Global atmospheric change can induce physiological changes in plants that alter plant quality for herbivores.  Numerous studies have shown that such changes can alter invertebrate behavior and performance.  However, our understanding of plant-mammalian herbivore interactions under any scenario of global atmospheric change is minimal.  Plant-mammal interactions are widespread and important across many ecosystems, and are subject to the indirect effects of atmospheric change.  We studied the two-way interaction of nutrient availability and elevated CO₂ on the chemistry of herbaceous plants and preferences for these plants by a mammalian herbivore, the prairie vole (Mictotus chrogaster). One grass (Elymus canadensis) and two forbs (Desmodium canadense and Echinacea pallida) were grown under all combinations of nutrient availability (low and high) and CO₂ (ambient and enriched).  Plants were harvested and presented to individual voles until all voles had received access to all treatment combinations.  Preference was measured as the mass of plant material consumed, corrected for body mass.
Results/Conclusions:

Food preferences of voles differed between species and treatments (spp. x nutrient x CO₂: F_2,186=4.02, p=0.0196). Overall, preference was highest for E. pallida and lowest for D. canadense.  At high nutrient availability, E. canadensis was preferred over E. pallida under ambient CO₂, but preferences for these species did not differ under enriched CO₂.  At high nutrient availability, E. canadensis plants grown under enriched CO₂ were consumed 20% less than ambient-grown plants.  For E. pallida, plants grown in high nutrient availability and enriched CO₂ treatments were consumed 24% more than control-grown (low nutrient and ambient CO₂) plants. There were no within-species treatment effects for D. canadense. The treatment-related differences in preference for E. canadensis and E. pallida were mirrored by significant changes in crude protein content (spp. x nutrient x CO₂: F_3,172= 4.15, p=0.007), fiber/nitrogen (spp. x nutrient x CO₂: F_3,172=6.45, p<0.001) and carbon/nitrogen (spp. x nutrient x CO₂: F_3,172=8.30, p=0.0001) ratios.  These data suggest that nutrient and CO₂-mediated changes in plant chemistry have the potential to alter the relative consumption of plant species by a generalist mammalian herbivore.  Future work should focus on the response of other mammalian herbivores to these changes in plant quality and how mammals mediate the effects of global atmospheric change on plant richness and abundance in natural systems.