SMALL ISLANDS AND THE EQUILIBRIUM THEORY OF INSULAR BIOGEOGRAPHY.

The biotas of islands, long a favorite province of biogeographers and evolutionary biologists, have received much attention within the last few years. For example, Carlquist (1965, 1966a, 1966b, 1966c, 1967a, 1967b) has considered many aspects of insular biology including thel dispersal mechanisms which' transport disseminules to islands, the effect of island ecology on establishment, and the evolutionary phenomena that are evident once species have become established. Among the subjects which interested Carlquist as well as earlier students is the relationship between biotic diversity and both island size and distance from the source region. For most island systems one can detect a typical area-diversity pattern, with larger islands supporting the richest biota. Also, evident is an inverse relationship between species number and distance from the source. Islands that are far removed have fewer species than similar-sized islands that are more proximate. Thus the biota of an island reflects the biotic diversity of the source region, the size of the island, and the distance from the source. More recently the problem of insular biogeography has been dealt with differently. MacArthur and Wilson (1963, 1967) have suggested that the relationships between diversity, size, and distance can be approached more mathematically. Th'ey have proposed that the biota of an island contains an equilibrium number of species, with the equilibrium number controlled by a balance between immigration rate and extinction rate. The slop,e of the immigration curve is distance-dependent, implying that the immigration rate for far islands is low. The extinction curve is area dependent, with a steeper slope (hence higher rate) on small islands. Both rates are controlled by the number of species present on the island. These relationships are illustrated in Figure 1. From this figure it can be seen that as the biota of the island approaches the equilibrium number, the immigration rate drops and the extinction rate rises.. The point of intersection of the curves indicates the "equilibrium number" of species or that fraction of the "species pool" represented at equilibrium. On large islands the slope of the immigration curve will be steeper, hence the immigration curve will intersect the extinction curve further to the right and the equilibrium number will increase. As distance from source increases (and immigration rate consequently decreases), species number decreases because the immigration curve will intersect the extinction curve further to the left. The contributions of both Carlquist's and MacArthur and Wilson's work are significant. The phenomena of dispersal, immigration, colonization, extinction, and speciation which attend the insular syndrome have been more clearly defined, new approaches have been suggested, and much discussion has been engendered. We have found it interesting to examine certain aspects of the equilibrium model in the light of Carlquist's data on dispersal and the ecological factors which affect establishment (1965, 1967a). Some inconsistencies are evident. In addition, it appears to us that the application of the model can provide erroneous conclusions in certain instances. Thus in the present paper we would like to deal with some of these problems, specifically: (1) the atypical situation presented by exceptionally small is-