Herbivores override climate control of grassland production in Yellowstone National Park

Understanding the factors regulating temporal variation in grassland annual aboveground net primary production (ANPP) is dominated by studying the effects of climate, particularly water, in ungrazed grassland. However, the overwhelming majority of the Earth's grasslands are grazed by large herbivores, which have large effects on ANPP and interact with climate in unknown ways. Here, we analyzed an eight-year dataset of ANPP across a 26-year period that included widely variable climatic conditions and consumption rates by herds of elk (Cervus elaphus), bison (Bison bison), and pronghorn (Antilocapra americana) at 25 grassland sites in Yellowstone National Park (YNP). We found that ANPP was primarily a positive function of consumption rate and secondarily affected by a nonlinear temperature effect, with ANPP declining in hot years. Water balance (WB, a measure of soil moisture available to plants) had no significant effect on ANPP. Examining the difference between grazed minus ungrazed (fenced) ANPP (i.e., grazer stimulation) at 13 grassland sites revealed that herbivores increased average ANPP by 20%, with variation across sites and years driven by the amount grazed, temperature, and interactions of temperature with local environment and WB. We found a surprising negative main effect of WB on ANPP stimulation, likely because grazing ameliorated moisture stress in dry years by reducing transpirational moisture loss. Our results demonstrate that Yellowstone grazers override the well-documented positive effect of moisture on grassland ANPP, which highlights the need to understand how together climate and herbivory regulate production in the world's other grassland ecosystems.