Distributions of Galapagos Ground Finches Along An Altitudinal Gradient: The Importance of Food Supply

I investigated the abundance, diet and food supply of ground finch species (Geospiza Gould) at different altitudes over a calendar year on Isla Pinta, Galapagos. The primary purpose of the study was to evaluate which of three alternative hypotheses best explains the present distributions of species along the vegetation gradient: habitat structure, food supply, or current interspecific competition. Isla Pinta was specifically chosen because Lack (1947) thought that on this island one species, G. difficilis, was competitively restricted to the highlands by the morphologically very similar species, G. fuliginosa. Four of the five species present were broadly distributed among habitats, and their altitudinal ranges overlapped considerably. The fifth, G. scandens, was rare and little was learned of its habit. G. difficilis ate mainly litter arthropods and gastropods, especially at high elevations. The remaining three species, G. fuliginosa, G. fortis, and G. magnirostris, were primarily granivorous, but each species consumed a different range of seed sizes. Diet overlap between species was highest when food production was high, and lowest when food was scarce. Seasonal declines in overlap seemed to be mainly due to finches' response to asynchronous food production and a gradual broadening of the available food spectrum. In the dry season the abundance and distribution of the four common species matched to varying degrees the abundance and distribution of the foods they consumed. Effects of current competition on these distributional patterns were not detected. Most Geospiza species did not seem to possess strict habitat requirements, and it is unlikely that variation in habitat structure greatly influenced distributions. One possible exception to this is G. difficilis, where litter supply may be important. Overall, the data suggest that while competition may be occurring between species, variation in food supply is currently a far more important determinant of distributions on Pinta.