Appearances can be deceiving : pollination in two sympatric winter-flowering Aloe species

Abstract

Aloe marlothii and A. greatheadii var. davyana are two sympatric winter-flowering succulents that occur in the summer rainfall regions of northern and north-eastern South Africa. Both have flower characteristics that are strongly suggestive of bird pollination, although their nectar differs in volume and concentration. We conducted pollinator exclusion experiments to determine the importance of birds and insects as pollinators of these Aloe species. For both species fruit set and the number of seeds per fruit were higher in control treatments (all pollinators) and lower in treatments that excluded all pollinators. The contribution of insect pollinators to fruit set in A. marlothii was low (3–4%), like that of no pollinators (0–2%) whilst that of all pollinators (14–19%) was significantly higher, suggesting that generalist avian pollinators, which visited flowers in large numbers, are the most important pollinators. In A. greatheadii var. davyana fruit set in the absence of pollinators was also very low (2–6%), while the contribution to fruit set by insects (36–51%) was similar to that of all pollinators (55–55%), confirming the importance of honeybees to pollination. Clear understanding of both flower and nectar characteristics, and observations of flower visitors, are therefore required before an accurate prediction of pollinator type can be made.