Analysis of multi-year nutrient enrichment experiments carried out on 45 global grassland sites show that an addition of an increasing number of nutrients leads to a reduction in plant species diversity, and competition for multiple belowground resources promotes plant species diversity. Theory suggests that the presence of multiple limiting resources within an ecosystem allows for trade-offs between species, promoting the potential for coexistence. William Harpole and colleagues test this theory in plant communities, using data from the international Nutrient Network collaboration. They compile data from multi-year nutrient enrichment experiments carried out on 45 grassland sites on five continents to show that the addition of an increasing number of nutrients leads to a reduction in plant species diversity. The findings suggest that competition for below-ground resources promotes plant species diversity. Niche dimensionality provides a general theoretical explanation for biodiversity—more niches, defined by more limiting factors, allow for more ways that species can coexist1. Because plant species compete for the same set of limiting resources, theory predicts that addition of a limiting resource eliminates potential trade-offs, reducing the number of species that can coexist2. Multiple nutrient limitation of plant production is common and therefore fertilization may reduce diversity by reducing the number or dimensionality of belowground limiting factors. At the same time, nutrient addition, by increasing biomass, should ultimately shift competition from belowground nutrients towards a one-dimensional competitive trade-off for light3. Here we show that plant species diversity decreased when a greater number of limiting nutrients were added across 45 grassland sites from a multi-continent experimental network4. The number of added nutrients predicted diversity loss, even after controlling for effects of plant biomass, and even where biomass production was not nutrient-limited. We found that elevated resource supply reduced niche dimensionality and diversity and increased both productivity5 and compositional turnover. Our results point to the importance of understanding dimensionality in ecological systems that are undergoing diversity loss in response to multiple global change factors.

Addition of multiple limiting resources reduces grassland diversity