Ribes (Grossulariaceae) pollination in northern California: strong overlap in visitor assemblages despite floral diversity

The genus Ribes displays extensive floral diversity. The pollinator shift model suggests that such diversity is an outcome of species shifting to specialize on new pollinators. To test this model, I surveyed the flower visitors of 14 Ribes species at 44 sites in northern California and southern Oregon. Visits to sympatric species in other genera were also counted at each site. For purposes of analysis, visitors were placed into one of ten functionally equivalent groups. Ribes species were chosen to maximize differences in flower size and form; ten floral traits were measured for each species to characterize these differences. Ordination and correlation approaches were used to compare visitor assemblages, both within and between species, as well as to examine the match between differences in flower morphology and visitor assemblages. I found only weak support for the pollinator shift model. Despite the marked differences between Ribes flowers, most of the 14 species were visited primarily by bees. In general, differences in floral morphology were poor predictors of differences in visitor assemblage, but flower depth and width were both correlated with the ordination of species in visitor space. Most Ribes species are not strong pollination specialists. Visitor assemblages varied as much between sites for individual species as they did between different species. Although shifts between pollinators may account for some of the differences between Ribes flowers, it appears that other factors play a greater role in floral diversification.