Local ecological conditions can determine whether species interactions drive evolutionary and community structural change. We investigated whether pollinator-mediated interactions between co-flowering plants varied with plant density, co-flowering neighbor identity, and flowering season. We conducted a field experiment, in which flowering time and floral neighborhood were manipulated in a factorial design. Early- and late-flowering Clarkia unguiculata plants were placed into arrays with Clarkia biloba neighbors, non-congeneric neighbors, additional conspecific plants, or no additional plants as a density control. We compared whole-plant pollen limitation of seed set, pollinator behavior, and pollen deposition among treatments.
Interactions mediated by shared pollinators depended on the identity of the neighbor and possibly changed through time, although flowering-season comparisons were not as robust due to low Early-season plant survival. Interactions with conspecific neighbors were likely competitive late in the season. Interactions with C. biloba appeared to involve facilitation or neutral interactions. Interactions with non-congeners were more consistently competitive. The community composition of pollinators varied among treatment combinations. Pollinator-mediated interactions involved competition and likely facilitation, depending on co-flowering neighbor. Experimental manipulations helped to reveal context-dependent variation in indirect biotic interactions.