Land Bridges and Ecology in Bat Distribution on Islands off the Northern Coast of South America

The zoogeography of insular faunas has always posed special problems. Prominent among these is the question of faunal composition. Whether an island is of the continental or of the oceanic type, its fauna usually differs significantly from that of nearby mainland areas. This phenomenon, in any given case, appears to be determined by two main factors. The first of these, which has received the most attention, is the past history of the island in terms of former land connections, or, if these have been absent, of ease of overwater dispersal. This latter factor may involve either absolute distance from the mainland or the presence or absence of special aids to overwater transport. Under the last heading would be classed favorable hurricane paths and large rivers which could propel natural rafts to the island in question. The second factor is the ecology, past and present, of the island and of the mainland source from which the insular fauna must come. It is evident that unless the ecologies of two islands are at least approximately similar, a faunal comparison is not likely to elucidate the means by which they were colonized. Under these circumstances it is often very difficult to determine whether the water barrier as such or the ecological difference has prevented certain forms from colonizing the island. For some time I have been much concerned about the relative roles of these two major factors as applied to the West Indian mammal fauna, particularly as they concern its largest component, the order Chiroptera. The general consensus, with which I am in hearty agreement, is that the entire mammal fauna has reached these islands across water (Simpson, 1956). It is also clear that the bat fauna represents, on the whole, an accumulation of invaders which have reached the Antilles at various times. This is shown by the extremely uneven degree of differentiation shown by the various species and higher groups. At one extreme there is an endemic subfamily, the Phyllonycterinae, while at the other, some West Indian bats are indistinguishable from their mainland representatives. An interesting feature is that