When can two plant species facilitate each other's pollination?

Facilitation occurs when an increase in the density of one species causes an increase in the population growth rate or the density of a second species. In plants, ample evidence demonstrates that one species can facilitate another by ameliorating abiotic conditions, but the hypothesis that pollination facilitation - in which the presence of one flowering species increases pollinator visits to a second species - can also occur remains controversial. To identify the necessary conditions for pollination facilitation to occur, we constructed population models of two plant species that share the same pollinator and compete for establishment sites, and we assumed that heterospecific pollen can interfere with successful seed set. We found that facilitation for pollination occurs only when the pollinator visitation rate is an initially accelerating function of the combined numbers of flowering plants of both species in a patch. The presence of a second species can allow populations of a focal species either to persist for a longer amount of time before going extinct ("weak facilitation") or to persist indefinitely at a stable equilibrium density ("strong facilitation"). When only a single plant of either species can occupy a site, the plant species with the higher initial density can experience strong facilitation but will eventually out-compete the other species. However, when site occupancy was not exclusive, strong facilitation sometimes led to coexistence of the two species. Increasing the extent of pollen carryover increased the range of initial population densities leading to strong facilitation. In light of our theoretical results, we discuss the apparent rarity of pollination facilitation in nature.