Leveraging modern coexistence theory to understand diversity maintenance requires understanding how various biotic interactions contribute to the net competitive effects species have on each other. One understudied but ubiquitous interaction between outcrossing flowering plants is the indirect interaction that arises from sharing pollinators. Pollinator foraging behaviors can affect plant coexistence in two ways: inconstant foraging has the potential destabilize plant coexistence, whereas preferential foraging change fitness inequalities between competitors.
In the Clarkia (Onagraceae) system of the Kern River Canyon in southern California, previous work has shown that shared pollinators of four outcrossing Clarkia are inconstant, but prefer C. xantiana over the other three species (C. speciosa, C.cylindrica, C. unguiculata) . To quantify how these pollinator behaviors affect Clarkia interactions, we established reciprocal competition plots of all four species in a community naturally dominated by C. speciosa. We planted a focal individual of each species into each of 160 meter-squared plots. Each plot was seeded with a background of one of four Clarkia species at one of five densities. In addition to measuring demographic variables of each individual, we supplementally pollinated one flower. This eliminated pollen limitation, thereby allowing for estimation of the effect that pollinators had on Clarkia interactions.
Results/Conclusions
We estimated demographic parameters of each of the four Clarkia species using model selection with generalized linear mixed effects models. The four Clarkia species had differential survival in the experiment, where C. speciosa exhibited significantly higher average survival from germination to fruiting compared to the rest of the three species. Survival to fruiting and the number of fruits produced by all species significantly declined with the background density of the plot, but where survival was not explained by the interaction of background and focal competitor identity, the average number of fruits produced by C. xantiana and C. cylindrica varied with the background competitor. Variation in pollen limitation and seed production of C. xantiana, the species preferred by pollinators, was not explained by the background competitor or the background seeding density of the plot. In fact, we found no evidence of pollen limitation C. xantiana in this experiment; as such, C. xantiana was not pollen limited when it was rare in frequency. We conclude that the level of pollinator inconstancy does not destabilize Clarkia interactions, but their frequency-independent preferential visitation to C. xantiana has the potential to change the fitness inequality between species.