The ecology of pollen limitation in buzz-pollinated Rhexia virginica (Melastomataceae)

1 Pollen limitation of female fertility has often been documented in animal -pollinated plants, but seldom have the ecological mechanisms responsible b een investigated. We examined factors influencing pollen limitation in Rhex ia virginica (Melastomataceae), a species in which pollen transfer depends on bumblebees capable of buzz pollination. Experimental studies were conduc ted in two populations at Lake Matchedash, southern Ontario, Canada, where the species occurs at the edge of its familial range. 2 Supplemental hand pollination of flowers increased their probability of f ruit set compared with open-pollinated control flowers by an average of 57. 6%. Pollen limitation was assessed on a per flower basis because the median floral display size at Lake Matchedash was one flower. Pollen limitation w as prevalent throughout the 3-4-week blooming period in one population, whe reas in the other it was only evident at the beginning of flowering, despit e their close proximity. 3 Bumblebee visits (primarily Bombus impatients) to R. virginica flowers we re infrequent and variable in their occurrence. Visitation was recorded on 14 days during flowering. The median number of visits was 0.65 bees per hou r; on 6 days there was virtually no bee activity, but on 2 days visitation rates were high. Variation in pollinator activity was apparently unrelated to local weather conditions. 4 Field experiments demonstrated that the poricidal anthers of R. virginica dispense pollen gradually, with only 10.2% of pollen removed from flowers during a single bumblebee visit. This level of pollen removal is lower than reported in other flowering plants. On average, 47.3% of pollen remained i n anthers at the end of anthesis. 5 Investigation of the relation between pollen dispersal and pollen limitat ion demonstrated a significant negative correlation between the proportion of pollen removed from anthers on a given day and the intensity of pollen l imitation. It appears that the pollen-dispensing mechanism of R. virginica and infrequent visitation by bumblebees compromise pollen dispersal, causin g pollen limitation in Ontario populations.