This blog post is provided by Anthony M. Lowney and Robert L. Thomson from the Fitzpatrick Institute of African Ornithology, University of Cape Town and tells the #StoryBehindThePaper for the paper ‘Ecological engineering across a spatial gradient: Sociable weaver colonies facilitate animal associations with increasing environmental harshness’, which was recently published in the Journal of Animal Ecology.
Sociable weavers (Philetairus socius) are small passerine birds, endemic to southern Africa. They weigh no more than 15g, but working as a group they are capable of constructing some of the largest bird-built structures in the world. These huge haystack structures are an iconic feature of the Kalahari and each is unique in shape and size. Larger colonies may contain hundreds of individual nesting chambers and house hundreds of weavers all year-round, roosting and breeding in the nests. A wide range of other species, both avian and non-avian, have also been documented using these colonies: large birds of prey nest on top of weaver colonies, while smaller birds roost in weaver chambers. African pygmy falcons (Polihierax semitorquatus) do not build their own nests and instead exclusively use chambers in weaver colonies for their own nesting and roosting purposes. Kalahari tree skinks (Trachylepis spilogaster) are found in greater numbers on trees containing a weaver colony compared to trees without, and large cats have been observed climbing to the top of colonies to use them as vantage points.
Different species gain different benefits from sociable weaver colonies. Nesting chambers provide insulation against ambient temperatures, being cooler in summer and warmer in winter, a crucial buffer against extreme temperatures in the harsh Kalahari. The soils below weaver colonies are highly enriched with nutrients, and this is expected to have knock-on effects on the surrounding vegetation and the animals that feed on it. Tree-climbing mammals and large nesting birds can use the top of the colonies as a safe refuge or as a vantage point. The structures are maintained constantly, and as a result, they can persist in a landscape for decades, with reports that some are over a century old. Therefore, older and larger colonies may act as landmarks for the animals that use them.
Animals that alter the availability of resources and create habitats are often termed “ecosystem engineers”. Surprisingly, birds are often overlooked as engineers, especially as their nests come in various shapes, forms and sizes. Nests burrowed underground can alter vegetation structural complexity and vertebrate fauna presence, while large communal nests may provide resources for multiple species creating localised biodiversity hotspots. Identifying ecosystem engineers has scientific value, but the real test is to determine how big an impact they have on the wider community and if this impact changes through space and/or time. It has been predicted that the importance of such positive interactions will likely increase in harsher and more stressful environments. This prediction comes from the stress gradient hypothesis, but studies testing this have mainly focussed on plant interactions and ecologists have been relatively slow in applying this hypothesis to animal communities.
Our previous study in this system tested if the importance of weaver colony resources changed over time at our main study site Tswalu Kalahari. We found that weaver colonies created localised biodiversity hotspots and that this was maintained across the seasons. We found fascinating evidence that large mammals used colonies for shade, foraging, and territorial behaviours (scent marking, fighting). We also found an overwhelming increase in the number of arboreal mammals using the top of weaver colonies, with species richness being 36 times greater on trees with colonies compared to trees without. Animals climbing to the top of the platforms included small-spotted genet (Genetta genetta), slender mongoose (Galerella sanguinea), cheetahs (Acinonyx jubatus), and African wild cats (Felis lybica). Clearly these sociable weaver colonies were an important resource to animal communities in these environments, but we wanted to check further how the importance of weaver colonies to local animal communities changed over a large spatial gradient.
We monitored invertebrates, reptiles, birds, and mammals that visited trees that contained weaver colonies and compared these to nearby trees of the same species, similar size, and structure, but without a colony. We used pitfall and pan traps to compare invertebrates, and we recorded and counted reptiles at a given tree. To compare birds, we undertook point counts at the colony and non-colony trees and conducted night visits to sociable weaver colonies to determine which other species were roosting in weaver colonies. We also used camera traps to monitor ground-dwelling, arboreal and tree-climbing mammals. These surveys were repeated at eight different sites across a >1000 km aridity gradient from the relatively mesic savanna Kalahari, across the dry Kalahari to the extremely dry Namib desert and the results were compared against rainfall and vegetation cover across this gradient. If the resources weaver colonies provide positively influence the local animal community, then we would expect to find more animals using colony trees compared to non-colony trees. Additionally, to find support for the prediction of the stress gradient hypothesis, we would expect a greater positive impact of colony trees compared to non-colony trees at sites with lower rainfall and vegetation cover – meaning that food availability was also limited.
Our surveys revealed that weaver colonies create localised biodiversity hotspots throughout their range. A greater number of invertebrates, reptiles, birds, and mammals were all observed at colony trees compared to non-colony trees. Furthermore, animal abundance changed as the environment changed with more animals being observed at wetter sites. However, trees containing sociable weaver colonies maintained localised animal diversity as the environment became harsher for terrestrial invertebrates and birds, something that non-colony trees failed to do. Therefore, our results were consistent with predictions of the stress gradient hypothesis. As a result, we provide one of the first tests of this hypothesis in terrestrial animal communities and provide evidence that facilitation and amelioration by ecosystem engineers may mitigate some of the extreme impacts of environmental harshness.
Read the paper
Read the full paper here: Lowney, A. M. & Thomson, R. L. (2022). Ecological engineering across a spatial gradient: Sociable weaver colonies facilitate animal associations with increasing environmental harshness. Journal of Animal Ecology, 00, 1– 15. https://doi.org/10.1111/1365-2656.13688