Background/Questions: Annual net primary production (ANPP) and decomposition in Earth's grassland ecosystems represent the energy flow critical for supporting agriculture and terrestrial foodwebs. Nutrient supplies and herbivore density are two major determinants of grassland ANPP at the local scale, and both of these factors are being altered by anthropogenic activities. Understanding the effects of altered nutrient supply rates and herbivory on ANPP is complex, because increased ANPP can lead to litter build-up which can suppress ANPP, creating a negative feedback cycle.
Methods: We replicated an experiment, a factorial combination of large vertebrate herbivore exclusion and fertilization (N, P, K, and micronutrients), at 44 sites spanning six continents to evaluate the relative strength and generality of herbivory and nutrient supply in controlling grassland ANPP and litter accumulation.
Results: Experimental nutrient addition and exclusion of large vertebrate herbivores led to sustained increases in ANPP, but herbivores consumed the same proportion of primary production under ambient and eutrophied conditions. ANPP responses to treatments varied widely among sites, but temporally-averaged ANPP more than doubled in response to nutrient addition at nearly 60% of sites and in response to herbivore exclusion at more than 20% of sites. Nonetheless, the overall additive effect of nutrients and herbivore exclusion on ANPP meant that after an initial period of transience, the increase in ANPP from eutrophication was similar in the presence and absence of herbivores. In contrast, there were strong interactions between nutrients and herbivores on litter accumulation, which was much higher in fenced and fertilized plots. The suppressive effects of litter accumulation led to a lower-than-expected effect of herbivore exclusion on ANPP.
Conclusions: Thus, these results demonstrate significant scope for future increases in global grassland ANPP in response to eutrophication that are unlikely to be consumed by vertebrate herbivores. However, the suppressive effects of litter accumulation in response to herbivores and eutrophication may induce long-term feedback cycles in ANPP.