Island Biogeography

Abstract

The ‘equilibrium theory’ produced the first precisely derived set of testable propositions about how biogeographic variables determine species number. A key concept is that the species on an island are at equilibrium, whereby immigrations of new species are balanced by extinctions of species already present. Islands vary considerably in the degree to which equilibrium is prominent: many are normally at equilibrium, but islands that are frequently disturbed tend not to be. A second key concept is species turnover: species identities change over time despite constancy in species number. Turnover can be common or rare, and where common may mainly involve ephemeral species of low abundance rather than the numerically dominant species. The ‘equilibrium theory’ also predicts more species on islands with greater area: this ‘species–area effect’ is a seemingly university ecological property not limited to islands. Habitat or elevation are sometimes more strongly related to species number or occurrence than area; typically area, habitat diversity, and elevation are strongly correlated. ‘Equilibrium theory’ predicts fewer species on distant islands. This ‘distance effect’ is less precisely documented; it may in part result from habitat poverty on far islands. The study of extinction on islands is becoming increasingly sophisticated and aids substantially in conservation efforts.