COMPETITIVE EXCLUSION BETWEEN THE RODENT GENERAMICROTUSANDCLETHRIONOMYS

Abstract

Comparative studies of the faunas occurring on islands and adjacent mainland areas have revealed instances of what have been interpreted as examples of competitive exclusion in a number of vertebrate groups, particularly birds (Amadon, 1953; Lack, 1942; Mayr, 1942, 1963; Serventy, 1951) and reptiles (Brown and Marshall, 1953; Underwood, 1962). In these cases it has been found that closely related forms with similar habits may occur sympatrically on a mainland area but often only one species has succeeded in becoming established on an adjacent island, because, it is assumed, of the poverty of available ecological niches. It further appears, in many instances, that the particular species which first reaches an island is the one which will become established there and once it has done so it can apparently prevent subsequent colonization by other closely related forms. Similar cases of competitive exclusion on islands have not been reported upon in detail for mammals, and it therefore seems desirable to place on record what appears to be a clear-cut example of ecological competition between the Holarctic rodent genera Micratus (North America: meadow vole; Britain: short-tailed vole) and Clethrionomys (North America: redbacked vole; Britain: bank vole). North American data apply only to Microtus pennsylvanicus and Clethrionomys gapperi due to the lack of detailed information on other species of these genera. The data presented here are based primarily on field studies carried out by the writer at various times over a 10-year period; otherwise, material has been drawn from the published works of other zoologists. On the mainland of temperate North America where both genera occur, Microtus pennsylvanicus is essentially a grassland form whereas Clethrionomys gapperi is virtually confined to forested, or at least, scrubby areas. In peak years of population density, Microtus may invade open woodlands in small numbers and may even occur in isolated grassy areas which are completely surrounded by forest. Scrubby areas, especially in swamps, may be invaded at such times. Clethrionomys, on the other hand, seems never to invade exclusively grassy areas, regardless of population density. This is not to imply that C. gapperi is incapable of living in grassland habitats, but no instance is known to the writer where this is the case. That Microtus may well be restricted to grasslands on the North American continent because it is excluded from the woodlands by Clethrionomys is quite apparent from the fact that on large, ecologically diversified islands such as Newfoundland where Clethrionomys is absent, Microtus occurs commonly throughout the woodlands as well as the grasslands (Cameron, 1958). Twenty-seven of the 43 specimens collected on the island were taken in forested areas; one specimen was collected in a mature spruce woodland 20 miles from the nearest known grassland area. Similarly, Microltus was found to be most abundant in upland coniferous woodlands on the Magdalen Islands (Quebec) in the summer of 1959. Thirty-six of the 50 specimens collected were taken in woodlands. Cletlrionomys does not occur there but the deer mouse, Peromyscus maniculatus, is native and presumably a limited degree of competition does exist between it and Microtus when the latter is at a peak in population density, as was the case in the summer of 1959 (Cameron, 1962a). On a number of smaller inshore islands in eastern Canada studied by the writer, Microtus was also found in heavily forested