Marginal distribution (biology)

The geographical limits to the distribution of a species are determined by biotic or abiotic factors. Core populations are those occurring within the centre of the range, and marginal populations (also called peripheral populations) are found at the boundary of the range.

The inability of a species to expand its range beyond a certain geographic area is because of some limiting factor or factors to which the species cannot successfully adapt. In some cases, geographical range limits are entirely predictable, such as the physical barrier of an ocean for a terrestrial species. In other cases the specific reasons why species do not pass these boundaries are unknown, however, ecology is the main determinant of the distribution of a species. The fitness of a species falls at the edges of its distributional range, with population growth and fitness falling to zero beyond where a species can survive.

For many species of invertebrate animals, the exact geographic range limits have never been precisely ascertained, because not enough scientific field work has been carried in many parts of the world to map distribution more precisely, therefore finding a range extension for species, especially marine species, is not an uncommon occurrence.

Marginal distributions can have conservation implications.