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. 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. These surveys were carried out over extended periods of time, and the results were compared against the local vegetation cover and ambient temperature. 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. We used camera traps to monitor ground dwelling mammals and also fitted camera traps up in trees to monitor arboreal and tree climbing mammals. 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 set out to test the impacts of sociable weavers on the surrounding animal community. 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. 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. Furthermore, the majority of studies have investigated the stress gradient hypotheses over spatial gradients/distance, but not temporal gradients/time. Studies that have tested this have mainly focussed on plant interactions, with ecologists being slow to apply these ideas to animal communities. This prediction stems from the stress gradient hypothesis. It has been predicted that the importance of such positive interactions will likely increase as environments become harsher and more stressful. 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. Two male cheetahs on a large sociable weaver colony (Photo: Liam Charlton)Īnimals that affect the availability of resources and create habitats are called “ecosystem engineers”. 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. African pygmy falcons ( Polihierax semitorquatus)do not build their own nests and use sociable weaver chambers for their own nesting and roosting purposes. 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. These massive haystack structures are an iconic feature of the Kalahari and are all unique in shapes and sizes. 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. 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.
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