Decomposing niche components reveals simultaneous effects of opposite deterministic processes structuring alpine small mammal assembly

Our knowledge of community assembly dynamics under multiple stressors is
limited because of opposite niche-based processes, i.e., limiting
similarity and habitat filtering could simultaneously occur, masking the
overall patterns. Alpine biomes provide the ideal systems to explore the
influences of co-occurrence processes as these communities usually face
multiple stresses such as resources limitation and habitat constraints.
However, the assembly processes of mammals in alpine areas have hardly
been exclusively studied. Here, we aimed to address how different
processes structured small mammal communities at the tree line transition
zone, which represents one of the most distinct vegetation boundaries
separating alpine from montane habitats. We compiled a regional dataset
including species list, phylogeny, and functional traits from field
collections across 18 mountains of southwest China and complemented them
with published data sources. The traits were decomposed into different
niche components to determine the respective effects of specific
stressors. Phylogenetic and functional diversity indices representing
evolutionary history, trait space, and pairwise species distance were
calculated and compared with null expectations. Linear mixed-effect models
were constructed to assess the increasing or decreasing tendencies of
diversity values against increasing elevation. The results showed that
phylogenetic and functional richness were strongly correlated with species
richness, unlike the distance-based indices which were uncorrelated with
species richness. There was no evidence found to support non-random
phylogenetic or overall trait patterns. However, the resource acquisition
niche tended to be more overdispersed (positive slopes), while the habitat
affinity niche tended to be more clustered (negative slopes) as habitats
became less productive and less vegetated. We conclude that limiting
similarity and habitat filtering simultaneously structure small mammal
communities in alpine areas. Altogether, the present study provides vital
insights into the complexity of co-occurring assembly processes by niche
decomposition, and highlights the importance of considering different
diversity dimensions when assessing community structure.