Hummingbird pollination and floral evolution of introduced Nicotiana glauca and native Epilobium canum: California island-mainland comparisons.

摘要

To examine how changes in pollinators affect plant reproductive success and floral evolution, I compared island and mainland populations of two hummingbird-pollinated plants, one introduced and one native. Nicotiana glauca (tree tobacco, Solanaceae) has recently colonized the Channel Islands from the California mainland, while Epilobium canum (California fuchsia, Onagraceae) has long-established island and mainland populations. Based on the largely untested hypothesis that characteristic island plant traits are in response to insular pollination, I tested the following predictions: (1) island plants will be visited by different pollinator species than mainland plants, which will lead to selection for different floral morphologies, and (2) island plants will be visited less frequently and experience pollen-limitation of reproductive success, which will lead to selection for increased ability to self-pollinate. I observed the composition and frequency of visits to plants, measured floral traits and selfing ability, and tested for phenotypic selection on traits across Santa Catalina and Santa Cruz Islands and three adjacent mainland sites from 1998 to 2002.; My results reveal island-mainland differences in floral biology, but do not support predictions for the pollinator conditions thought to select for these differences. This indicates that past selection and/or neutral processes contribute to differentiation of island plants from mainland relatives. In support of the first prediction, a novel and longer-billed island pollinator visited both N. glauca and E. canum, and island plants had longer or wider corollas, respectively, but contrary to predictions, island plants were not clearly under current selection by pollinators for longer or wider corollas. Contrary to the second prediction, neither plant species was visited less frequently on islands, nor did they experience pollen-limitation to reproductive success. In fact, visitation and pollination tended to be greater on the islands than on the mainland. Consistent with this lack of selection for selfing, island E. canum plants did not exhibit higher selfing ability than mainland plants. However, N. glauca island plants were more capable of selfing, especially on the most recently colonized island, which points to the importance of colonization events ( vs. selection after establishment) in determining patterns in island floral biology.