Too much of a good thing: Shrub benefactors are less important in higher diversity arid ecosystems

Abstract The biodiversity–ecosystem function literature provides a useful framework to examine many processes associated with species diversity in ecology. One such context is the maintenance of biodiversity by facilitation in arid ecosystems. Here, we examined the complex interactions between local plant species richness and the intensity of shrub facilitation for maintaining biodiversity in arid plant communities. A synthesis including a meta‐analysis was used to compile nearly 600 papers on positive interactions mediated by shrubs in dryland plant communities (search terms: shrub, positive, facilitat*) to examine whether interactions in these studies changed with reported local species richness. A total of 19 studies and 141 independent instances directly examined and reported facilitation of diversity measures in naturally assembled plant communities and provided estimates of local species richness. Synthesis . The net effect of increasing local plant species richness was negative and shifted the relative frequency of interactions with shrubs from positive to negative with increasing local species richness. This relationship suggests that increases in richness do not always enhance functions that maintain diversity in plants communities likely due to concurrent increases in the indirect negative interactions between species under shrubs or in changes in the local species pool.