ECOLOGICAL REPLACEMENT OF THE DEER MOUSE, PEROMYSCUS-MANICULATUS, BY THE WHITE-FOOTED MOUSE, PEROMYSCUS-LEUCOPUS, IN THE GREAT-LAKES REGION

The possible ecological replacement of the boreal, long-tailed Forest Deer Mouse Peromyscus maniculatus gracilis, by the White-footed Mouse, P. leucopus noveboracensis, in Wisconsin was studied by analysis of t he zoogeography (parapatric distribution and relict populations of P. m. gracilis in the south and on islands), and by surveying relative po pulation abundance on Washington Island, where competition probably co mmenced in 1987, upon the arrival of P. leucopus. In Wisconsin, the ge ographic range of P. leucopus is expanding northward, and P. maniculat us gracilis has vanished in the south and on the Door Peninsula. The W hite-footed Mouse thrives in brushy and dry forest habitats (often nes ting in ground burrows), and the Forest Deer Mouse seems both adapted to high tree branches and cavities, to cold weather, and to boreal (co nifer, mixed conifer) forests. On Washington Island, after an initial increase of P. leucopus and the concomitant near extinction of P. m. g racilis, both species have coexisted at low to medium densities during the mild winters of the past three years.