Mammals on mountainsides revisited: trait-based tests of assembly reveal the importance of abiotic filters

Aim: Mountains provide uniquely informative systems for examining how
biodiversity is distributed and identifying the causes of those patterns.
Elevational patterns of species richness are well-documented for many taxa
but comparatively few studies have investigated patterns in multiple
dimensions of biodiversity along mountainsides, which can reveal the
underlying processes at play. Here, we use trait-based diversity patterns
to determine the role of abiotic filters and competition in the assembly
of communities of small mammals across elevation and evaluate the
surrogacy of taxonomic, functional, and phylogenetic dimensions of
diversity. Location: Great Basin ecoregion, western North America Taxon:
Rodents and shrews Methods: The elevational distributions of 34 species of
small mammals were determined from comprehensive field surveys conducted
in three arid, temperate mountain ranges. Elevation-diversity
relationships and community assembly processes were inferred from
phylogenetic (PD) and functional diversity (FD) patterns of mean pairwise
and mean nearest neighbor distances while accounting for differences in
species richness. FD indices were calculated separately for traits related
to either abiotic filtering (β-niche traits) or biotic interactions
(α-niche traits) to test explicit predictions of the role of each across
elevation. Results: Trait-based tests of processes indicated that abiotic
filtering tied to a strong aridity gradient drives the assembly of both
low and high-elevation communities. Support for competition was not
consistent with theoretical expectations under the stress-dominance
hypothesis, species interactions-abiotic stress hypothesis, or guild
assembly rule. Mid-elevation peaks in species richness contrasted with
overall FD and PD, which generally increased with elevation. PD and total
FD were correlated on two of three mountains. Main conclusions: The
functional diversity of small mammal communities in these arid, temperate
mountains is most consistent with abiotic filters whereas support for
competition is weak. Decomposing FD into traits related to separate
assembly processes and examining ecoregional variation in diversity were
critical for uncovering the generality of mechanisms. Divergent patterns
among dimensions revealed species richness to be a poor surrogate for PD
and FD across elevation and reflect the effect of biogeographic and
evolutionary history. This first analysis of elevational multi-dimensional
diversity gradients for temperate mammals provides a versatile framework
for future comparative studies.