Effects of habitat fragmentation on pollinator diversity and plant reproductive success in renosterveld shrublands of South Africa

We examined the effects of habitat fragmentation on the pollinator diversity and reproductive success of seven perennial plant species in renosterveld shrublands in South Africa. We sampled pollinators in small (0.5-2 ha), medium (3-10 ha), and large (>30 ha) habitat fragments during the peak flowering period in spring and summer. We also compared fruit set and seed set in the seven plant species on different-sized fragments. Hand-pollinated controls were used to determine pollination deficits in three species. Seed-germination studies were done on two species to determine the effect of reduced seed set on reproductive output. Overall, the species richness of bees, flies, and butterflies did not vary significantly among different-sized fragments. However, the abundance of particular species of bees and monkey beetles (Scarabaeidae: Hopliini) was significantly affected by fragment size, together with other factors such as vegetation cover and the ratio of grass to shrubs. Fragment size and distance to large remnants of vegetation had a significant influence on seed or fruit set in four of the seven plant species examined. One species failed to set any seed in small- and medium-sized fragments. Comparisons of fruit and seed set among hand-pollinated and naturally pollinated plants confirmed that seed set was pollinator-limited in the orchid Pterygodium catholicum. Our results suggest that perennial plant species respond in different ways to fragmentation, and that populations on small fragments do not always experience pollination deficits. Existing classifications of pollination systems, in relation to their vulnerability to fragmentation, do not adequately explain our results. We argue that there needs to be a greater focus on the habitat requirements of pollinators to predict the effects of habitat fragmentation on pollination systems and plant reproductive success.