Mammalian herbivory exacerbates plant community responses to long-term increased soil nutrients in two Alaskan tundra plant communities

The interaction between bottom-up and top-down forces in regulating plant communities is a long-standing topic of interest in ecology. Factorial field experiments examining these factors have been relatively few, but recent meta-analyses provide predictions that can be tested in a range of ecosystems. We tested the prediction that added nutrients would reduce species richness and evenness, while herbivore activity would offset those changes in two tundra plant communities after 11 years. In moist acidic tundra (MAT), herbivores reduced richness more in fertilized plots when mammals were present compared with fertilized plots without herbivores. In dry heath (DH), evenness was significantly reduced in fertilized plots only when herbivores were present, also providing evidence that herbivores enhanced community changes caused by greater nutrient availability. The difference in response between MAT and the meta-analysis predictions appears to be driven by Betula nana, the species that dramatically increased with added nutrients in MAT. Unlike in similar studies and in DH, B. nana is not as palatable as most of the species in the community and is generally avoided by herbivores. These results highlight how the effects of herbivory and nutrients differ across communities and can be affected by the traits of the species present.