OOS 35-8
Nutrients and vertebrate herbivores interact to control global grassland productivity
Net primary production (NPP) in Earth's grassland ecosystems provides the energy critical for supporting agriculture and terrestrial foodwebs and represents an integrative measure of several planetary boundaries. At the local scale, grassland NPP is controlled by factors including nutrients and offtake by herbivores, yet anthropogenic activities are directly and indirectly altering these factors in grasslands, worldwide. Meta-analyses suggest that whereas fertilization tends to increase NPP, herbivores do not alter net production in ambient or fertilized plots. However, because meta-analytic studies rely upon experiments that vary greatly in their approach, duration, and location, such studies may have low power to determine the degree to which the role of nutrients and herbivores is contingent on the abiotic or biotic characteristics of the local environment. We made measurements of NPP in 75 grasslands in 17 countries on 6 continents to determine the primary constraints on NPP in unmanipulated grasslands. We also replicated an experiment, a factorial combination of large vertebrate exclosures and fertilization, at 33 of these sites to evaluate the relative strength and generality of herbivory and nutrient supply in controlling grassland NPP.
Results/Conclusions
We found that across the world's grasslands, NPP increased with soil %N and was highest at low elevation and in warm, wet climates where temperature is least variable. After three years of experimental treatments, fertilization increased NPP, but fences alone did not alter NPP. However, the increase in NPP was significantly higher (super-additive) when fertilizer was added in the absence of large vertebrate herbivores, compared to the effects of each of these treatments, individually. The unprecedented scale of our replicated direct observations and experimental manipulations suggests that nutrient supply is a primary limitation of NPP in the world’s grasslands, but that herbivores set constraints on NPP where nutrients are not limiting.