Experimental Zoogeography: Introductions of Mice to Small Islands

Microtus pennsylvanicus is the only rodent native to small islands in the Gulf of Maine. Peromyscus maniculatus is found on larger islands, even when rather remote, while Clethrionomys gapperi occurs only on the largest islands within 1 mile of the mainland. Population dynamics and rates of extinction and recolonization of resident Microtus populations were studied from 1962 to 1972. During this period, the ability of Peromyscus and Clethrionomys to establish and maintain insular populations was tested through experimental introductions of from one to eight pairs. Empirical determinations of demographic parameters provide data consistent with the MacArthur-Wilson model for propagule survivorship, and propagule sizes are of the same order of magnitude as those predicted by the model. Average survival (T1) of propagules and time to ultimate extinction (TK) of established populations of the three species also agree with theoretical expectations. The distribution of the three species in the Gulf of Maine is largely accounted for by the equilibrium theory of MacArthur and Wilson. Its ability to disperse across water, to establish reproducing populations, and to attain high densities in the available habitat allow Microtus to succeed as an insular species in spite of frequent extinction. These properties are viewed as extensions of its role as a fugitive species in mainland habitats. Peromyscus is found on larger islands where carrying capacity is sufficient to preclude extinction, while Clethrionomys is found only on the largest and least isolated islands. While competition with Microtus may contribute to the poor success of experimental introductions of Clethrionomys and might forestall natural colonization, absence of Clethrionomys from most islands is attributed primarily to limited dispersal. Because immigration to distant islands depends on rare events while low extinction rates allow oversaturation of large land-bridge islands, I conclude that small mammals reach equilibrium only on small coastal islands. The means of dispersal are determined by local conditions, and successful species may vary between island groups.