Characterizing isotopic variability of primary production and small mammals and the implications for reconstructing paleodiets over the last 5 MYR

Background/Question/Methods

Small mammal faunas in the Meade Basin, southwest Kansas, record evolutionary and ecological changes in the local community over the last 5 Myr in response to the evolution of modern, C₄–dominated grasslands 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 address this by measuring carbon and nitrogen isotope values in hair from rodent consumers and potential food resources (plants and arthropods) in the modern Meade Basin ecosystem. Consumer d¹³C values reflect consumption of C₄-based resources; d¹⁵N values indicate trophic level. Hair samples were taken from live-trapped individuals from three local macrohabitats (sagebrush grassland, shrub thicket, riparian woodland) that vary in proportions of trees, shrubs, herbs, and cool- and warm-growing season grasses (C₃ and C_4, respectively). We compared the results to data for live-trapped small mammals from northwestern Nebraska (mixed shortgrass prairie) and northern Minnesota (coniferous forest) as a C₃ only environment end member to determine how small mammals partition food resources among ecosystems with varying proportions of C₄plants.

Results/Conclusions

In Kansas, both C₃ and C₄ plants (n = 40) occurred in each macrohabitat, but neither type has δ¹³C values that vary in relation to macrohabitat. Although C₄ resources are available in all study areas, C₃ resources are the dominant energy source for all consumers. However, mammals captured in the sagebrush grassland consume more C₄-based resources. The δ¹⁵N values indicate higher level consumers integrate δ¹³C signatures of both primary producers (C₃ and C₄ plants) and primary consumers (arthropods), potentially obscuring distinct signatures of individual macrohabitats. Regional δ¹³C means (Kansas = -25.0±2.0, n = 27; Nebraska = -26.1±1.7, n = 48; Minnesota = -27.0±1.2, n = 84) indicate the proportion of C₄ plants increases in small mammal diets with increasing C₄ abundance on the landscape from Minnesota to Kansas. Separate ANOVAs of mammal diets (δ¹³C) from grouped by state indicate statistically significant differences among regional communities. Thus, small mammal diets reflect macrohabitat and regional differences in C₃ and C₄ plant compositions and long-term paleodiet changes in the fossil record should be discernable from d¹³C value of fossils. Our results provide an isotopic baseline for understanding similar data from other modern ecosystems and can be useful for future conservation management.