Outcrossing rates in a rare “ornithophilous” aloe are correlated with bee visitation

Abstract

Pollinator taxa may vary in their contribution to plant outcrossing rates and hence differ in their influence on plant fitness. In plants that have ornithophilous floral traits (large, deep-tubed orange-red flowers with dilute nectar), it is expected that birds should be the most effective agents of outcrossing. However, bees often pollinate plants with ornithophilous floral traits and may contribute to their outcrossing. To assess the contribution of bees to outcrossing rates and inbreeding in the rare self-compatible treelet Aloe thraskii, we compared outcrossing rates (t) and adult inbreeding coefficients (F), in bird-excluded and open-pollinated plants. We then tested whether outcrossing rates, and inbreeding depression (δ) estimated using Ritland’s genetic marker-based approach, varied according to bee and bird visitation rates, plant height, flowering population size, population density, and isolation distance across eight populations. Outcrossing rates did not differ between progeny from open-pollinated and bird-excluded inflorescences. Outcrossing rates increased with bee visitation, but not bird visitation. Adult inbreeding coefficients were low (F = 0–0.1), and inbreeding depression estimates were high (δ = 0.59–1). Estimates of inbreeding depression did not vary according to population aggregation. These results highlight the contribution to outcrossing that insect pollinators can make to outcrossing rates of rare plant species that are seemingly adapted for bird pollination.