Pollinators cause stronger selection than herbivores on floral traits in Lobelia cardinalis (Lobeliaceae).

Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits. We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores--slugs, weevils and caterpillars--affected the relationship between floral traits and fitness. We detected stronger total selection on four traits and correlational selection (γ(ij)) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date. In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.