Similarity among Avifaunas of the Galapagos Islands

Abstract

Similarity among avifaunas is calculated for both historical and current distributions of resident land birds of the Galapagos Islands. Multiple regression analysis indicates the variation in avifaunal similarity is accounted for primarily by variation in the similarity in composition of island floras and, to a much smaller degree, by isolation among islands. Thus, plant similarity and interisland distance seem the principal identifiable factors influencing avifaunal resemblance. Similarities among islands based on numbers of plant species, area, and elevation do not have a detectable effect. Variation in plant similarity among the Islands is explained largely by isolation, with area and elevation having a reduced, but significant, influence. Close to half of the variation in avifaunal similarity is unaccounted for by plant similarity and isolation. Characteristics of islands with avifaunas that deviate most from the predicted relationship suggest that small, outlying islands in a zone of infrequent and irregular rainfall may have populations of birds prone to extinction. This would have the effect of reducing similarity with most other islands. Compounding this, islands that are upwind most of the year in the prevailing trade winds of the region probably do not receive as many colonizers from other islands as do islands that are downwind. Average avifaunal similarity among congeners is less than among noncongeners. This would be expected if ecological incompatibility exists among congeneric species of the Galapagos, or if there is differentation owing, in part, to geographic isolation.