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 data set 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) did not
affect 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.