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