Comparative Ecology of Galapagos Ground Finches (Geospiza Gould): Evaluation of the Importance of Floristic Diversity and Interspecific Competition

Abstract

Distribution, abundance, diet, and beak morphology of the six Geospiza finch species were studied at eight sites on seven Galapagos islands. The resulting information was used to test the theories of Lack and Bowman that interspecific competition (Lack) and floristic and food differences among islands (Bowman) determine the ecological and morphological characteristics of the finches. Both factors were found to be important in different aspects of finch ecology. Most of the 21 populations studied spent more time foraging for seeds and fruits than for insects. Most populations had generalized diets and nine out of 18 pairs of sympatric populations had diets overlapping by 50% or more. Diet breadth was positively correlated with variety of available foods (classified by an index comprising measures of size and hardness), but was not correlated with abundance or biomass of sympatric populations of finches (potential competitors). Competitive release, in the form of large population biomass on islands with few congeners, was not found. Nor was population biomass related to variety of available food. Overlap in diet between two species was greatest when the species were most similar in abundance and body size. Interspecific competition is indicated by the distribution and morphology of the finches. The absence of four combinations of species cannot be attributed to change: (1) G. conirostris and G. fortis, (2) G. conirostris and G. scandens, (3) G. difficilis and G. fuliginosa, and (4) G. scandens and G. difficilis. These absences cannot be attributed to the absence of suitable foods. and sites with similar floras and suites of food types do not necessarily have similar ground finch faunas. The ecological and morphological similarity of members of each of the four pairs suggests an inability to coexist for competitive reasons. In contrast, where only two species occur together on an island they tend to differ markedly in size (beak and body) and diet. Species with large beaks can eat larger and harder seeds and fruits than species with small beaks, and as a consequence they tend to have more diverse diets. Species with large beaks also eat medium—sized seeds and fruits quicker than species with small beaks. Species with small beaks may be able to deal with small soft seeds more efficiently than species with large beaks, but the data are not clear on this point. In addition, small species with small beaks have a metabolic advantage over large species with large beaks. Variety of available foods influences the large species more than the small species: the two largest species, G. magnirostris and G. conirostris, occur only on sites where plants producing large and hard seeds and fruits are plentiful. Food variety apparently does not influence the degree of diet overlap. The number of plant species on an island statistically determines the number of land—bird species in general, and Geospiza species in particular, which occur on that island. The influence is probably mediated through food supply, since 53% of the variation in Geospiza species diversity among the eight sites was accounted for in a multiple linear regression analysis by diversity of available seeds and fruits. We conclude that food supply and interspecific competition have jointly determined the ecological course of the radiation of Darwin's finches and the resulting pattern of species diversity. Interisland variation in vegetation favored the initial steps of differentiation. Competitive interactions among species influenced later stages by determining which ecological types could coexist on an island with a given array of foods.