The dimensionality of plant-plant competition.

To avoid extinction, every species must be able to exploit available resources at least as well as all other species in its community. Theory predicts that, the more distinct the niches of competing species, the more species that can coexist together. However, both theoretical and experimental studies define a priori the nature and number of resources over which species presumably compete. It therefore remains unclear whether species in empirically realistic contexts actually fill all of the niches available to them. Here, we show how the interactions between co-occurring species can be used to quantify their implied "niche dimensionality": the effective number of axes over which those species show niche differentiation. We then apply this approach to data from 12 plant assemblages distributed across the globe. Contrary to expectation, we find that their niche dimensionality was much lower than the number of species. However, data from two high-resolution experiments show that changes to the local environment reshuffle plants' competitive roles and hence act to increase the assemblages' emergent niche dimensionality. Our results therefore reinforce the notion that homogeneous environments are less capable of maintaining high diversity while also highlighting how environmental variation shapes species' niches and hence moderates their long-term survival.