The time to extinction of a colonizing propagule of lizards increases with island area

Abstract

The question of what causes species extinction and how to prevent it is a crucial one for both basic and applied ecology. Three kinds of island-biogeographical field studies have made particularly important contributions towards this problem. First, a variety of bird studies suggest for small populations a negative relation between extinction time and population size; smaller islands typically have smaller populations (see ref. 1 and H. L. Jones, J. M. Diamond and M. Gilpin, manuscript in preparation). Second, in a study on arthropods2, experimentally reducing the area of already saturated mangrove islands caused numbers of species to decrease, presumably because of reduced population sizes. Third, in a study of mammals3 in which small, variably sized populations of colonists (propagules) were experimentally introduced onto islands, larger propagules survived longer. In the present study with lizards, both island area and propagule size were varied experimentally in an attempt to separate their effects. The results show: (1) for a given species, time to extinction increased with island area in a monotonic fashion; (2) above a certain island area, rapid colonization occurred; below that threshold, propagules went extinct quickly; and (3) propagule size had no effect.