Effects of nutrient enrichment on pairwise niche and fitness differences in annual grassland plants

Background/Question/Methods

Changes in environmental conditions can simultaneously affect the growth rates and interactions of co-occurring species. Eutrophication is notoriously detrimental to plant species coexistence, an effect that is widely hypothesized to result from changes in the outcome of species competitive interactions. Coexistence theory offers a framework for quantifying nutrient impacts in terms of shifts in species niche and fitness differences. However, there is a lack of empirical evidence showing how changes in limiting factor availability regulate coexistence and diversity by affecting these two processes. Here, we parameterized annual plant competition models with data from a nutrient addition experiment under greenhouse conditions to test how eutrophication influenced species niche and fitness differences. Specifically, we tested how enrichment by nitrogen, phosphorus, and potassium, both independently and as multiple-nutrient additions, impacted coexistence metrics.

Results/Conclusions

Nutrient enrichment had strong effects on both niche and fitness differences for all species pairs. Nutrient enrichment usually destabilized coexistence by increasing fitness differences or reducing niche differences, but also stabilized coexistence under certain conditions. This outcome was influenced by species-level growth responses to nutrient inputs. Nutrient limitation directly constrained fitness by limiting fecundity and increasing sensitivity to interspecific competition. Our findings demonstrate the utility of using coexistence theory to understand community responses to global changes by providing empirical evidence that niche and fitness differences responded to nutrient enrichment. Our findings also offer an improved mechanistic understanding of plant community changes in response to nutrient enrichment.