Tree cover, understory complexity and heaps shape fine-scale microhabitat partitioning in sympatric rodents

Habitat heterogeneity promotes species coexistence by increasing opportunities for niche differentiation. Temperate forest small mammals provide an excellent model for investigating how fine-scale habitat structure influences spatial segregation. In this study, we examined fine-scale microhabitat associations of sympatric forest rodents, Clethrionomys glareolus and Apodemus spp., within a heterogeneous deciduous woodland in central Italy. We reanalysed a historical dataset using contemporary statistical approaches to assess habitat associations, based on a 289-trap grid (2-m spacing, 12 days) and 20 additional traps monitored for 23 days in woody debris heaps and control plots. A total of 176 individuals of Apodemus spp. and 65 of C. glareolus were captured within the trapping grid, whereas C. glareolus was recorded near heaps significantly more frequently (49.1%, n = 57 captures) than Apodemus spp. (32.8%, n = 116). Multivariate ordination (RDA), GLMMs and GLMs consistently identified vegetation structural complexity as the main driver of habitat segregation. Clethrionomys glareolus was associated with woody debris heaps and higher tree cover, whereas Apodemus spp. preferred herb-rich, open understory habitats and occurred significantly less frequently near heaps. Independent evidence from the spatial distribution of the asp viper (Vipera aspis), the principal ambush predator of both rodents, further indicated that heaps function as focal microhabitats, with nearly 90% of recently fed vipers recorded within 5 m of these structures. These results support the hypothesis that fine-scale habitat heterogeneity promotes coexistence by reducing spatial niche overlap. Maintaining structural complexity, including woody debris and heterogeneous understory layers, may therefore contribute to sustaining small mammal diversity in Mediterranean forest ecosystems.