Island Colonization by Lesser Antillean Birds

Abstract

-We present an analysis of bird distribution in small islands in the northern Lesser Antilles colonized principally from Guadeloupe. In spite of great differences among the islands in soils, rainfall, and vegetation, their avifaunas are strikingly uniform. We found that species inhabiting coastal scrub on the source island performed better as colonists than inhabitants of interior rainforest, suggesting that humid forests in the target islands would hold drastically impoverished bird communities. This proved not to be the case. Diversities in the small-island rainforest communities were compensated by the substitution of coastal scrub species for missing forest counterparts and the expansion of vertical foraging zones. In progressing from species-poor to species-rich communities in equivalent habitat, the number of trophic guilds remains constant, while the number of species per guild and the tightness of species packing increase. We conclude that the faunal uniformity of islands colonized from Guadeloupe results from nonuniform dispersal abilities coupled with ordering ecological constraints: versatility in habitat occupancy, trophic status and size in relation to guild neighbors.-Department of Biology, Princeton University, Princeton, New Jersey 08540; Division of Biological Sciences, University of Missouri, Columbia, Missouri 60521; and Department of Zoology, University of Florida, Gainesville, Florida 32601. Accepted 2 August 1976. MODERN biogeography has enjoyed considerable success in accounting for the numbers of species of birds, lizards, and other taxa found on the islands of various archipelagoes. Nearly all the interisland variation in bird species numbers in the southwest Pacific, for example, can be explained by a simple empirical formula that takes into account each island's area, elevation, and distance from New Guinea, the ultimate source of colonists in that region (Diamond 1973). Equilibrium theory (MacArthur and Wilson 1963) maintains that insular faunas are stabilized by a dynamic balance of colonizations and extinctions, and suggests the statistical limits within which the number of species should vary on any given island. However, equilibrium theory tells us nothing about how different or how alike the faunas of two nearby islands should be. If they were totally different we should be surprised. We should also be surprised if they were totally alike, for the equilibrium model rests on random processes, limited to be sure by certain statistical constraints. To phrase the question in general terms, we ask whether the interaction of colonization and extinction leads to chaos, a rigidly ordered community structure, or to some more perplexing intermediate condition. In at least one group of islands, the Lesser Antilles, the answer is clearly that the composition of insular avifaunas is highly deterministic. In a previous publication one of us (Terborgh 1973) showed by regression analysis that the species lists of both the smaller ( 300 but < 1,600 kM2) islands were approximately 90% determined by their locations. Another result of this analysis was that interisland differences in habitat in the Lesser Antilles exert such a minor influence on species composition that it is scarcely detectable. This was particularly astonishing in the case of the northern Lesser Antilles, a group of 10 islands lying to the north and northwest of Guadeloupe, and to the east of the Virgins (Fig. 1). These islands comprise 2 natural groups, an inner chain of 5 high, wet volcanic peaks covered with rainforest, and an outer chain of 5 low, dry coral platforms covered with xeromorphic scrub. Though the islands of the two chains are of very nearly the same size and within sight of one another, it is hard 59 The Auk 95: 59-72. January 1978 This content downloaded from 157.55.39.212 on Sun, 09 Oct 2016 05:49:50 UTC All use subject to http://about.jstor.org/terms 60 TERBORGH ET AL. [Auk, Vol. 95 PUERTO RICO ANOUILLA ST. MARTIN MONA S D CDS T. BARTHELE MY o ) Y VIRGIN 19S. SABA ZIBARBUDA ST. EUSTATIUS SE o ANTlGUA ST. KITTS/ ST. KITTSZ 4MONTSERRAT GUADELOUPE NEVIS XS \?U0()GUADELOUPE /E GRANDE TERRE O