These demographic effects are things such as the death of a fertile female in a small population or a litter of all males or the inability to find a mate; just random chances that are no problem at all in a big population or never occur. But those are also exacerbated by genetic factors such as inbreeding depression, low juvenile survival, susceptibility to disease. The interplay of those factors can lead to local extinction.

Michael Gilpin and Michael Soule elaborated upon Mark Shaffer¹s original idea of the four causes of extinction, and they developed a paradigm of what they called an extinction vortex, which is sort of a feedback loop where, as the population gets small, it loses genetic variation, and that effects the reproduction and the population gets smaller still: going into what they call an extinction vortex.

And then of course, environmental effects such as three severe droughts in a row can wipe out one or several populations over a large area and wipe them out almost immediately. And if there are no individuals nearby to come back and repopulate, then the species will go extinct locally.

These and other factors are the basis of the Theory of Island Biogeography, which you probably know about, that was developed by MacArthur and Wilson in 1967. They studied species on islands and developed a mathematical relationship between the number of species and the area of the island. Larger islands support more species. They developed a concept of turnover rates such that as species go extinct on an island they have a chance to be replaced by other species that can disperse onto the island. The closer that an island is to a mainland (or a source of dispersing individuals) the more rapidly those extinct species can be replaced.

Narrative Index

Table of Contents

BioForum Index

AE Partners Collection Index

Activities Exchange Index