The importance of flies for pollination at high elevation: a case study of Erica caffrorum.

Abstract

Pollinator communities typically vary across elevation gradients. In particular, the contribution of bee pollination typically declines at higher elevations in favour of other pollinators, such as birds, butterflies and flies. Erica caffrorum occurs at high-elevation sites in the South African Drakensberg Mountains and has floral traits suggestive of pollination by short-tongued insects. Most Erica species with similar floral traits studied to date occur in the Cape Floristic Region at low elevations and are pollinated by honey bees. I present a case study of pollination in Erica caffrorum, including characterization of the breeding and pollination system, quantification of the associated floral traits, and experimental determination of the significance of flower colour and scent for pollinator attraction. Hand-pollination experiments confirmed that similar to most Erica species, E. caffrorum is self-incompatible. At five study sites, between 1800 and 2500 meters above sea level, pollinator observations revealed that most visitors were not honey bees but Diptera, which were identified into morphospecies and identifications verified using DNA barcoding. Assessment of pollinator importance based on pollen loads and visitation frequency identified Muscidae, Rhiniidae and Scatophagidae as the most important pollinators. Analysis of floral scent using GC-MS indicated that benzaldehyde and 2-methyl butanoic acid dominated the odour bouquet across all populations. In bioassays, a significant preference for white colour in the presence of scent was found for flies across all three experimental sites and for other visitors at one of two sites, but no other consistent preferences for colour or scent alone were detected. Across three sites, a combination of acid and benzaldehyde attracted significantly more flies than the unscented control. A similar pattern was observed for acid alone but not for benzaldehyde. This study provides novel evidence for short-tongued fly pollination in an Erica species, mediated through a combination of floral scent and colour. The similarity in traits between E. caffrorum and several other distantly related plant species in the highelevation ecosystems of the Drakensberg mountains suggests that the pollination of scented, small, white flowers by short-tongued flies found here likely represents a more widespread pollination system whose importance in southern African mountain regions is currently underestimated.