Cosmopolitan (distribution)

In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. Such a taxon is said to exhibit cosmopolitanism or cosmopolitism. The opposite extreme is endemism.

The term pandemism also is in use, but not all authors are consistent in the sense in which they use the term; some speak of pandemism mainly in referring to diseases and pandemics, and some as a term intermediate between endemism and cosmopolitanism, in effect regarding pandemism as subcosmopolitanism. This means near cosmopolitanism, but with major gaps in the distribution, say, complete absence from Australia. Terminology varies, and there is some debate whether the true opposite of endemism is pandemism or cosmopolitism.

The term "cosmopolitan distribution" usually should not be taken literally, because it often is applied loosely in various contexts. Commonly the intention is not to include polar regions, extreme altitudes, oceans, deserts, or small, isolated islands. For example, the housefly is nearly as cosmopolitan as any animal species, but it is neither oceanic nor polar in its distribution. Similarly, the term "cosmopolitan weed" implies no more than that the plant in question occurs on all continents except Antarctica; it is not meant to suggest that the species is present in all regions of every continent.

Another concept in biogeography is that of oceanic cosmopolitanism and endemism. Though there is a temptation to regard the World Ocean as a medium without biological boundaries, this is far from reality; many physical and biological barriers interfere with either the spread or continued residence of many species. For example, temperature gradients prevent free migration of tropical species between the Atlantic and Indian-plus-Pacific oceans, even though there is open passage past continental masses such as the Americas and Africa/Eurasia. Again, as far as many species are concerned, the Southern Ocean and the Northern marine regions are completely isolated from each other by the intolerable temperatures of the tropical regions. In the light of such considerations, it is no surprise to find that endemism and cosmopolitanism are quite as marked in the oceans as on land.

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