INTERSPECIFIC COMPETITION, ISLAND BIOGEOGRAPHY AND NULL HYPOTHESES.

Abstract

We conducted a field study of some of Darwin's Finches (Geospiza species) in order to assess the relative importance of interspecific competition and habitat features in determining the observed biogeographic, ecological and morphological characteristics of these species (Abbott et al., 1977). Strong et al. (1979) have criticized one of our methods and have reanalyzed a small portion of our data. They employed stochastic models to generate expected beak size differences between sympatric species, and then compared expected with observed differences. Finding a generally close correspondence between expected and observed differences, they concluded that random processes are sufficient to account for the observations, and that therefore there is no need to invoke deterministic processes such as competition as we had done. Strong et al. (1979) obtained the same results and drew the same conclusion from analyses of beak size differences among birds on the Tres Marias islands of Mexico and the Channel islands of California. Simberloff and his associates have also drawn the same conclusion from a series of other analyses performed in like manner (Connor and Simberloff, 1978; Simberloff, 1978). We take issue with the procedures Strong et al. (1979) have used in their analyses and with the way in which our statements and interpretations have been represented. We identify five problems in their analyses and five sources of confusion in the interpretation of results. We find no evidence in their analyses or arguments to change our previous conclusion that interspecific competition has played a role in the adaptive radiation of Darwin's Finches. Finally, we draw attention to some unsolved problems in biogeography, concerning principally the separation of potentially conflicting effects of different processes such as dispersal and competition.