Saturation of Bird Communities in the West Indies

Abstract

The paper is directed to interpreting the set of data displayed in figure 1, which shows that the number of bird species in two widespread West Indian habitats reaches a ceiling (saturation) that holds over a wide range of island species numbers. Four hypotheses are evaluated in an effort to understand the result: (1) Island communities are as tightly packed as they can be without breaching the limits of similarity; (2) island bird species are habitat specialists; (3) island species display an upper limit of tolerance of interspecific competition; (4) island species respond to reduced competition by broadening habitat spectra and other forms of ecological release. West Indian guilds are found to be less tightly packed than their mainland counterparts, eliminating hypothesis 1 in the sense of absolute limits to packing, but not foreclosing the possibility that island species are competitively excluded at lower packing levels than mainland species. The number of species per island increases in tight correlation (r2 = .90) with the number of available habitats, and large islands do contain numbers of species with restricted habitat ranges. Nevertheless, when tested with the opportunity to expand into atypical habitats containing reduced numbers of competitors most species do so, suggesting that habitat ranges in rich faunas are largely delineated by diffuse interspecific competition. A good deal of evidence suggests that island species are unable to invade richer mainland communities or, in some cases, even the richest large islands. We speculate that the evolution of ecological versatility at the expense of competitiveness leads to coadaptation of the fauna throughout the archipelago for the level of diffuse interspecific competition characteristic of the largest islands. Various forms of ecological release (expansion of abundance, habitat spectra, vertical foraging ranges) could then serve on smaller islands to maintain the observed constancy of within-habitat species numbers over a wide range of island species numbers.