Habitat selection among allied species is one of the key supports for niche theory in ecology. Line transects and systematic sampling was used to study habitat selection by Chevrier's field mouse and the South China field mouse from June to August 2009. The two species are sympatric in the Tangjiahe Nature Reserve, Qingchuan County, Sichuan Province, China. We compared habitat plots used by both mammals and control plots reflecting the environment at large to investigate their habitat use patterns. 140 plots were measured totally. Of which, 49 were used by Chevrier's field mouse, 41 were used by South China field mouse and the rest 50 were control plots. 21 environmental variables potentially influencing their habitat selection were measured.
In univariate analysis, 16 variables were significantly different between plot groups, indicating the two mammals both have a distinct significant habitat preference. In addition, their selected habitats overlapped to some extent: they both preferred for habitats with gentle slopes, lower canopy height, significant fallen leaf ground coverage, a relatively higher proportion of open land, larger tree diameter at breast height, greater tree height, moderate shrub bamboo and herb density, and larger herb coverage when compared with the control plots. Significant differences in habitats selected indicated the two species have distinct habitat selection patterns. Chevrier's field mouse occurred more frequently in habitats with lower elevations (about 1400 m) and taller herbs (about 16-30 cm), while South China field mouse occurred in habitats with higher elevations (about 1900 m) and shorter herbs (<15 cm). Moreover, Chevrier's field mouse preferred for habitats characteristic of evergreen and deciduous mixed broad-leaved forests, a shorter distance to water sources (<50 m), vegetation in earlier successional stage (a small growth of trees) and shorter bamboo height(<1 m). South China field proved to be more of a generalist with no prominent selectivity for the above ecological factors. This indicated the former's habitat is more decentralized while Chevrier's field mouse's habitat is more centralized or its habitat use is more specialized.
Stepwise discriminant function analysis showed the four variables of elevation, vegetation stage of succession, canopy, and herb height contributed more to habitat separation between the two small mammals than others. The higher discriminant function coefficients indicated the two species could coexist sympatrically.
Habitat-use strategies are often cited as the means by which sympatric species avoid competition. Occupying separate microhabitats is common among sympatric species. It seemed that these two small mammals can coexist and evolve in the same region contributing to their temporal and spatial separation in habitat use. Since they have similar body sizes and breeding dates and share the same food sources, we hypothesize habitat separation between the two species may not have resulted from differences in physiological and ecological requirements, but from ecological or evolutionary adjustment to reduce interspecific competition.Based on our findings, different habitat management strategies should be adopted for the two species.