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 birds (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. Post-doc Anina Coetzee and MSc student Samantha McCarren are investigating how sunbirds influence flower colour evolution in bird-pollinated Erica species. Such processes may now be threatened by habitat fragmentation in many parts of the Cape Floristic Region, and understanding these effects is the goal of PhD student Daniël Cloete’s research. At the border between urban and natural fragments, supplementary feeding, which is increasing in popularity, may impact sunbird-plant mutualisms. Monique du Plessis’ MSc study will assess the effect of feeders on sunbirds and bird-pollinated Erica species.
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. This project investigates the origin and maintenance of these flower colour polymorphisms. Specifically, we are asking what role plant community context and sunbird foraging behaviour play in generating intra-specific lower colour diversity.
African nectarivorous birds are thought to be able to detect ultra-violet (UV). Thus, UV colouration might be used by bird-pollinated flowers in the Cape Floristic Region 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, 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, in the Nature’s Valley area. This is a good area to address this question because it naturally comprises of 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..
Activities in 2019
- Post-doc Anina Coetzee completed pollination experiments on co-occurring Erica species, which showed that bird-pollinated Erica species are highly dependent on sunbirds for seed production and that hybridisation between species is limited by post-pollination isolation barriers.
- Anina’s research was communicated to the public through an article in African Birdlife and presentations at bird clubs and museums.
- Samantha McCarren completed data collection on flower reflectance of a diversity of plant families, and choice experiments on sunbirds and insects to test their flower preferences.
- Monique du Plessis completed her first year of field experiments, during which she conducted seasonal bird surveys in 18 gardens and their bordering fynbos veld. She alternated presence and absence of feeders between the gardens, comparing bird abundance of sites with and without feeders. In doing so, she will be able to test whether the presence of feeders attract nectarivorous birds away from the natural veld.
- Monique also measured Erica visitation rate in the bordering fynbos veld. She collected visitation rate data at varying distances from gardens to test whether the presence of feeders affects bird visitation pattern, potentially decreasing Erica pollination.
- Daniël Cloete completed his data processing and analysis following a year of fieldwork in the Nature’s Valley area, where he collected data on bird species presence and relative abundance, Protea flowering phenology, and bird-pollinated Protea and Erica nectar traits and seed set.
- Daniël extracted data from his high-resolution aerial photos captured by drone to determine the distribution and density of bird-pollinated Protea species in each of the study patches.
- Daniël’s preliminary analyses suggest that the fynbos-specialist endemics, Cape Sugarbird and Orange-breasted Sunbird, are both negatively affected by fragmentation. 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 accepting of fragmentation, and may even benefit because of their flexibility to use resources from the surrounding non-fynbos matrix.
- Anina presented her research results at the European Society for Evolutionary Biology in Turku, Finland, and at an Erica Symposium in Cape Town.
- Monique was awarded a competitive Joan Wrench Scholarship from SANBI for her MSc study.
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.
DST-NRF CoE grant; Claude Leon Foundation, The Botanical Education Trust; Biotechnology and Biological Sciences Research Council; South African National Botanical Institute; Joan Wrench Scholarship, Harry Crossley GreenMatter Scholarship.
Research team 2019
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)