N-enrichment induced biodiversity loss can be explained by reductions in competitive intransitivity: Evidence from a decade-long grassland experiment

Abstract

Recent theoretical and empirical studies demonstrate that intransitive competition is an important mechanism promoting species coexistence and biodiversity maintenance. Few studies, however, have examined how intransitive competition is affected by N enrichment and spatial scale in natural communities. Based on species abundance data from a decade-long field experiment with a control and six levels of N addition rate in semi-arid grassland, we examined the effects of fertilization treatment and spatial scale on competitive intransitivity by using a recently developed Markov chain model approach. Although the relative strength of competitive intransitivity was low (I < 0.3) it was common across all, common and rare species, and over different species pools, with significant competitive intransitivity being detected amongst 60–74 % of the plots in control and nutrient treatment communities. The overall species richness consistently increased with increasing competitive intransitivity among all species and common species; however, the positive effects of intransitivity among rare species on diversity among rare species occurred only at small to moderate spatial scales. For abundance-based species groups, a negative linear relationship between competitive intransitivity and N fertilization rate was common. For trait-based functional groups, the negative effect of high N inputs on competitive intransitivity was found in seven of eight functional groups. For all species groups, the degree of competitive intransitivity generally declined with increasing spatial scale. Our study presents strong evidence that a high degree of competitive intransitivity could promote species coexistence by reducing the negative effect of N fertilization on biodiversity in natural communities. Given that the global N deposition rate is projected to increase in this century, our study sheds new light on the effects and underlying mechanisms of increased N inputs on biodiversity maintenance in semi-arid grasslands.