Mammal functional diversity increases with vegetation structural complexity in two forest types

Enhanced understanding of relationships between functional diversity (FD) and environmental gradients is crucial given accelerating rates of land-cover change and disturbance worldwide. Functional diversity measures the abundance, range and distribution of traits in a community, and links species diversity with ecosystem function. Several studies have related bird or invertebrate FD to environmental gradients, but information on the responses of mammal FD to vegetation structural diversity at scales of management relevance is scarce. We addressed this knowledge gap by examining responses of ground-dwelling mammal FD to vegetation structural complexity in wet (high-productivity) and dry (low-productivity) eucalypt forest in the Otway Ranges, southeast Australia. In dry forest, we expected a positive relationship between FD and vegetation structural complexity because more resources should enable species with a greater diversity of traits to co-exist. We expected negative correlations in wet forest, where competitive dominance may drive a decrease in FD as structural complexity increases. Ground-dwelling mammals were surveyed using camera traps, and we used five traits to construct four FD indices (richness, evenness, divergence, and dispersion). Six vegetation structure variables were used to calculate two indices of vegetation structure, and we used linear mixed models to relate functional diversity and species richness to vegetation structural complexity (the total abundance of structural attributes) and heterogeneity (the level of contrast or patchiness in structure), in wet and dry forest. Camera traps detected ten native ground-dwelling mammal species. All FD indices were positively correlated with vegetation structural complexity, but only functional dispersion responded to structural heterogeneity. Contrary to expectations, relationships between FD and structural complexity were consistent in both forest types, and we suggest that low levels of functional niche occupancy prevented competitive dominance in wet forest. Species richness did not respond to any predictor variables, and is unlikely to be a useful surrogate of ground-dwelling mammal FD. Our results indicate that forest managers may sustain ecosystem functions performed by ground-dwelling mammals by conserving structurally complex vegetation.