THE DISTRIBUTION OF GEOSPIZA DIFFICILIS IN RELATION TO G. FULIGINOSA IN THE GALÁPAGOS ISLANDS: TESTS OF THREE HYPOTHESES.

David Lack (1947, 1969) argued that interspecific competition for food has played a significant role in the adaptive radiation of Darwin's Finches. He regularly invoked competition to explain morphological and distributional patterns in the finches. An important example is the altitudinal distribution of Geospiza difficilis in relation to G. fuliginosa (Fig. 1). Low elevation, dry islands in the Galapagos are occupied by one of the two species, but never both. Islands possessing a highland vegetation zone may have both species but here they are altitudinally segregated, with G. fuliginosa occupying the lowlands and G. difficilis the highlands. Lack noted the morphological similarity of these finch species and suggested that G. difficilis had been excluded on low islands or altitudinally restricted on high islands through competition for food by G. fuliginosa wherever the species had come into contact. Bowman (1961), however, suggested instead that the unique food requirements of the two species and differences in food supply among islands, once known, would suffice to explain the pattern. The ecology of these two species was recently investigated over a calendar year on Isla Pinta where they coexist (Schluter, 1982a, 1982b). Surveys conducted on this island and on Islas San Salvador and Fernandina (unpubl. observ.) revealed that G. difficilis and G. fuliginosa are not allopatric as Lack believed. Their altitudinal ranges overlap extensively year-round although they are not coextensive (Fig. 2). Detailed observations of behavior and abundance strongly suggested that interspecific competition between them is presently weak. Food supply was probably limiting over part of the year, but interspecific aggression (e.g., territoriality) was not observed, and their diets and feeding positions were very different. The distributions and abundance of both species along the elevational gradient on Isla Pinta could easily be interpreted as being determined by the availability of their different foods. The question of whether past competition has influenced the distribution of G. difficilis in relation to G. fuliginosa remains. Significantly, G. difficilis varies in size and shape from island to island. For example on Isla Genovesa the species resembles the missing G. fuliginosa (Lack, 1947; see also Grant, in press). Possibly G. difficilis has adapted to exploit lowland environments on islands without a highland zone, and it is this lowland form which cannot coexist with G. fuliginosa. Observed differences in sympatry might also be the result of past competition. Lack's competition hypothesis is therefore still consistent with the observed insular distributions of these two species. Here we report the results of a field study which tested the argument that competition has influenced the distributions on low islands, against two alternative hypotheses based on Bowman's (1961) suggestions. We use information gained on diets, density and food supply of G. difficilis and G. fuliginosa on Pinta (Schluter, 1982b) to predict attributes of the two species in allopatry. The three hypotheses make different predictions, and these are tested with field data. The allopatric populations we used were G. difficilis on Genovesa and G. fuliginosa on Marchena (Fig. 2).