Small mammal species richness is directly linked to regional productivity, but decoupled from food resources, abundance, or habitat complexity

Aim: Species richness is often strongly correlated with climate. The most commonly invoked mechanism for this climate-richness relationship is the more-individuals-hypothesis (MIH), which predicts a cascading positive influence of climate on plant productivity, food resources, total number of individuals, and species richness. We test for a climate-richness relationship and an underlying MIH mechanism, as well as testing competing hypotheses including positive effects of habitat diversity and heterogeneity, and the species-area effect. Location: Colorado Rocky Mountains, USA: two elevational gradients in the Front Range and San Juan Mountains. Methods: We conducted standardized small mammal surveys at 32 sites to assess diversity and population sizes. We estimated vegetative and arthropod food resources as well as various aspects of habitat structure by sampling 20 vegetation plots and 40 pitfall traps per site. Temperature, precipitation and net primary productivity (NPP) were assessed along each gradient. Regressions and structural equation modelling were used to test competing diversity hypotheses and mechanistic links predicted by the MIH. Results: We detected 3,922 individuals of 37 small mammal species. Mammal species richness peaked at intermediate elevations, as did productivity, whereas temperature decreased and precipitation increased with elevation. We detected strong support for a productivity-richness relationship, but no support for the MIH mechanism. Food and mammal population sizes were unrelated to NPP or mammal species richness. Furthermore, mammal richness was unrelated to habitat diversity, habitat heterogeneity, or elevational area. Main conclusions: Sites with high productivity contain high mammal species richness, but a mechanism other than a contemporary MIH underlies the productivity–diversity relationship. Possibly a mechanism based on evolutionary climatic affiliations. Protection of productive localities and mid-elevations are the most critical for preserving small mammal richness, but may be decoupled from trends in population sizes, food resources, or habitat structure.