This project examines the importance of Sociable Weaver Philetairus socius nests to Kalahari animal and plant communities. The objectives are to investigate the diversity of animals associated with the nests, the interactions between these species, and to gain insights into the life histories of associated species. We also aim to understand how the ‘ecological engineer’ potential of these nests may have community-wide impacts on structure and function, and how this impact may change across environmental gradients.

Evidence of the importance of facilitation in communities has accumulated, which challenges traditional emphasis of negative interactions in ecology. In particular, facilitative interactions are predicted to increase in importance in stressful environments and may become a crucial component of the adaptive responses of communities under stress. Ecological engineers – species that modify habitats and ameliorate abiotic stress for other species – are a key research focus. Identifying and understanding the impact of ecological engineers is vital, especially in arid environments that are expected to become harsher with global climate change.

Pygmy Falcons Polihierax semitorquatus are the most controversial user of the weaver colonies. They never construct their own nests, depending entirely on weaver colonies, which is a unique obligate nesting association. Pygmy Falcons also, albeit rarely, prey on weaver nestlings and even adults, suggesting a semi-parasitic relationship between the species. We study the natural history and ecology of Pygmy Falcons, and assess whether the falcons provide benefits to the weavers.

As colonial breeders, Sociable Weavers bring material back to their nest trees in the form of faeces, feathers, and carcasses. This nutrient input results in weaver nests being islands of fertility in the landscape. We study how this alters the soil chemistry, as well as soil nematode and plant communities. We further investigate the effect of this fertile island on host tree seedlings, host tree productivity and the potential costs of supporting such a huge nest.

Activities in 2022

  • Olufemi Olubodun led the 12th field season at Tswalu Kalahari following the individually marked pygmy falcon population and collecting his last PhD thesis data. Tswalu received good rains but these came late in the falcon breeding season, leaving reproductive output lower. Olufemi followed 26 Pygmy Falcon pairs and ringed 21 nestlings.
  • Daniel Rossouw joined the research group for his Honours project where he investigated whether pygmy falcon presence at weaver colonies negatively effects the quantity and quality of sleep. Daniel worked with existing video and recorder data from the field site, but also performed a presentation and playback experiment on Tswalu Kalahari.
  • Data collection towards the Kalahari Endangered Ecosystem Project (KEEP), a collaboration with WITS, UP, UNISA and UWC researchers, continued. These long-term data will contribute to identifying the impacts of global change on the Kalahari ecosystem.
  • Robert Thomson, Timothy Aikins Khan and Olufemi all presented talks on various aspects of the project at the 15th Pan-African Ornithological Conference held at Victoria Falls, Zimbabwe in November..

Highlights:

  • Olufemi received a British Ornithologists Union funding award for his research investigating the movement ecology of the pygmy falcon. The award was used to buy the small GPS loggers which are placed on falcons to record their exact position every 30 minutes for a 2 to 3-week period. 
  • Robert and Anthony Lowney published a second paper in Journal of Animal Ecology showing the importance of Sociable Weavers as ecological engineers across an aridity gradient in the weaver range. This study highlights these Sociable Weaver colonies appear to buffer conditions as the environment becomes increasingly harsh and maintain biodiversity of several animal taxa.
  • Timothy’s first PhD thesis chapter was published in Journal of Arid Environments. He explored the “Islands of fertility” created by savanna trees and how these are amplified by sociable weaver colonies, with Prof. Michael Cramer and Robert as co-authors. 
  • The project again hosted the Conservation Biology Masters programme field course. The students collected data to contribute to Timothy’s study on the costs and benefits to trees hosting a sociable weaver nest.
  • Anthony and Olufemi published a short note in African Herp News that describes evidence of boomslang foraging during the night on pygmy falcon chicks.

Impact of the project

This project provides unique insights into the community ecology and between-species interactions in the Kalahari. It highlights fascinating natural history stories and brings attention to this unique system. We quantify the ecological engineering role of the Sociable Weaver and determine the potential role of Sociable Weaver nests in a warming and increasingly arid Kalahari. The outputs of this project also contribute to eco-tourism information that enhances the experience of visitors to landscapes within the distribution of the Sociable Weaver.

Key co-supporters

DSI-NRF CoE grant; Tswalu Foundation; University of Cape Town launching grant; Suzuki South Africa.

Research team 2022
A/Prof. Robert Thomson (FIAO, UCT)
Dr Anthony Lowney (U. Hartpury, FIAO associate)
Prof. Michael Cramer (Biological Sciences, UCT)
Dr Mariette Marais (ARC – Plant Protection Research Institute, Pretoria)
Dr Bryan Maritz (UWC)
KEEP team (led by Prof. Andrea Fuller, and Prof Graham Alexander, both WITS)

Students: Timothy Aikins Khan (PhD, UCT); Olufemi Olubodun (PhD, UCT); Daniel Rossouw (BSc Hons, UCT)..