In this study, I compared male and female reproductive success of island and mainland populations of two plant species, Opuntia stricta (Cactaceae) and Centrosema virginianum (Fabaceae) on the west coast of Florida. During the 1980, 1981, and 1982 breeding seasons, both male success, as measured by pollen dispersal, and female success, as measured by fruit or seedset, showed a significant reduction in island populations relative to mainland populations of these species despite the close geographic proximity of the study sites. The pollinator species that visited Opuntia stricta on the island site were a subset of the mainland pollinator assemblage. A complete shift in pollinating species of Centrosema virginianum occurred between the mainland and the island. On the island site, pollination was less effective for this suedes and less reliable for both species than on the mainland sites. Observational and experimental data suggested that pollinator scarcity was the cause of reduced reproductive success on islands. On more distant, oceanic islands, these effects may be even more extreme and could either filter out inappropriate colonists or act as an agent of directional selection.