a long period from other populations of the same species, it will tend to evolve divergently, eventually forming a separate species. This may happen when plants or animals disperse from their normal range as, for example, when a land animal colonizes an oceanic island. It may also occur when a formerly continuous habitat is disrupted by climatic or geological events. Land upheavals may divide a single river system in two, or a change to a more arid climate may cause forests to shrink, dividing once continuous cover into a series of isolated patches.

Endemism is maintained by barriers to dispersal of the species concerned. These are areas of inhospitable habitat which individuals cannot easily cross. Just what these barriers are depends on the particular species, its habits, and powers of dispersal. Dry land is a barrier to aquatic species, while the sea is a barrier to most land animals and plants. Areas of acidic soils and rocks prevent dispersal of plants adapted to limestone, while arid lands are barriers to species of moist forests.

Generally, the longer an area has been isolated from other similar areas, the higher the proportion of species which are endemic. Ancient islands such as Madagascar and New Zealand have a very high rate of endemism among the terrestrial species that live there.

The geographic range of animals and plants usually changes over time, so that endemism is not fixed. Today, human activities are undoubtedly the most important factor affecting the distribution of species. However, biologists tend to consider the natural ranges of species when talking of endemism. For example, the Madagascar Periwinkle Catharanthus roseus (from which the leukaemia drug vincristine is derived) is considered an endemic of the island despite now being naturalized throughout most of the tropics.