Marginal distribution (biology)

In some cases, geographical range limits are entirely predictable, such as the physical barrier of an ocean for a terrestrial species.

This is based on the assumption that the habitat is most ideal at the centre of a distribution and ecological conditions decline towards the margin.

Reduced gene flow between central and peripheral populations also limits the genetic diversity at the margins.

High selection pressure, due to a less than ideal habitat at the margin, furthermore reduces genetic diversity.

In Europe, for example, the geographical spokes sticking out of the continent in the south - the Iberian Peninsula, Italy and the Balkans served as refugia for warmth-adapted species during the Ice Ages.

This is often seen in organisms with high water demands, whose survival and reproduction is limited by dry conditions.

[15] There are many other abiotic factors that can determine a species range, including dissolved oxygen, conductivity, alkalinity and pH.

Where two similar species share an overlapping range, competition often causes the distributions to shift to exclude one of the two.

[24] Humans have also initiated many range expansions by introducing species to new locations both intentionally and accidentally.

[25] In most cases combinations of factors are responsible for limiting the geographic range edge of species.