TSWALU FOUNDATION | CURRENT RESEARCH

Sociable weavers as ecological engineers

Sociable Weaver

SOCIABLE WEAVERS AS ECOLOGICAL ENGINEERS

 

Principle Researcher/s, Supervisors and Primary Academic Affiliation:

Dr Robert Thomson (FIAO, UCT), Dr Anthony Lowney (FIAO, UCT), Prof Michael Cramer (Biological Sciences, UCT), Dr Mariette Marais (ARC – Plant Protection Research Institute, Pretoria); PhD students: Timothy Aikins Khan (UCT), Olufemi Olubodun (UCT).

Key co-supporters: DST-NRF Centra of Excellence grant; Tswalu Foundation; University of Cape Town launching grant, Suzuki South Africa.

This project examines the importance of Sociable Weaver (Philetairus socius) nests to Kalahari animal and plant communities. The team aims to understand how the ‘ecological engineer’ potential of these nests may have community-wide impacts, and how this impact may change across environmental gradients. Their 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. The team also focuses on the Pygmy falcon (Polihierax semitorquatus) and investigates the costs and benefits of these falcons on the weaver colonies.

Facilitative interactions are predicted to increase in importance in stressful environments. In harsh environments, like the Kalahari, facilitation may become a crucial component of the adaptive responses of animal and plant communities. Ecological engineers – species that modify habitats and ameliorate abiotic stress for other species – are likely to be key species. 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 are the most ‘controversial’ user of the Sociable Weaver nest colonies. They never construct their own nests, depending entirely on chambers in the weaver nests for roosting and breeding: a unique obligate nesting association. These falcons also prey on weaver nestlings and even adults, suggesting a semi-parasitic relationship between the species. This project aims to describe the natural history and ecology of Pygmy falcons, and to characterise their interactions with Sociable weavers; do falcons provide benefits to weavers or are they vertebrate parasites? And how does their presence impact the ecological engineering role of weaver colonies.

The Kalahari has two iconic tree species, the Camelthorn (Vachellia erioloba) and the Shepherd’s tree (Boscia albitrunca). These trees frequently host Sociable weaver colonies. The team investigates the costs and benefits that these weavers have on their host trees. Sociable weavers forage in the landscape and bring plant and insect material back to their nest trees which is deposited as faeces. This nutrient input at weaver nests results in these sites being islands of fertility in the landscape. We study how this alters the soil chemistry, but also the impacts on plant communities and soil nematode communities.

This project is providing unique insights into the community ecology and between-species interactions in the Kalahari. Fascinating natural history stories have come to light, and brought attention to a unique system. The project aims to quantify the ecological engineering role of the Sociable weaver and determine the predicted role of their nests in a warming and increasingly arid Kalahari. Lastly, the outputs of this project will also contribute to available eco-tourism information that enhances the experience of visitors to landscapes within the distribution of the Sociable weaver.

Sociable Weaver Image by Anthony Lowney

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