UC Santa Cruz

Why are there so many angiosperm species compared to other plant groups? Why is angiosperm diversity so high in the tropics? Extensive evidence suggests that plant-pollinator interactions is a significant cause of angiosperm diversification. One important mechanism for pollinator-driven speciation is the transition between pollination systems, known as pollination shifts. Pollination shifts may have played a significant role in shaping tropical diversity. However, the promoting ecological factors and evolutionary processes still need to be fully understood. My dissertation focuses on understanding pollination shifts using a tropical plant system (genus Costus; family Costaceae). I studied the Neotropical Costus clade because it exhibits many parallel shifts from orchid bee to hermit hummingbird pollination. Furthermore, bee- and hummingbird-pollinated Costus species are associated with different suites of floral traits. I used various approaches, from field sampling and experimentation, quantitative floral trait analyses, and pollinator quality comparison in hyper-diverse Neotropical ecosystems in Central and South America. I examined how intrinsic (floral traits) and extrinsic (elevation, pollinator assemblage, pollinator quality) factors may drive pollination shifts and how floral traits change during a pollination shift. In chapter one, I analyzed the variation and association between floral traits and pollinator assemblages in Costus guanaiensis var. tarmicus along an elevational gradient. I found substantial divergence in floral traits and pollinator assemblages across the elevational gradient, establishing the necessary preconditions for pollinator-driven floral divergence. An association between floral and bee trait variation suggests adaptation to the local bee fauna along the elevational gradient. In addition, I found floral traits that might work in hummingbird fitting and attraction in the highest sampled site, and hummingbirds composed the pollinator assemblage there. The beginning of a bee-to-hummingbird pollination shift in the highest-studied site was not ruled out. In chapter two, I investigated whether differences in pollinator quality between bees and hummingbirds could promote bee-to-hummingbird pollination shifts. I compared heterospecific and conspecific pollen loads transferred to the flowers' stigma of bee- and hummingbird-pollinated species and assessed the plant-plant pollinator-mediated interactions in bee- and hummingbird- pollinated Neotropical communities. I found that bees and hummingbirds are high-quality pollinators, transferring large conspecific pollen loads but little heterospecific pollen loads to the stigma. Commensalism prevails among the bee-pollinated community, whereas facilitation prevails among the hummingbird-pollinated community. Neither difference in pollen transfer nor the ecological processes driving the bee- and hummingbird-pollinated communities appear to promote bee-to-hummingbird pollination shifts. In chapter three, I investigated the function of floral traits as anti-bee and pro-bird traits in Costus malortieanus. I artificially modified the size, shape and color of C. malortieanus flowers to resemble a hummingbird-pollinated Costus flower. I exposed the modified flowers to orchid bees and hummingbirds in the field. I found evidence for anti-bee floral traits in the Neotropical Costus clade. Overall, my findings contribute to understanding bee-to-hummingbird pollination shifts beyond temperate systems, giving insights into processes of major adaptive shifts that contribute to tropical plant speciation.