Experimental Zoogeography of Islands: Effects of Island Size

A controlled experiment performed on 8 small mangrove islands constituted an exact test of several biogeographic hypotheses which had rested largely on unsatisfying statistical treatments. The islands were censused for arboreal arthropods; each contained a different subset of a species pool of °500. Sections of the islands were then removed and censuses retaken after a waiting period; this procedure was repeated on four islands a second time. The results were consistent with a model which posits the islands as originally in a state of dynamic equilibrium between immigration and extinction, then re—equilibrating quickly when forced into an oversaturated condition. It was specifically demonstrated that: (1) species number increases with island size alone, independent of habitat diversity; (2) the increase with area is faster on separate islands than on increasing subsections of one island; (3) the area effect and the underlying dynamic equilibrium are not due only to an increased sample of transients and vagrants on larger islands, though there are a number of species which are particularly prone to quick extinction and which contribute disproportionately to the observed high turnover rates; and (4) prediction of the effects of decreased area on species composition can be stochastic at best, not deterministic. The equilibrium theory prediction of higher turnover rate on smaller islands could not be conclusively tested because of small sample size. Several species interactions suggested by the distributional data are so subtle that it is apparent that much more intensive work will be required to demonstrate even the existence of interactions, let alone whether they are important. It is clear that much of the dynamic equilibrium and its associated turnover in this system can be ascribed to individual species characteristics and the effects of a rigorous physical environment.