Bird pollination in the Cape Floristic Region
Why do flowers come in so many different colours, both within and between species? This deceptively simple question is still surprisingly difficult to answer. The Cape Floristic Region’s bird pollination systems provide an ideal opportunity to address it in the context of natural communities of co-flowering species. Claude Leon Post-doc Anina Coetzee is tackling this question in collaboration with Claire Spottiswoode and Colleen Seymour. Specifically, she is testing the hypothesis that flower colour variation within species arises from selection for convergence in flower colour with other species in the local community, to benefit from shared signalling to pollinators. Such ecological 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.
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 different colour morphs both within and between species. These species are pollinated predominantly by just one bird species, the Orange-breasted Sunbird Anthobaphes violacea, so the prevalence of these poly-morphisms is a mystery. This project is investigating the origin and maintenance of these flower colour polymorphisms: what role does plant community context and sunbird foraging behaviour play in generating the intra-specific colour diversity of this group of plants, and how might habitat transformation influence these ecological and evolutionary processes?
Flower colour polymorphisms may evolve for two opposing reasons. Firstly, if co-existing species experience competition for the attention of pollinators, then colour divergence may promote assortative foraging by pollinators. Secondly, when a plant species receives low pollinator visitation rates, it may benefit from mimicking the signals of co-existing species in order to attract more pollinators. Evidence of such honest signal mimicry, as opposed to deceptive mimicry, is very rare in nature. A cause for concern is that habitat fragmentation may change sunbird behaviour in such a way that the evolutionary process maintaining this diversity in ericas is changed completely
PhD student Daniël Cloete is working in and around the Tsitsikamma section of the Garden Route National Park to directly address the effect of habitat fragmentation on the fitness of bird- pollinated plants. Daniël’s research aims to investigate how bird pollination relationships are affected by habitat fragmentation, and to test 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 sugarbirds of Protea and Erica species across 17 fynbos patches, natural and fragmented, in areas located in the vicinity of Nature’s Valley. This is a good area to address this question because it naturally comprises of a matrix of forest, fynbos and coastal thicket, now 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 are affecting, and will further affect, ecosystem function and services in the Cape Floristic Region.
Activities in 2018
- Anina Coetzee completed the data collection for hybrid pollination experiments on three Erica species. This showed that bird-pollinated Erica species are dependent on sunbirds for seed production and that hybridisation between species is limited by post-pollination mechanisms.
- The patterns found in this study so far suggest that Erica species have possibly evolved different flower morphologies and similar flower colours to facilitate their coexistence and their sharing of sunbird pollinators. In order to test these mechanisms, behavioural experiments have been planned and preliminary tests conducted.
- Anina’s research was communicated to the public through an article in African Birdlife and presentations at bird clubs.
- Daniël completed his data processing and analysis following a year of fieldwork in the Nature’s Valley area, where he seasonally collected data on bird species presence and relative abundance, Protea flowering phenology, and bird-pollinated Protea and Erica nectar traits and seed set, for each of 17 study patches.
- 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 Promerops cafer 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 Cinnyris afer and Amethyst Sunbirds Chalcomitra amethystina are more accepting of fragmentation, and may in fact benefit because of their flexibility to use resources from the surrounding non-fynbos matrix.
- Anina’s paper on urban nectarivorous birds was published in Journal of Avian Biology.
- Anina presented the results of the study at the International Association of Vegetation Science Symposium in Montana, United States, where she received second place for the Best Young Scientist Oral Presentation. She also presented this project at the conference of the South African Association of Botanists in Pretoria.
- Anina recruited two Masters Students to work on projects involving the ecology and conservation of sunbirds in 2019. She also trained a prospective postgraduate student from rural Limpopo as part of her fieldwork.
Impact of the project:
The unique sunbird-Erica mutualism will allow us to gain key insights into the mechanisms by which individual 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 on pollination systems such as this will inform the development of guidelines for maintaining biodiversity and ecosystem functioning.
DST-NRF CoE grant; The Botanical Education Trust; Biotechnology and Biological Sciences Research Council; South African National Botanical Institute; Harry Crossley Green Matter Scholarship.
Dr Anina Coetzee (FIAO, UCT)
Prof. Claire Spottiswoode (FIAO, UCT and U. Cambridge)
Dr Phoebe Barnard (FIAO, UCT)
Dr Mark Brown (Nature’s Valley Trust)
Prof. Peter Ryan (FIAO, UCT)
Dr Colleen Seymour (SANBI)
Student: Daniël Cloete (PhD, UCT)