Stable isotope analysis highlights contrasting use of C3 and C4 plants resources by prairie small mammals

Background/Question/Methods

We characterized the diets of small prairie mammals using stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) in fur and in particular quantified the use of C₃ and C₄ plants. Stable isotopes of carbon in fur record dietary input and can be used to distinguish between the consumption of two sets of plants, those that use the C₃ photosynthetic pathway and those that use the C₄ pathway. In many prairie ecosystems, C₄ plants include warm season grasses such as big bluestem and little bluestem. C₃ prairie plants include some cool season native grasses along with introduced grasses and a diverse native prairie forb community. Small mammals such as voles and mice are often described as having a diet of leaves and seeds of both monocots and dicots along with some arthropods. Although diet has been inferred through a variety of methods, stable isotopes of carbon in fur provide a unique snapshot into the consumption of plants from these two photosynthetic pathways. Since voles are often referred to as grazers, we hypothesized that voles would rely to a greater extent on the high biomass warm season C₄ grasses characteristic of North American prairies.

Results/Conclusions

In contrast to our hypothesis, we found that both vole species, Microtus pennsylvanicus and Microtus ochrogaster, had δ¹³C ratios indicating a diet high in C₃ plants such as cool season grasses and forbs while both mice species, Peromycus maniculatus and Peromyscus leucopus, had values indicating a greater reliance on warm season C₄ grasses such as big and little bluestem and Indian grass. Both Peromyscus species had values of δ¹⁵N that potentially indicate the consumption of insects. The inclusion of fur samples from historic museum specimens of prairie voles (M. ochrogaster), a species that has experienced population declines throughout Minnesota, showed similar values to populations present in the same region of the state today.  This consistency of δ¹³C stable isotope values over time and consistency across prairies despite differences in the proportion of C₃ and C₄ vegetation, suggest voles are selective foragers. High densities of voles occur regularly in prairies and our results reinforce their potential importance in structuring plant communities by preferentially foraging on a subset of plant species present. The similarity in δ¹³C of both voles suggests competition from meadow voles (M. pennsylvanicus) may have played a role in the decline of prairie voles in some habitats.