The earth is undergoing rapid changes in biodiversity because of human activities. Specifically, humans have dramatically increased the availability of nutrients in terrestrial and aquatic ecosystems. In grasslands this eutrophication causes loss of plant species diversity. Surprisingly, we lack a mechanistic understanding of how nutrient enrichment decreases plant diversity even though alternative hypotheses were suggested decades ago. Here, we present a novel manipulation of experimental grassland plant communities that restores light to the species in the lower canopy that are thought to decrease in diversity due to deeper shading following the increase in aboveground productivity caused by eutrophication.

Results/Conclusions

We found that addition of light to the grassland understory reduced competition for light, sustained seedling establishment and maintained plant diversity despite the additional nutrient inputs. While other processes such as competition for soil resources, acidification or accumulation of plant litter can also contribute to diversity loss they played no detectable role in our study. Our results advance a long running debate in community ecology by providing a direct experimental demonstration of the importance of competition for light as a mechanism of plant diversity loss. Our work explains the particular threat of eutrophication to plant diversity and emphasizes the need to develop conservation policies and management procedures that control nutrient enrichment if plant diversity is to be preserved.