Disjunct distribution

In biology, a taxon with a disjunct distribution is one that has two or more groups that are related but considerably separated from each other geographically. The causes are varied and might demonstrate either the expansion or contraction of a species' range.

Also called range fragmentation, disjunct distributions may be caused by changes in the environment, such as mountain building and continental drift or rising sea levels. It may also be due to an organism expanding its range into new areas, by such means as rafting, or other animals transporting a propagule to a new location; seeds consumed by birds and animals can be moved to new locations during migrations, and those seeds can be deposited in fecal matter. Other conditions that can produce disjunct distributions include flooding, changes in wind, stream, and current flows, and other anthropogenic introduction of introduced species either accidentally or deliberately through agriculture and horticulture.

Disjunct distributions can occur when suitable habitat is fragmented, which produces fragmented populations, and when that fragmentation becomes so divergent that species movement between one suitable habitat to the next is disrupted, isolated population can be produced. Extinctions can cause disjunct distribution, especially in areas where only scattered areas are habitable by a species; for instance, island chains or specific elevations along a mountain range or areas along a coast or between bodies of water like streams, lakes and ponds.

There are many patterns of disjunct distributions at many scales: Europe - East Asia, Europe-South Africa (e.g. genus Erica), Mediterranean-Hoggart disjunction (genus Olea), amphi-Pacific distribution (Australia - South America), Asa Gray disjunction (eastern North America and East Asia), etc.

This kind of disjunct distribution of a species, such that it occurs in Iberia and in Ireland, without any intermediate localities, is usually called "Lusitanian" (named after the Roman Province Lusitania, corresponding roughly to modern-day Portugal).

Examples of animal species with a Lusitanian distribution are: the Kerry slug Geomalacus maculosus and the Pyrenean glass snail Semilimax pyrenaicus. Plant species with this kind of distribution include several heather species (Calluna spp.) and the strawberry tree (Arbutus unedo).

The theory behind the name "Lusitanian" is now discredited; it posited that there was an ice-free land mass that served as a refugium off of the south-west of Ireland during the Quaternary (last) glaciation. In this refugium, relic fauna and flora from a previous ice-free period survived until the present warmer interstadial period. Although the theory is no longer accepted, the term Lusitanian is still used as a descriptive term for faunal elements such as the Kerry slug.

Recently a better explanation of the occurrence of the Kerry slug and similar faunal elements in southwestern Ireland has been developed. This new theory is supported by two recent discoveries: the genetic similarity of much of Ireland's fauna to that of northern Spain, and the genetic similarity of much of Ireland's human population to that of northern Spain.