Species Number and Compositional Similarity of the Galapagos Flora and Avifauna

The flora and avifauna of the Galapagos Islands are used to reexamine quantitative analyses of species numbers and compositional similarities. Conflicts in the results of previous analyses of the Galapagos flora are reconciled and are shown to be the result of using different species numbers and physiographic parameter measurements. The idea that Galapagos plant species numbers largely reflect conditions in the archipelago during the Pleistocene glaciations (i.e., that relaxation times are very long) is criticized, and the more parsimonious alternative, that they reflect recent conditions, is proposed. Generalizations about factors determining species numbers that are based on multiple regression and correlation are precarious. The number of botanical collecting trips to each of the Galapagos Islands is a better predictor of species numbers than are area, elevation, or isolation. Two null hypotheses concerning the determination of floral and avifaunal compositional similarities among the Galapagos Islands are proposed and tested, through the application of a new method of similarity analysis employing the "expected number of taxa shared" (E t s ) and its variance (V E t s ). These hypotheses view compositional similarities as resulting from the stochastic dispersal and persistence of a pool of species equally likely to colonize (Null Hypothesis I), and of a pool of species with different dispersal and persistence abilities (Null Hypothesis II). Although both hypotheses are found to be inadequate models of compositional similarity in the Galapagos, the results suggest that a substantial proportion of compositional similarity can be considered a consequence of stochastic processes of dispersal and persistence, and that compositional similarity arises indirectly as a result of similar species numbers rather than directly in a fashion determined by the physical environment or species interactions. Analyses of the distribution of congeneric bird species in the Galapagos, based on similarity analyses, are also criticized.