The deermouse, Peromyscus maniculatus gracilis, and the white-footed mouse, P. leucopus noveboracensis, cohabit tracts of woodland in parts of eastern Ontario. In view of the close ecological similarity of these two forms as suggested by the literature, this is an apparent exception to the principle of Gause.
In 1962 and 1963 a program of live-trapping on grids and traplines was conducted in an area of sympatry in Carleton County, Ontario, to investigate the habitat relationships of these forms.
The population of P. m. gracilis was at a relatively high level throughout the study. This form occurred in all wooded areas with the exception of a small deciduous woodlot. In addition, a breeding population of P. m. gracilis was located on a barren, overgrazed field on which scattered, low rock piles occurred.
The population of P. l. noveboracensis was at a low level and formed about 30 per cent of the total Peromyscus population of the area. This form was distributed more frequently along the peripheries than within the wooded areas cohabited by P. m. gracilis. P. l. noveboracensis also occurred alone in the one deciduous habitat available in the study area.
Rock cover was demonstrated to be an important factor in the selection of habitat by both P. m. gracilis and P. l. noveboracensis.
It was concluded that P. l. noveboracensis was selecting habitat on the cohabited areas despite the high population level of P. m. gracilis. It was further concluded that the characteristics of the habitat selected by P. l. noveboracensis were associated with conditions found at the edges of coniferous woods, and also found in deciduous woods and more open, shrubby areas. It was also concluded that P. m. gracilis preferred conditions associated with the damper coniferous areas.
Thus, it was postulated that the habitat condition selected by P. l. noveboracensis was a drier, warmer microclimate than that obtaining in the coniferous areas around and within which P. m. gracilis occurred. It was thus concluded that the apparent cohabitation of these two forms did not provide an exception to the principle of Gause.