Nutrients destroy niches

Background/Question/Methods

A general theoretical prediction is that greater niche dimensionality, expressed as greater numbers of limiting factors, can allow greater species diversity. The corollary prediction is that reducing the number of limiting factors will cause loss of diversity. In particular, if species coexist through competition for multiple limiting nutrients, then nutrient deposition is expected to relax limitation by those elemental nutrients, while exacerbating limitation by other factors. The consequence is niche destruction for previously dominant species not adapted to the novel conditions. The few experimental tests of the Niche Dimension hypothesis show consistent support for the predicted effects of diversity loss through eutrophication. Here we report the results of a “meta-experimental” test of niche destruction using a coordinated network—The Nutrient Network—of nutrient-addition experiments. Specifically, we tested the prediction that the addition of greater numbers of limiting nutrients should lead to increasing loss of species diversity. Using consistent methodology and protocols, we applied factorial additions of nitrogen (N), phosphorus (P) and potassium plus micronutrients (K) to replicate blocks at 39 grassland sites in seven countries.

Results/Conclusions

An important assumption underlying the Niche Dimension hypothesis is that communities be limited by multiple factors; we found evidence for multiple limitation by at least three nutrients: nitrogen, phosphorus and potassium (N, P and K plus micronutrients). The average effect of decreasing multiple nutrient limitation by adding combinations of N, P and K was to decrease plant diversity. Addition of greater numbers of limiting resources drove a negatively-correlated joint response of decreasing diversity and increasing biomass. Loss of diversity was greatest when limiting nutrients were added in combination rather than singly and the effect tended to be multiplicative. Different nutrients contributed to producing the greatest relative responses at different sites, but intriguingly, the identity of the nutrient causing greatest relative diversity loss was most often not the same as the nutrient causing the greatest relative biomass increase. These results provide indirect support for the hypothesis that high plant diversity can be maintained by tradeoffs between species for multiple limiting nutrients. Our results also suggest that the effects of eutrophication on species diversity may differ from its effects on production—that the relative importance of multiple constraints over production and coexistence may differ.