Environmental Heterogeneity as a Differential Driver for Density of Two Sympatric Rodent Species

Environmental heterogeneity drives patterns of species diversity by shifts in population abundances. High levels of heterogeneity may be associated with the disruption of ecosystem structure and, in turn, with the decline of specialist species due to a reduced availability of the habitats or resources they need. We analysed two taxonomically close and sympatric rodent species with differences in niche breadth: the forest specialist Apodemus flavicollis and the generalist A. sylvaticus . We set our study along a gradient of environmental heterogeneity across different developmental stages of a deciduous oak forest originally managed through coppicing. We assessed the densities of Apodemus species through spatially explicit capture-recapture analyses in 12 grids during a three-year campaign. We quantified environmental heterogeneity in each grid based on direct measurements of stand structural attributes, vascular plant species composition and functional traits, distinguishing between overstorey and understorey heterogeneity. We tested the responses of the species densities to these components of heterogeneity through linear mixed models. The two Apodemus species responded differently to environmental heterogeneity. The generalist species showed a weak decreasing trend as overstorey heterogeneity, that is, tree species diversity, increased, while the specialist species was favoured by tree diameter heterogeneity. Conversely, the highly heterogeneous understorey linked to recent coppicing was associated with higher densities of the generalist species, but understorey heterogeneity variables had no significant effect on the density of the specialist one. Our findings suggest that the response of biotic populations and communities to environmental heterogeneity may depend on the species' niche breadth and on the degree of environmental heterogeneity.