Do anthropogenic effects change the ecosystem services provided by nectarivorous birds? The Cape Floristic Region hosts over 300 plant species that depend on only eight species of nectar-specialist sunbirds and sugarbirds. This unusually asymmetrical mutualism provides an ideal system to investigate the pivotal role that pollinators play in the evolution and conservation of plants. This project investigates how sunbirds influence flower colour evolution in bird-pollinated ericas, and how supplementary feeding along the urban fringe and ongoing habitat fragmentation are threatening these processes in many parts of the Cape Floristic Region.

The genus Erica is one of the most diverse in the fynbos biome, and its many bird-pollinated species are striking for the high levels of flower colour polymorphism. Some Erica species have up to five colour morphs yet are pollinated predominantly by just one bird species, the Orange-breasted Sunbird Anthobaphes violacea. Former post-doc Anina Coetzee and PhD student Samantha McCarren are investigating the origin and maintenance of these flower colour polymorphisms. Specifically, they are asking what role plant community context and sunbird foraging behaviour play in generating intra-specific flower colour diversity.

African nectarivorous birds are thought to be able to detect ultra-violet (UV) reflectance. Thus, UV colouration might be used by bird-pollinated flowers to increase visibility for their pollinators. However, nectar-robbing insects might also use this channel for foraging decisions and consequently there may be selection against UV signals. Samantha McCarren is quantifying UV reflectance in bird-pollinated flowers in the Cape Floristic Region, and using choice experiments to test whether sunbirds or insects exhibit a preference for certain flowers based on their UV reflectance.

PhD student Daniël Cloete is writing up his study of the impacts of habitat fragmentation on bird-pollinated plants in the Tsitsikamma section of the Garden Route National Park. Daniël has measured pollination by sunbirds and Cape Sugarbirds Promerops cafer of Protea and Erica species across 17 fynbos patches, both natural and fragmented, to test whether certain thresholds of patch size and isolation exist where pollination services by birds start to break down. This is a good area to address this question because it naturally comprises a mosaic of forest, fynbos and coastal thicket, further fragmented by agriculture, plantations, alien infestations, farmland and urban areas. Insights from Daniël’s research will hopefully shed light on how threats, including land-use change, alien invasive vegetation and climate change might affect ecosystem function and services in the Cape Floristic Region.

At the border between urban and natural fragments, supplementary feeding of nectar-feeding birds, which is increasing in popularity, may impact sunbird-plant mutualisms. MSc student Monique du Plessis assessed the effect of feeders on sunbirds and bird-pollinated Erica species. By taking advantage of the tell-tale sign left when sunbirds probe Erica flowers, she was able to assess flower visitation rates at varying distances from nectar feeders in gardens bordering natural areas.

Activities in 2021

  • Anina Coetzee took up a teaching position at Nelson Mandela University, George campus. She is starting new projects on bird-pollination systems in the southern Cape, in collaboration with Dr Colleen Seymour.
  • PhD student Samantha McCarren recorded flower reflectance of a diversity of Erica species, and found that ultra-violet signals are absent or rare in ericas pollinated by short-proboscid insects, rodents, or wind. It occurs in some bird-pollinated ericas and choice experiments with sunbirds showed that they can learn to discriminate flowers based on their UV reflection. However, sunbirds do not show a preference for UV colouration.
  • Monique du Plessis was awarded her MSc degree with Distinction. Her experiment in the Cape Peninsula added supplementary feeders to gardens bordering fynbos veld. This showed that feeders seem to attract some nectarivorous birds away from the natural veld towards gardens. One of the two Erica species monitored were visited less often by sunbirds when feeders were present.
  • PhD student Daniël Cloete’s analyses show that the fynbos specialist endemics, Cape Sugarbird and Orange-breasted Sunbird, are both negatively affected by habitat fragmentation. Their numbers are much reduced in smaller fynbos patches. By contrast, more generalist species such as the Southern Cinnyris chalybeus and Greater Double-collared Sunbirds C. afer, and Amethyst Sunbirds Chalcomitra amethystina are more common in small fragments, and may even benefit because of their flexibility to use resources from the surrounding non-fynbos matrix.

Highlights

  • Anina Coetzee published a paper in Functional Ecology with Colleen Seymour and Claire Spottiswoode, on a geographic mosaic of flower colours in bird-pollinated ericas in the Cape fynbos. Using fieldwork and visual modelling, Anina showed that in some circumstances, natural selection drives bird-pollinated Erica species in different plant communities to mimic one another by converging on the same flower colour, to better attract the sunbirds that pollinate them. However, when the reproductive parts of different species are similar in size, natural selection instead favours colour divergence to avoid receiving pollen from the wrong species. Together these two processes appear to have helped to drive the remarkable diversity in flower colour within and between Erica species.
  • Anina also published a paper in Ostrich with Phoebe Barnard and Anton Pauw, describing the distribution of supplementary nectar-feeding in cities in the Cape Floristic Region.
  • Monique du Plessis published a paper in Global Ecology and Conservation, together with Colleen Seymour, Claire Spottiswoode and Anina Coetzee, on her interesting results about the effects of supplementary nectar-feeding on birds and plants.
  • Samantha McCarren published a paper in Journal of Pollination Biology with Anina Coetzee and Jeremy Midgley, on her work on sunbirds’ responses to UV signals in flowers.
  • Anina and Monique presented their research on supplementary nectar-feeding at the Fynbos Forum conference.

Impact of the project

The unique sunbird-Erica mutualism will allow us to gain insights into the mechanisms by which bird behaviour affects community ecology. It provides an opportunity to address knowledge gaps, particularly because human disturbance may directly interact with evolutionary processes in this system. Insights into the effects of habitat transformation and supplementary feeding on pollination systems such as this will inform the development of guidelines for maintaining biodiversity and ecosystem functioning.

Key co-supporters
Claude Leon Foundation; Biotechnology and Biological Sciences Research Council; SANBI Joan Wrench Scholarship; Harry Crossley GreenMatter Scholarship; Smuts Memorial Postdoc Fellowship.

Research team 2021
Dr Anina Coetzee (FIAO, UCT)
Prof. Claire Spottiswoode (FIAO, UCT / U. Cambridge)
Dr Colleen Seymour (SANBI, FIAO)
Dr Phoebe Barnard (FIAO, UCT)
Dr Mark Brown (UKZN)
Prof. Peter Ryan (FIAO, UCT)
Prof. Jeremy Midgley (Biological Sciences, UCT)

Students: Daniël Cloete (PhD, UCT); Monique du Plessis (MSc, UCT); Samantha McCarren (MSc, UCT)