We investigate a fundamental question: how does competition for pollinators affect species’ coexistence? Here, we quantified the role of competition for pollinators in promoting or limiting the coexistence of five annual plant species in Switzerland. We did so by measuring the fecundity of focal individuals growing against a density gradient of each competitor species. We experimentally mitigated the effects of competition for pollinators by supplementing natural pollination via hand pollinating for a subset of focal plants, relative to paired control plants that received only natural pollination. We then quantified the effects of competition for pollinators on coexistence by fitting competition models to our experimental data. Finally, we investigated the outcome of competition using Coexistence Theory.
Results/Conclusions
Under Coexistence theory, coexistence requires that species’ niche differences (e.g., in resource use) are sufficient to overcome their competitive (fitness) differences. We investigated how competition for pollinators affects both niche and fitness differences by comparing competition coefficients estimated from data on hand-pollinated versus control plants. We report three key results. First, competition for pollinators promotes plant coexistence for some species pairs by increasing the strength of intraspecific relative to interspecific competition such that species limit themselves more than their competitors. Second, for other species pairs, competition for pollinators weakens plant coexistence by reducing intraspecific relative to interspecific competition, thus conferring a competitive advantage to one species. Third, competition for pollinators simultaneously modifies both niche and fitness differences; thus, the outcome of competition depends on whether niche differences overcome fitness differences when species compete for pollinators. The key insights here are that competition for pollinators acts symmetrically to exploitative and apparent competition and is equally able to either promote or weaken coexistence.