Effect of competition for pollination on the ecology and evolution of a hummingbird-pollinated plant, Ipomopsis aggregata

Since it was recognized that the delivery of pollen is a resource for which plants can compete, co-flowering plant species have become model systems for studying competition. Many studies have documented divergence in floral traits within and between species and attributed this divergence to competition for pollination. An untested prediction of such studies is that competition for pollination can influence pollinator-mediated selection on floral traits. To test this prediction, I examined the effect of competition for pollination with Castilleja linariaefolia on the ecology and evolution of Ipomopsis aggregata. I first determined that C. linafiaefolia competes with I. aggregata for pollination, and that this interspecific competition was stronger than intraspecific competition. I then determined that C. linariaefolia can compete with I. aggregata by the mechanism of interspecific pollen transfer.; To determine if this competition can influence selection on floral traits, I measured phenotypic selection on floral traits of I. aggregata in populations with and without C. linariaefolia. Directional selection on corolla length, corolla width, and flower number was stronger in populations with C. linariaefolia, suggesting that competition for pollination can influence selection on floral traits. In addition, the phenotypic correlation between corolla length and width, which determines the shape of I. aggregata's flowers, was significantly stronger in populations without C. linariaefolia. This change in shape may allow flowers to maximize conspecific pollen deposition while minimizing heterospecific pollen deposition.; I then measured the effect of the density of neighboring conspecifics and competitors on pollination, reproduction, and selection on floral traits of I. aggregata in artificial arrays and natural populations. If competition in pollination selects for divergence in floral traits, and the strength of competition is density-dependent, then selection on these traits should be influenced by the density of conspecifics and competitors. However, the density and interspersion of plant species had weak and inconsistent effects on pollination, reproduction, and selection on floral traits of I. aggregata. These results suggest that the strength of competition for pollination in the I. aggregata-C. linariaefoblia system may instead be influenced by variation in these species' abundances at larger spatial scales that were examined in this study.