Ecological engineering across a temporal gradient: Sociable weaver colonies create year‐round animal biodiversity hotspots

Abstract

1. Animal distribution in a landscape depends mostly on the availability of resources.This can be facilitated by other species that have positive effects on local species diversity and impact community structure. Species that significantly change resource availability are often termed ecosystem engineers. Identifying these species is key but predicting where they have large or small impacts is an even greater challenge.2. The stress-gradient hypothesis predicts that the importance of facilitative interactions that shape community structure and function will increase in stressful and harsh environments. In most environments, conditions will fluctuate between harsh and benign periods, yet how the impacts of ecosystem engineers will change in different conditions has received little attention. Monitoring for extended periods will increase the understanding of how engineers may mitigate the extreme differences between changing seasons.3. We investigated the role of sociable weavers Philetairus socius as ecosystem engineers and examined how the association of species to weaver colonies may vary across a seasonal (temporal) gradient. Sociable weavers build large colonies that are home to hundreds of weaver individuals but also host a wide range of other animal species. We investigated the use of weaver colonies by terrestrial and arboreal vertebrates and birds throughout a calendar year, encompassing harsh and benign periods. 4. We demonstrate that the presence of sociable weaver colonies creates centres of animal activity. Colonies were used by the local Kalahari animal community for foraging, shade, territorial behaviours and roosting sites. Furthermore, animal activity increased with increased primary productivity, but this was not restricted to weaver colonies, suggesting that the importance of colonies does not directionally change across environmental conditions. Our results were not consistent with predictions of the stress-gradient hypothesis across a temporal gradient.5. We demonstrate the importance of sociable weavers as ecological engineers and the significance of their colonies in structuring the surrounding animal community. Colonies appear to provide a range of different resources for different species. Sociable weaver colonies have large ecological importance to local animal communities and, by mitigating environmental stress, may be increasingly important as human-driven climate change advances.

Ecological engineering across a spatial gradient: Sociable weaver colonies facilitate animal associations with increasing environmental harshness

“…Unless otherwise stated, we used ‘tree’ as the sampling unit (each response variable was calculated per tree), to evaluate our response variables (see below). We used colony presence (present/absent, our primary variable of interest), the stress components (NDVI or rainfall) and the tree characteristics (PCA1 loading score) as explanatory variables in each model (Lowney & Thomson, 2021). Due to the strong correlation between NDVI and rainfall, both stress components could not be used in the same model.…”

“…Data transformation was applied to Shannon diversity (square root) and analysed using an LMM. Sociable weaver observations were removed all from the point count data (Lowney & Thomson, 2021).…”

“…GLMMs investigating the probability that a colony is occupied were fitted with a binomial distribution, whereas those investigating the number of heterospecific species and species richness were fitted with negative binomial and Poisson distributions respectively. Data transformation was applied to Shannon diversity (square root), and these were analysed using an LMM (Lowney & Thomson, 2021).…”

“…We removed all events where individuals that were deemed to 'pass by' and not interact with the tree and/ or colony from the analyses. If individuals triggered both terrestrial and arboreal cameras, we only then used the data from the arboreal camera (Lowney & Thomson, 2021).…”

“…Soils directly below colonies have particularly increased nutrient levels (Prayag et al, 2020) and this could result in direct effects on the local vegetation and animals. Weaver colonies have been shown to act as a resource to multiple species within the environment (Bolopo et al, 2019; Lowney & Charlton, 2017; Maclean, 1970; Rehn, 1965; Rymer et al, 2014) and maintain this impact throughout the year (Lowney & Thomson, 2021). Their facilitative role across their distribution remains unknown (Figure 1).…”