Endemism describes taxa that are distributed on particular areas. It represents a basic feature of geographic distributions: species are rarely cosmopolitan and most species and even supraspecific taxa are confined to restricted regions. Endemism occurs on a variety of spatial scales, from areas as large as continents to small areas as islands or mountain tops. The puma (Puma concolor) is endemic to the Americas, occurring from Canada to Patagonia. Several plant families are endemic to the Neotropics. The volcano rabbit or teporingo (Romerolagus diazi ) is endemic to some volcano tops (2800-4250 m) from the central part of the Transmexican Volcanic Belt (Mexico). Many researchers in the past focused on species endemic to small areas and thus endemism has been incorrectly associated with 'rarity'.
Organisms can be endemic on different taxonomic levels; usually the size of the area depends on the category of the taxon, with genera having larger areas than species, and families having larger areas than genera. This situation, however, is not comparable between different taxa: the distribution of a plant species may correspond to the distribution of an insect family. Conventionally, some authors have used the term 'endemic' for taxa restricted to a single biogeographical region, 'characteristic' for those taxa shared by two regions, 'semicosmopolitan' for taxa inhabiting three or four regions, and 'cosmopolitan' for taxa inhabiting five or more regions.
The restriction of taxa to particular areas is a consequence of both historical and ecological factors. Historical events are invoked to explain how a taxon became confined to its present range. Vicariant events caused by drifting continents, long-distance dispersal, and extinction are some of these events. On the other hand, ecological explanations are invoked to explain the present limits of endemic taxa. Abiotic (temperature, altitude, soil) and biotic factors are commonly considered.
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