Selection on floral traits across variable plant and pollinator communities: A generalist plant and its specialist pollinator

Background/Question/Methods Interspecific interactions are a major source of selection shaping adaptive evolution. Community composition frequently varies geographically, and changes in the species present in a community can alter the strength and direction of selection on community members' traits. In this study, we are examining how selection on floral traits changes across variable plant and pollinator communities. Claytonia virginica L. (Portulacaceae) is an abundant spring ephemeral wildflower in the eastern United States that is pollinated successfully by multiple insect taxa. Andrena erigeniae (Andrenidae) is a solitary vernal bee specialized on C. virginica pollen. There is variation in C. virginica for flower color, with white-flowered plants having higher levels of secondary compounds in the pollen compared to pink flowered plants; these compounds are known to have antiherbivore properties. The abundance of co-flowering plant species varies between populations, from near monocultures of C. virginica to extremely diverse spring ephemeral communities. We predict that higher A. erigeniae abundance will lead to selection for pinker flowers, because pink-flowered pollen contains fewer secondary compounds that the larvae would have to process. We also predict that the pink-flower host search image will make this selection even stronger in diverse floral communities because pink-flowered species are rare in spring ephemeral communities. In spring 2008, we documented seed set and pollinator fauna for C. virginica in four populations in southwest Michigan.

Results/Conclusions A. erigeniae was the single most abundant pollinator species in all populations, but varied in frequency, accounting for 26% to 71% of visits in different populations. Populations differed significantly in the number of A. erigeniae visiting flowers (F = 3.40, p = 0.03). Populations with a greater proportion of visits by A. erigeniae also had higher seed set (F = 31.3, p = 0.03), suggesting that A. erigeniae may be a more effective pollinator. In spring 2009, we will characterize the pollinators and co-flowering plant diversity of six C. virginica populations. In addition, we will measure selection on C. virginica floral traits, including size and color, in all populations.