Direct competition is often invoked as the main type of interaction structuring ecological communities and determining species coexistence. However, a variety of biotic interactions extrinsic to direct competition can affect the likelihood of species persistence in a community. For example, in the flowering plant species Clarkia xantiana ssp. xantiana (Onagraceae), co-occurrence with congeners has been shown to increase plant fitness due to interactions with shared pollinators—a facilitative interaction via shared mutualists. Whether such pollinator sharing between flowering plants promotes or inhibits coexistence depends in part on pollinator foraging behaviors, such as constancy and preference.
To determine if and when pollinator-mediated interactions of four Clarkia species can promote coexistence, I measured pollinator preference and constancy in experimental arrays of Clarkia in 2015 and 2016. In each array treatment, I placed fresh-cut Clarkia stems in a cleared 5m x 5m patch situated in a community of four naturally co-occurring Clarkia species. I varied the number of Clarkia stems (10, 20, 40, and 100 stem-treatments in 2015; 12, 24, 40, 64, and 100-stem treatments in 2016) and the diversity of Clarkia species in each treatment (1, 2, and 4 species of Clarkia) and observed bee pollinator movements in the arrays, identifying each pollinator to genus when possible.
I observed a total of 3,907 pollinator visitation events in 31 array treatments (2015) and and 3,671 pollinator visitation events in 41 array treatments (2016). In both 2015 and 2016, pollinator constancy was high on average, but significantly decreased with the diversity of Clarkia species in the arrays. Declining constancy in increasingly diverse arrays indicates that living in close proximity to (heterospecific) congeners may increase the competitive interactions between Clarkia species due to the increased likelihood of heterospecific pollen transfer. Pollinators also exhibited significant preference for one species of Clarkia over the other three, Clarkia x. xantiana, in both years. In 2015, the rate of pollinators visiting the arrays increased significantly with date, but was unrelated to the diversity or density of the array treatments. In 2016, the rate of pollinators entering the arrays increased significantly with the density of the array treatments, but not with date or diversity of the arrays. Taken together, these results indicate that the potential for Clarkia to benefit from the joint attraction of pollinators increases with the density and homogeneity of a patch. As such, co-occurring with congeners should be facilitative when Clarkia spatially aggreggate by species in a community.