KALAHARI APARTMENT BLOCKS

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 temporal gradient: sociable weaver colonies create year-round animal biodiversity hotspots’, which was recently published in the Journal of Animal Ecology.

Sociable weavers (Philetairus socius) are small passerine birds, no bigger than 15 cm, yet working together, they construct large colonial nests that make up some of the largest bird-built structures in the world. These massive haystack structures are an iconic feature of the Kalahari and are all unique in shapes and sizes. Larger colonies can contain hundreds of individual nesting chambers and house hundreds of birds year-round, while a wide range of other species have also been documented using weaver colonies. African pygmy falcons (Polihierax semitorquatus)do not build their own nests and use sociable weaver chambers 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 cheetahs (Acinonyx jubatus) have been observed climbing to the top of colonies to use them as vantage points.

An acacia tree with a large sociable weaver nest, that takes up around half of the canopy of the tree, set against the colours of the setting sun in the sky
Sociable weaver colony in a camelthorn tree (Photo: Anthony Lowney)

Sociable weaver colonies provide different benefits for different species. Nesting chambers are recorded as being cooler on hot summer days and warmer during the cold winter nights, providing a crucial buffer against extreme ambient temperatures in the harsh Kalahari. The soils directly below the colonies are also highly enriched with nutrients, which could have knock-on effects for the local vegetation and the animals that feed on this vegetation. The colony structure itself creates a platform that tree climbing mammals and large nesting birds can use as a safe refuge. The colonies are constantly maintained, meaning that they can persist in a landscape for many years, with reports that some are over one hundred years old. Therefore, larger and older colonies may act as landmarks for animals that use them.

A cheetah sitting on top of a sociable weaver nest in the top of a tree, looking out over the landscape. Another cheetah is seen lower in the tree underneath the nest
Two male cheetahs on a large sociable weaver colony (Photo: Liam Charlton)

Animals that affect the availability of resources and create habitats are called “ecosystem engineers”. Although identifying these engineers is fun and has scientific value the real challenge is predicting how big an impact these species have on the broader community and if this impact changes through time and/or space. It has been predicted that the importance of such positive interactions will likely increase as environments become harsher and more stressful. This prediction stems from the stress gradient hypothesis. Studies that have tested this have mainly focussed on plant interactions, with ecologists being slow to apply these ideas to animal communities. Furthermore, the majority of studies have investigated the stress gradient hypotheses over spatial gradients/distance, but not temporal gradients/time. Birds are also often overlooked as ecosystem engineers which is surprising as their nests come in many shapes and forms, and are built with the evolved purpose of protecting offspring. Large communal nests likely provide resources for species that gravitate towards these structures, and nests burrowed underground that alter vegetation structural complexity and vertebrate fauna.

We set out to test the impacts of sociable weavers on the surrounding animal community. We monitored mammals and birds 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 camera traps to monitor ground dwelling mammals and also fitted camera traps up in trees to monitor arboreal and tree climbing mammals. To compare bird species, we carried out point counts at both colony and non-colony trees and conducted night visits to sociable weaver colonies to determine which other species were roosting in weaver colonies. These surveys were carried out over extended periods of time, and the results were compared against the local vegetation cover and ambient temperature. If the resources or conditions provided by weaver colonies positively influenced the local animal community, then we would expect to find more animals using colony trees than non-colony trees. Further, to find support for the prediction of the stress gradient hypothesis, we would expect greater positive impact of colony trees compared to non-colony trees during times when vegetation cover was limited – meaning that food availability was also limited – and/or when temperatures were at their most extreme.

Our surveys demonstrated that weaver colonies create localised biodiversity hotspots with more animals being observed at colony trees than non-colony trees. Colonies were used by large mammals for foraging, shade, and territorial behaviours (scent marking, fighting). There was also an overwhelming increase in the number of arboreal mammals using weaver colonies, with species richness being 36 times greater in colony trees. These included cheetah, African wild cat (Felis lybica), small spotted genet (Genetta genetta) and slender mongoose (Galerella sanguinea).

A camera trap photo of a greater kudu in the shade underneath a weaver colony nest
Camera trap image of a greater kudu (Tragelaphus strepsiceros) using a sociable weaver colony for shade.

With regards to the birds that used weaver colonies, we found that 39 of the 45 surveyed colonies featured other species. These included pygmy falcons, acacia pied barbets (Tricholaema leucomelas), ashy tits (Melaniparus cinerascens), and scaly feathered finches (Sporopipes squamifrons). However, outside this particular study we also found red-headed finches (Amadina erythrocephala) breeding in weaver chambers, spotted eagle-owls (Bubo africanus) breeding on top of colonies, and barn owls (Tyto alba) nesting inside colony cavities. Elsewhere, within the sociable weaver range, rosy-faced lovebirds (Agapornis roseicollis) are known to breed in weaver colonies and several raptor species have been observed using weaver colonies as platforms to nest on.

An African pygmy falcon perching on a stick with a small lizard in its beak, below the sociable weaver nest
African pygmy falcon bringing prey back to the colony for its chicks (Photo Anthony Lowney)

Overall animal activity changed as the environment changed. More animals became more active as the landscape became greener and more animals sought shade during high temperatures. However, as the number of animals using colony trees increased, so did the number of animals that used non-colony trees, meaning that the difference between colony and non-colony trees did not change. As a result, we failed to find broad support for the stress gradient hypothesis. However, the number of animals using colony trees for shade increased as the temperature increased, which was not found in non-colony trees, suggesting that for some environmental stressors, the positive associations with weaver colonies may increase as conditions become more severe.

Our findings add to the growing literature that ecosystem engineering provides significant resources to other community members. Sociable weavers and their colonies play a pivotal role in structuring the surrounding animal community, resulting in year-round biodiversity hotspots. As human-driven climate change advances, the importance of these structures may increase, especially to those species that use them to escape the harsh Kalahari temperatures.

A sunset image with very deep yellow sky, that silhouettes a sociable weaver nest, and three weaver birds flying back up the nest
Sociable weavers returning to roost at sunset (Photo Anthony Lowney)
Read the paper

Read the full paper here: Lowney, A. M., & Thomson, R. L. (2021). Ecological engineering across a temporal gradient: Sociable weaver colonies create year-round animal biodiversity hotspots. Journal of Animal Ecology, 00, 1– 15. https://doi.org/10.1111/1365-2656.13544

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