Immigrant Selection, Predation, and the Distributions of Microtus pennsylvanicus and Blarina brevicauda on Islands

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Abstract
The insular distributions of Microtus and Blarina in the Thousand Island Region [New York, USA and Ontario, Canada] result from the combined effects of immigrant selection (selection for the more vagile phenotypes) and predatory exclusion. Blarina is a relatively poor immigrator and thus is restricted to the near islands (those < 700 m); Microtus inhabits even the most distant islands and occupies habitats considered atypical for the species. On the near islands, Microtus is restricted to its typical habitat (grasslands) by Blarina, which appears to prey most heavily on th juvenile voles. The primary means of immigration of mammals in the Thousand Island Region is across the river''s ice cover in winter. Consistent with the immigrant selection hypothesis, winter track surveys revealed that Microtus used the ice more frequently, traveled further, and dispersed over a greater range of air temperatures than did Blarina. Treadmill experiments indicated that the maximum dispersal distances at winter temperatures (0.degree.-5.degree. C) are significantly greater for Microtus than Blarina, and support a bioenergetic model predicting that larger mammals should have greater immigration abilities (average weights of Microtus and Blarina .apprxeq. 40 and 18 g, respectively). As predicted by the immigrant selection hypothesis, insular body size of Microtus and Blarina increases with isolation. Predatory exclusion of Microtus by Blarina was evidenced by an inverse correlation between Microtus and Blarina densities on both insular and mainland sites. Introductions of Blarina onto islands inhabited by Microtus resulted in declines in Microtus densities, extinction of Microtus on 1 island, and a drastic reduction in juvenile survivorship. Thus, because of its broad niche and superior immigration abilities, Microtus is an excellent colonizer of depauperate (small or isolated) islands in comparison to most other small mammals. On more speciose islands, Microtus may be excluded from all but its optimal habitat by predation. Only on the larger islands can Microtus coexist with Blarina by using the refugium of its optimal habitat, grasslands. Crowell''s (1983) recent suggestion that niche shifts of insular species were unduly attributed to competitive release seems valid. As the interrelationships between Microtus and Blarina clearly demonstrate, niche expansions of some insular species may result from the absence of predators (not competitors) on depauperate islands. Immigrant selection probably merits much more attention from community ecologists than it has received. Evidence from mammalian communities of the Thousand Island Region and other archipelagoes suggests that immigrant selection may be of general importance and that community structure and, perhaps, body size of mammals on islands result from the combined effects of selective forces operating during, as well as subsequent to immigration.