Human activity has affected grassland biodiversity through the addition of both nutrients and grazing. Theory predicts that these factors could balance each other because they have opposing effects on light limitation, and this international collaboration across 40 experimental sites on six continents — from the 41 Nutrient Network (NutNet) cooperative — puts the theory to the test. The results demonstrate a consistent counteracting effect, with nutrient addition and herbivores jointly controlling plant diversity via light: nutrients reduce ground-level light thereby reducing plant diversity, and herbivores increase plant diversity by reducing competition for light among plants. This work will contribute towards more accurate modelling of the effects of grazing practices and nitrogen deposition on biodiversity in the world's grasslands. In a second paper in this issue of Nature, Yann Hautier et al. studied the influence of eutrophication in the NutNet grassland sites and show that the use of fertilizers is not only a threat to grassland biodiversity but also to the stabilizing effect it has on ecosystem functioning.