Diversity begets diversity in competition for space

Competition can profoundly affect biodiversity patterns by determining whether similar species are likely to coexist. When species compete directly for space, competitive ability differences should theoretically promote trait and phylogenetic clustering, provided that niche differences are otherwise minimal. Yet many sessile communities exhibit high biodiversity despite minimal reliance on niche differentiation. A potential explanation is that intransitive competition ('rock-paper-scissors' competition) not only promotes species richness but also fosters coexistence among highly dissimilar species with different competitive strategies. Here, we test this hypothesis using a combination of empirical and analytical approaches. In an experimental system comprising 37 wood-decay basidiomycete fungi grown in nutrient-rich agar media, pairwise displacement was maximized when species had widely different competitive traits and divergent evolutionary histories. However, when these interactions were embedded in models of species-rich communities, high levels of intransitivity ultimately overwhelmed the pairwise relationships, allowing the weakest and most dissimilar species to survive. In line with theoretical expectations, these multispecies assemblages exhibited reduced functional and phylogenetic diversity, yet the smallest losses were likewise observed in species-rich communities. By demonstrating that species richness can act as a self-reinforcing buffer against competitive exclusion, these results contribute to our understanding of how biodiversity is maintained in natural systems.