Information on the interrelationships of Mus musculus, Peromyscus leucopus, Peromyscus maniculatus bairid, and Microtus ochrogaster was accumulated during the study of 429 25x25 m randomly selected plots in Vigo County, Indiana. P. leucopus apparently inhibited Mus musculus when the two occurred together. This may be the factor that limits Mus to cultivated areas, since P. leucopus does not flourish there. Mus and P. m. bairdi reacted in different ways in different habitates. In grassy and weedy fields, and in winter wheat they occurred together about as often as would be expected by chance, but both species were more abundant when together than when alone. In soybeans, cultivated fields, corn, and corn stubble the two occurred together less often than would be expected by chance, and both were less abundant than when alone. Both ecological separation and competitive exclusion appeared to be acting. In wheat stubble P. m bairid occurred at higher rates and Mus at lower rates when the two were together, probably because Mus had to invade and compete with the already established P. m. bairdi. Mus and M. ochrogaster, and P. leucopus and M. ochrogaster occurred together in weedy and grassy fields about as often as expected, and the species concerned were unaffected or were more abundant when together than when alone, apparently indicating similar habitat requirements but different niches. P. m. bairdi and P. leucapus occurred together much less often than would be expected by chance, and average numbers per plot were lower than together, than when either occurred alone. Apparently both ecological and competitive exclusion were occurring. P. m. bairdi and M. ochrogaster were ecologically separated, with M. ochrogaster in better cover, and P. m. bairdi in poorer cover. When together they did not affect each other appreciably.
© 1967 by the Ecological Society of America.