Species Movement in Zoogeography

Abstract

Theories of the relationships involved in species movement between different zoogeographic regions or subregions are examined, and the conclusion reached that it is not necessary to invoke the concept of relative species dominance as a determinant of the direction of species movement in many cases. Alternative views are put forward, using techniques similar to those developed by Macarthur & Wilson in their study of island biogeography. The theory of the relationships between different zoogeographic regions or subregions has not been examined in detail, although I have elsewhere (Horton, 1973a, b, 1974) looked at some aspects of the problem. Darlington (1957) examined these relationships qualitatively and suggested that the main factor determining direction of species movement between -such regions was the relative dominance of the groups in the areas concerned. Simpson (1950) documented the interchange of species between the nearctic and neotropical regions and also reached the conclusion that dominance determined the direction of species movement. I have suggested (Horton, 1974) that it is not necessary to use the concept of dominance to explain Darlington's and Simpson's findings, but that the patterns of climatic change in different regions provide a sufficient explanation. It should, nevertheless, be possible to make some generalizations about patterns of dispersal and this study is an attempt to find such generalizations. Macarthur & Wilson (1963, 1967) expressed the hope that the methods they developed for the study of island biogeography could be extended to encompass other aspects of zoogeography. This study uses similar techniques modified to allow for an interchange of species between two regions, rather than the one way flow of species from the mainland to an island. Let us consider two areas, A and B, separated by a barrier. Let each contain ten species and let each satisfy the following conditions: (1) no previous contact between A and B; (2) both A and B have reached equilibrium with other areas, i.e. they are holding an equilibrium number of species with extinction balancing immigration; (3) all species in A and B are equally likely to