HOW FAITHFULLY DO SMALL MAMMALS RECORD C4 GRASS ABUNDANCE IN GRASSLAND ECOSYSTEMS? ISOTOPIC INSIGHTS FROM MODERN AND FOSSIL COMMUNITIES

How faithfully do small mammals record C4 grass abundance in grassland ecosystems, and how strongly is this influenced by biotic (interactions with large and small bodied competitors) and abiotic (seasonal precipitation) variables? We gained isotopic (carbon and nitrogen) insights into this complex question via two modern case studies based in Kenyan savanna ecosystems, and a 2.4 million year chronology from the North American Great Plains. The first Kenyan case study focused on a small mammal community dominated by a single herbivorous pouched mouse, and an understory comprised mainly of C4 grasses (97% biomass). We used long term large herbivore exclosure plots to quantify how removal of megafauna influenced diet of the pouched mouse, in both wet and dry seasons. Understory primary production is strongly regulated by precipitation, and we observed an interaction between season, and the effect of large herbivore competitors on rodent diet. Yet, even when we account for both effects, the rodents showed strong selection for relatively rare C3 vegetation, and their diets significantly underestimated C4 grass abundance. The second Kenyan case study focused on a more diverse small mammal community, and we compared the diets of the 5 most common co-occurring taxa. The understory in this ecosystem is more equitably divided between C4 grasses and C3 herbs/forbs, and the small mammal taxa ranged from primarily insectivorous to herbivorous. We measured strong taxonomic differences, and calculated individual diets that spanned the full C3-C4 gradient. Although at the broader community-level, small mammal diet was skewed towards C3 plant resources (either via direct consumption of vegetation, or predation upon C3-consuming insects). Finally, to determine if small mammal diets suggest temporal change in the abundance of C₄ biomass in a region, we compared data from modern and 2.4 Ma rodents (24 species) in western Nebraska. Mean carbon isotope values of the modern rodent community was higher than for fossil rodents. This could suggest an increase in the relative abundance of C₄ grasses in Nebraska since 2.4 Ma, but based upon our modern Kenyan dataset we cannot rule out effects of changing competitive interactions or climatic conditions on diet, or shifts in the carbon isotope values of C3 plants in response to abiotic factors.