Carbon isoscapes of rodent diets in the Great Plains USA deviate from regional gradients in C₄ grass abundance due to a preference for C₃ plant resources

Diet is an ecological attribute that species may adjust to cope with changing environments and may indicate how a population responds to changes in resource availability. In the Great Plains, plants utilizing the C₄ photosynthetic pathway may increase in abundance in the future because of their high tolerance for warm and dry environments, which are projected to increase. How increased C₄ abundance will influence grassland food webs remains unknown. Here, we evaluate how rodent diets vary relative to C₄ plant biomass at the regional scale of the southern Great Plains in the U.S. We measured δ¹³C values of hair from 534 individuals of 14 rodent species. Resulting isoscapes of δ¹³C values were statistically compared to three proxies for local abundance of C₄ grasses across the region. Overall, diets of most rodent species were dominated by C₃-derived resources with a few species relying on C₄ resources. Ordinary least squares linear regressions indicate that proxies for regional abundance of C₄ grasses explain very little of the variance in δ¹³C values in hair for the entire rodent community, however the isoscape of rodent hair δ¹³C values does broadly correspond to that for δ¹³C values of soil organic matter. The difference between median δ¹³C values of granivores and folivores is significantly and highly correlated with SOM δ¹³C values. Results from this study can be used as a baseline for characterizing dietary shifts in response to environmental change both in the geological past and in the future, and they identify which dietary categories may be most sensitive to future changes in the regional abundance of C₄ grasses.