A thermal gradient of trait similarity across North America

The associated data files and R script are all that is required to reproduce the analyses in the manuscript "A thermal gradient of trait similarity across North America," currently in review at <i>Global Ecology and Biogeography</i>. <br>The following .csv files contain data required to reproduce the analysis, with raw unprocessed small mammal data from NEON, small mammal data that has been put through our QC code, mammal trait data compiled from different sources, and site-level environmental covariates: <b>raw_NEON_mammal_data.csv, final_NEON_mammal_data.csv, mammal_traits.csv, site_covariates.csv</b><b><br></b>The following .csv files contain metadata describing the corresponding data file: <b>final_NEON_mammal_data_meta.csv, mammal_traits_meta.csv, site_covariates_meta.csv. </b>Each file lists the data sources. Please see documentation at neonscience.org for more information on the NEON small mammal trapping data.<br>The following .R file is an R script, tested under R/3.3.3, that contains code to reproduce the analyses in the manuscript: <b>code_supplement.R</b><br>Below is the abstract:<b>Aims: </b>Temperature is widely regarded as a major driver of species richness, but the mechanisms are debated. Niche theory suggests temperature may affect richness by filtering traits and species in colder habitats while promoting specialization in warmer ones. However, tests of this theory are rare because niche dimensions are challenging to quantify along broad thermal gradients. Individual-level trait data are now available from long-term monitoring networks spanning large geographic extents, enabling trait-based tests of theory at large spatial scales. <b>Location: </b>Continental United States <b>Time period: </b>2015 <b>Taxon: </b>Rodentia <b>Methods: </b>Here we examine variation in body size among 23 communities of North American rodents sampled across the National Ecological Observatory Network (NEON), ranging from northern hardwood forests to subtropical deserts. We quantify body size similarity among species using a metric of overlap that accounts for individual variation, and fit a structural equation model to disentangle the relationships between temperature, productivity, body size overlap, and species richness. <b>Main conclusions: </b>We document a latitudinal gradient of declining similarity in body size among species towards the tropics and overall increase in the dimensions of community-wide trait space in warmer habitats. Neither environmental temperature nor net primary productivity directly affect rodent species richness. Instead, temperature determines the community-wide niche space that species can occupy, which in turn alters richness. We suggest a latitudinal gradient of trait space expansion towards the tropics may be widespread and underlie gradients in species diversity.