Species existence and coexistence under nutrient enrichment in the Park Grass

Numerous studies have shown that nutrient enrichment causes the loss of plant diversity in different grasslands across the globe. Thus far, three main hypotheses (niche dimension, competitive asymmetry, and soil acidification) have been proposed to account for this general phenomenon, but our knowledge of the underlying mechanisms remains rather vague. To reveal the cryptic mechanisms, we analyzed the famous long-term Park Grass Experiment (1856-) under modern coexistence theory by fitting Lotka-Volterra competition models with time-series data from the different treatments (15 different combinations of nutrient addition fully crossed with four levels of soil pH) to quantitatively test the three competing hypotheses. Supportive of the competitive asymmetry and soil acidification hypotheses, both nutrient addition and soil acidification overall decreased intrinsic population growth rates (r) and intensified competitive differences or asymmetries dramatically, which mostly favored grasses over forbs and legumes (and also forbs over legumes). These changes in r and competitive differences or asymmetries are generally consistent with the abundance changes of different functional groups following the various treatments. Moreover, the altered r (determining species existence) and competitive differences (affecting species coexistence) effectively explained the diversity loss and recovery (after nitrogen addition was withheld). However, while nutrient addition significantly decreased per-capita intra- and inter-specific competition (which indicates that belowground competition becomes less important when soil nutrients are more abundant), it did not decrease niche differences as predicted, poorly supporting the niche dimension hypothesis. These findings advance our understanding of fundamental mechanisms driving the response of plant communities to nutrient deposition in nature. Significance StatementAn unresolved fundamental scientific mystery with the crucial applied value in ecology is what causes the general loss of biodiversity following nutrient enrichment in the Anthropocene. In this study, we combined the recent advance in coexistence theory with the longest-running experiment existing in the world to wrestle with this conundrum. Our major results, which highlight the critical role of both existence and coexistence, are helpful in settling the debate between the three popular hypotheses and also, for the first time, provide quantitative explanations for the general findings in numerous nutrient-addition experiments. Our study shows the importance of applying modern coexistence theory to more quantitatively explain, predict and cope with the responses of ecological communities to global change factors in nature.