Indirect interactions between neighboring plants for pollination service vary temporally

Indirect biotic interactions can change seasonally with changes in the abundance of other community members. In this way, pollinator sharing between plant species may range from competition to facilitation depending on the seasonal pollinator community. I examined this phenomenon with experimental arrays in which flowering time of Clarkia unguiculata plants was crossed factorially with four floral neighborhood treatments including a manipulation of C. unguiculata density, arrays with other Clarkia, and arrays with plants in other genera. I measured pollinator composition, number of pollinator visits, and pollen limitation of seed set. Clarkia unguiculata plants growing with other Clarkia species (congeners) were less pollen-limited, indicated by the difference in seed set between pollen-supplemented (S) and unsupplemented (U) plants, than those growing among conspecifics in only the late season, suggesting late-season facilitation (95% CI for seed set (S-U) conspecifics = (69.9, 88.9); 95% CI for seed set (S-U) congeners= (24.8, 44.6)). The outcome of pollinator-mediated interactions thus varied temporally, and this could be due to differences in pollinator effectiveness among pollinator species rather than pollinator visit frequencies. The identities of pollinators varied between flowering times (Pearson Χ2 = 114.8, d.f. = 4, P < 0.001), and early- and late-season pollinators may have responded differently to floral neighborhoods. Evidence, for temporal variation such as found here in the outcome of an indirect interaction mediated by mutualists, offers a partial mechanism for the success of a species within a community.