SPATIAL VARIATION IN STABLE ISOTOPE ECOLOGY OF SMALL MAMMALS IN WESTERN NORTH AMERICA

Mammal species richness across the modern landscape of North America reveals a clear biogeographic diversity gradient: areas of greater topographic complexity have higher species richness than areas of low topographic relief. Research investigating the origins of this pattern reveals a volatile history of mammal diversification in western North America over the last 30 million years, suggesting that complex interactions among landscape, climate, ecology and evolutionary processes are driving diversity gradients over broad temporal and spatial scales. Ecological processes that influence diversification include species accommodation across heterogeneous habitats and niche differentiation. In order to understand the history of resource use and habitat preference of fossil mammals during past climate and landscape change, relationships between environment and ecology must first be assessed within modern species.

We investigated the isotope ecology of extant rodents in order to assess niche breadth and overlap across environmental gradients. Making up more than half of the modern diversity in North American mammals, rodents display a pronounced topographic richness gradient and high ecological diversity. With strong habitat associations, relatively small home ranges and low biomass consumption, these small mammals have significant potential for capturing local-scale landscape heterogeneity. This research focuses on the rodent family Heteromyidae, which includes extant kangaroo rats and pocket mice and over 80 fossil species that have diversified across the changing western North American landscape since 30 Ma. Using stable isotopic ratios of carbon, nitrogen, oxygen and hydrogen from hair samples, we evaluated the diets and habitat preferences of two geographically widespread species, Dipodomys ordii and Perognathus parvus, across spatial and elevational gradients. Study localities range in vegetation composition from no C₄ plants to 68% C₄ plants, mean annual precipitation from 180 to 1000 mm, mean annual temperature from 3° to 18°C and altitude from 150 to 3000 m. This study elucidates patterns of intra- and interspecific variation in isotope ecology that can be utilized to interpret stable isotopes of fossil rodents in relation to changing landscapes and climates of the Neogene.