DOI: 10.5061/dryad.wm37pvmk5
title: Mammals on mountainsides revisited: trait-based tests of assembly reveal the importance of abiotic filters

description: 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.
description: Species occurrence data for 34 species of small mammal were
 obtained through field work by the authors and numerous collaborators.
 Occurrences have grouped into 100-m elevational bins and presences
 interpolated between the highest and lowest record of
 observation. Species trait data were compiled data from
 existing databases and the literature for 23 traits.  These were organized
 under four niche axes: environmental filtering-related traits (ie. beta
 traits; n = 8), biotic interaction-related traits (ie. alpha-traits; n =
 12), life history (n = 2), and body size. 2% of trait values were
 estimated using regression or genus-level averages.
 Phylogenetic relationships and branch lengths (in millions of
 years) were obtained from the "best dates” supertree of Fritz et al.
 (2009) pruned to the 34 rodents and shrews in our dataset, with two
 polytomies resolved using the "multi2di" function in R package
 'ape'. Climate data (mean annual temperature
 and mean annual total precipitation) were calculated using PRISM data
 (prism.oregonstate.edu) extracted from points along each 100-m contour
 line across the elevational extent of the mountains. An index of aridity
 was caluclated according to the formula MAP/(MAT+10).
 Diversity values were calculated for several indices of
 phylogenetic and functional diversity using available R packages and
 standard procedures.
description: Files stored here include the complete species trait matrix
 (Great_Basin_mammal_trait_data_Kohli_etal_JBI.csv) and fully dichotomous
 phylogeny (GBTree.di_Kohli_etal_JBI.nex) used in all analyses, as well as
 a summary of species occurrences by elevational bin per mountain (3
 MtnName_ElevBins files), the mean climate data
 (ElevBin_Climate_Kohli_etal_JBI.csv), and calculated diversity metrics and
 statistics for each elevational bin (DiversityValues_Kohli_etal_JBI.csv). 
 The Diversity Values file contains columns for Mountain, elevational bin
 (ElevBin), species richness (Richness), and the calculated index values,
 SES, and p-value of the SES value for 8 metrics: mean phylogenetic
 distance (MPD), mean nearest phylogenetic distance (MNPD), and mean
 functional distance (MFD) and mean nearest neighbor functional distance
 (MNND) for all traits (.all), beta-traits related to environmental
 filtering (.ef), and alpha-traits related to biotic interactions
 (.bi). For more detailed information, see ReadMe.txt
 file.