Pollinators and seed predators generate conflicting selection on Dalechampia blossoms

Plant–pollinator relationships are often mediated by floral traits that advertise the presence or amount of rewards. However, herbivores may also use these traits to find their hosts. In Dalechampia scandens, we tested whether floral advertisements that attract pollinators were also used by seed predators, and whether this generated conflicting selection pressures. We studied the influence of natural variation in the size of showy bracts, amount of reward, and two shape traits on pollinator visitation, pollen arrival on stigmas, seed production and seed predation. We then built a multivariate fitness function for these traits to estimate selection generated by pollinators and seed predators. Blossoms with larger bracts were visited by bees more frequently and received more pollen on their stigmas. Seed predators laid more eggs on blossoms with larger bracts and also on blossoms later producing more seeds. Consequently, selection for larger bract size exerted by pollinators was counteracted by the selection exerted by seed predators. As a result, net selection on bract size tended to be stabilizing. Additionally, we found selection on traits that increased the rate of self-pollination (assuming uniform seed quality). These results illustrate the importance of both mutualists and antagonists in floral evolution, as well as the value of taking an integrative approach to assessing selection on floral traits.