Bird pollination in the Cape Floristic Region
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 colour polymorphism in their flowers. Some Erica species have up to five colour morphs yet are pollinated predominantly by just one bird species, the Orange-breasted Sunbird Anthobaphes violacea. Post-doc Anina Coetzee and MSc 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 working in and around the Tsitsikamma section of the Garden Route National Park to assess how habitat fragmentation affects the fitness of bird-pollinated plants. Daniël is testing whether certain thresholds of patch size and isolation exist where pollination services by birds start to break down. To do so, he is measuring pollination by sunbirds and Cape Sugarbirds Promerops cafer of Protea and Erica species across 17 fynbos patches, both natural and fragmented. 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 is assessing 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 is able to assess flower visitation rates at varying distances from nectar feeders in gardens bordering natural areas.
Activities in 2020
- Post-doc Anina Coetzee started experiments to test the role of sunbird foraging behaviour in flower colour evolution. She also tested whether flower colour can serve as a nectar robber avoidance mechanism.
- MSc student Samantha McCarren completed data collection and analysis on flower reflectance of a diversity of ericas, and choice experiments on sunbirds and insects to test their flower colour preferences. Her preliminary analyses suggest that UV colouration is absent or rare in ericas pollinated by short-proboscid insects, rodents, or wind. It occurs in some bird-pollinated ericas, but sunbirds do not show a preference for UV colouration. However, when presented with different rewards they can learn to discriminate flowers based on their UV reflection. Ericas that are pollinated by long-proboscid flies show bright UV colouration, and these flies exhibit a strong preference for flowers with UV colouration.
- Monique du Plessis completed her field experiments for her MSc, conducting bird surveys in gardens and their bordering fynbos veld. She alternated presence and absence of feeders between the gardens, comparing bird abundance at sites with and without feeders, to test whether feeders attract nectarivorous birds away from the natural veld. She also measured Erica visitation rates in the bordering fynbos. By collecting visitation rate data at varying distances from gardens she was able to test whether the presence of feeders affects bird visitation pattern, potentially decreasing Erica pollination.
- 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.
- Anina was awarded the Smuts Memorial Botanical Post-doctoral Fellowship and accepted a teaching position at Nelson Mandela University from 2021.
- Anina published results on reproductive isolation barriers in bird-pollinated ericas, as well as on the interdependence between nectarivorous birds and their associated Proteaceae species.
- Monique submitted her MSc dissertation.
- Samantha successfully applied to upgrade her MSc to a PhD from January 2021.
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.
Smuts Memorial Botanical Post-doctoral Fellowship; Biotechnology and Biological Sciences Research Council; South African National Botanical Institute, Joan Wrench Scholarship; Harry Crossley GreenMatter Scholarship.
Research team 2020
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 (Nature’s Valley Trust)
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)