Browsing by Author "Botha, Pieter Willem"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    The world without birds : an experimental test of the ecological significance of pollinating birds for plant communities
    (Stellenbosch : Stellenbosch University, 2017-03) Botha, Pieter Willem; Pauw, Anton; Hui, Cang; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Since pollinators are declining in many parts of the world, it is vital to understand the consequences of pollinator loss. How will plant communities be affected? If pollinators are keystone mutualists, then their loss may cause cascading extinctions with far-reaching consequences for communities. Alternatively, loss of a pollinator may have little effect, since many pollination systems are generalised and the typical structure of plant-pollinator interaction networks suggest flexibility and functional redundancy. Yet relatively few studies have assessed the effects of pollinator loss in the field. This thesis considers the consequences of bird pollinator loss for plant communities in the megadiverse Cape Floristic Region of South Africa, where bird pollinators are particularly important. I constructed 20 x 20 m cages to exclude birds, but not insects, from six plant communities in the Jonkershoek Nature Reserve near Stellenbosch. Communities without birds were compared to neighbouring communities with birds. The scale of the experiment made it possible to assess the demographic and community-level consequences of pollinator loss. Many Cape Proteaceae are pollinated by birds and reliant on seeds to regenerate after periodic fires. One such a species is the showy Leucospermum lineare (Proteaceae), the focus of Chapter 2. Leucospermum lineare was pollinated by Cape Sugarbirds. In the absence of birds, nectar volume and concentration increased, which seemed to amplify visitation by nectar thieving ants. Seed set was maintained without birds through autonomous selfing. However, seeds from bird-excluded plants had lower seed viability in laboratory trials. In the field, post-fire seedling recruitment was reduced significantly in plots where birds had been excluded, most likely as a result of inbreeding. Leucospermum lineare populations may thus decline without birds. A number of other bird-pollinated plant species in the study communities also saw declines in fecundity when birds were excluded (Chapter 3). Protea neriifolia (Proteaceae), a dominant plant in many of the communities, was mostly pollinated by Cape Sugarbirds and its seed set declined by half in the absence of birds. Erica plukenetii (Ericaceae) was pollinated by Orange-breasted Sunbirds and its fruit set was reduced substantially without birds. In Mimetes cucullatus (Proteaceae) seed set collapsed in the absence of birds. In contrast, Protea repens, also a dominant species, and Protea nitida maintained their seed set, likely thanks to insect pollination. Seedling recruitment of Protea species in the presence of bird pollinators was well above adult replacement levels after a fire swept the six year old vegetation; Protea seedling recruitment in the absence of birds could not be assessed. Overall my results suggest that extinction of bird pollinators may reduce the biomass and diversity of plant communities in the Cape Floristic Region. Plant fecundity can also be reduced by animals that eat flowers. I quantified the effects of florivory by rodents and baboons on Protea neriifolia. Seed cone production was reduced substantially. Striped mice, Rhabdomys pumilio, climbed up to 1.6 m high into P. neriifolia plants to feed on inflorescences. Rodents also severed inflorescences by gnawing through subtending stems.