DIETARY RECONSTRUCTIONS OF MODERN AND FOSSIL SMALL MAMMALS IN A GRASSLAND ECOSYSTEM OF SOUTHWESTERN KANSAS

Small mammal faunas in the Meade Basin, southwest Kansas record evolutionary changes in the local community over the last 5 Myr in response to the evolution of the modern, C₄–dominated grassland and long-term climatic changes. Carbon isotope compositions from these faunas provide a means to reconstruct resource utilization by small bodied consumers in spatially heterogeneous habitats, but interpretation of δ¹³C values of extinct species in such environments is difficult without understanding isotopic variation in comparable modern settings. We characterize carbon isotope variability of the modern ecosystem in the Meade Basin and compare data for modern small mammals with those for two Pliocene faunas from Meade (ca. 3.5 Ma).

We measured δ¹³C values of modern plants, herbivorous arthropods (orthopterans), and live-trapped rodents from three microhabitats (open sagebrush grassland, shrub thicket, riparian woodland). Microhabitats vary in proportions of trees, shrubs, herbs, and cool- and warm-growing season grasses (C₃ and C_4, respectively). Plants (n = 40) and arthropods (n = 25) were sampled within microhabitats as potential food resources. Neither C₃ nor C₄ plants have δ¹³C values that vary in relation to microhabitat and both plant types were found in each microhabitat. The most abundant arthropods mainly utilize C₃ plant resources. Five small mammal species (n = 27) have mixed diets derived from both C₃ and C₄ plant resources with diets including herbivory, or both plants and arthropods. P. maniculatus δ¹³C values do not vary with microhabitats. Dipodomys ordii (granivore) utilizes the most C₄ resources followed by Onychomys leucogaster (insectivore) and both were captured only in the sagebrush grassland. Although C₄ resources are available in all study areas, C₃ resources are the dominant energy source for all consumers. The Pliocene faunas (Rexroad Loc. 3 and Fox Canyon; total sample: n = 89 for 16 species), exhibit similar variability among species as the modern fauna and agree with reconstruction of landscape C₃:C₄ ration from paleosol carbonates. Fossil faunas likely result from time and space averaging to a greater degree than the modern samples, suggesting the need to monitor the modern ecosystems seasonally and at various spatial scales to characterize fully patterns of isotopic variability.