Optimal competitors: the balance of attraction and choices of mutualists, like pollinators, drives facilitation and may promote crop pollination

When two species use the same resource, this typically leads to competition, such as when different plants aim to attract the same mutualist pollinators. However, more flowers may also attract more pollinators to the area. Depending on which process dominates, one or both ‘competitors’ may actually benefit from the other’s presence: a phenomenon called ‘pollination facilitation’ in plant ecology. For example, it has been argued that strips of wildflowers planted next to crops may attract pollinators who ‘spill over’ into the crop. Here we mathematically examine the tradeoff between facilitation and competition in consumer attraction. We show that contrary to previous claims, no accelerating benefits of density per se are necessary for facilitation. Instead, under very general assumptions, an imbalance between local competition and joint attraction of consumers can generate a benefit to at least one ‘competitor’. This typically leads to maximal benefit to the crop at a low presence of highly attractive ‘wildflowers’ (i.e. plants that share pollinators with the crop). Our results generalize to many contexts where local competition may trade off with joint attraction of consumers. In such situations, the exact effect of competitor density on attraction of consumers can qualitatively shape outcomes, including facilitation or competition.