Pollination by short-tongued flies and associated floral traits: a novel pollination guild in the high-elevation Drakensberg of South Africa.

Abstract

The diversity of angiosperm flowers is largely attributed to adaptations to pollinators with differing sensory modalities and morphology. Plant-pollinator interactions vary along elevation gradients and global trends suggest that true flies (Diptera) are often dominant pollinators at high elevations. Despite this, relatively few studies within sub-Saharan Africa have assessed the importance of short-tongued flies as pollinators at high elevations. In this thesis, I investigate the role of short-tongued flies as important pollinators and quantify floral traits of a putative pollination guild of plants that co-flower at multiple high-elevation communities in the Drakensberg Mountains of southern Africa. For several species of the succulent genus Crassula, in its Drakensberg centre of diversity, pollinator exclusion and pollen-supplementation experiments indicated strong dependence on pollinators and an absence of pollen limitation. The insect visitor assemblage for guild members, as assigned using morphological characters, was dominated by saprophilous, short-tongued flies (Diptera), except for one species, C. vaginata, in which beetles (Coleoptera) dominated the visitor assemblages. The saprophilous, short-tongued flies was predominated by species of the families Muscidae, Sarcophagidae, Calliphoridae and Tachinidae, amongst other species of various families. Assessment of pollen loads on these floral visitors further supported their importance as pollinators. All studied guild members produced an unpleasant scent. Scent chemistry was dominated aliphatic acids in some species, while other species were dominated by various aromatics, terpenoids and nitrogen-containing compounds. These scents likely mediate attraction of short-tongued fly pollinators. Flowers of all studied plants were relatively small in size, corresponding to flower visitor dimensions. Analysis of spectral reflectance using colour vision models revealed colour contrasts between different floral parts and background gravel from which species predominantly grow from. Fly behaviour on flowers was suggestive of the presence of a reward, but only trace amounts of nectar were recorded. Overall, studied plant species exhibit similarities in floral colour, but exhibit variation in the scent chemistry. Despite the diversity of fly species among the visitor assemblages of studied plant species, I argue that these plant species, with the exception of C. vaginata, are functionally specialized for pollination by a group of short-tongued flies and ecologically generalized for visitation and pollination by diverse species of short-tongued flies. Moreover, these plant species belong to a new pollination guild that relies on saprophilous, short-tongued flies for pollination in the high-elevation Drakensberg Mountains. These findings provide novel insights into the pollination ecology of a new saprophilous, short-tongued fly pollination guild that has remained largely unexplored from southern Africa and emphasize the importance of saprophilous, short-tongued flies as important pollinators in high-elevation ecosystems.