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

Abstract

1. The spatial distribution of animals in a landscape depends mainly on the distribution of resources. Resource availability is often facilitated by other species and can positively influence local species diversity and affect community structure.Species that significantly change resource availability are often termed ecosystem engineers. Identifying these species is important, but predicting where they have large or small impacts is a key challenge that will enhance the usefulness of the ecosystem engineering concept.2. In harsh and stressful environments, the stress gradient hypothesis predicts that community structure and function will be increasingly influenced by facilitative interactions.3. To test this hypothesis, we investigate how the ecosystem engineering role and importance of sociable weavers Philetairus socius varies across a spatial gradient of harshness, for which aridity served as a proxy. These birds build large colonies that are home to hundreds of weavers and host a wide range of avian and non-avian heterospecifics. We investigated the use of weaver colonies on multiple taxa (invertebrates, reptiles, birds and mammals) at multiple sites across a >1,000 km aridity gradient. 4. We show that sociable weaver colonies create localized biodiversity hotspots across their range. Furthermore, trees containing sociable weaver colonies maintained localized animal diversity at sites with lower rainfall, an effect not as pronounced at sites with higher rainfall. 5. Our results were consistent with predictions of the stress gradient hypothesis, and we provide one of the first tests of this hypothesis in terrestrial animal communities. Facilitation and amelioration by ecosystem engineers may mitigate some of the extreme impacts of environmental harshness.

Harshness is not stress

“…Both stressful and harsh environments challenge homeostasis and survival, impacting fitness. Stress and harshness are often considered synonymous [4][5][6][7][8][9], but we maintain that the factors leading to stress differ from factors leading to harshness and that the resulting adaptive physiological responses are mutually exclusive.…”

“…Harsh environments with low rainfall and variable climate favor the evolution of complex animal societies with cooperative breeding [5,8,10]. Increasing harshness due to climate change, such as extended periods of heat and drought, has been identified as a major threat to biodiversity and ecosystem functioning [4,7]. However, there is no clear definition of harshness, which is often wrongly assumed to be the same as stress or long-term stress [4][5][6][7][8][9].…”

“…Increasing harshness due to climate change, such as extended periods of heat and drought, has been identified as a major threat to biodiversity and ecosystem functioning [4,7]. However, there is no clear definition of harshness, which is often wrongly assumed to be the same as stress or long-term stress [4][5][6][7][8][9]. To predict long-term consequences of harshness, we must identify which mechanisms evolved to cope with it, what their limits are, and when they might fail [11].…”

Earthen mounds ( heuweltjies ) of South Africa and their termite occupants: applicability of concepts of the extended phenotype, ecosystem engineering and niche construction

“…These large, persistent structures modify the environment in many ways through the formation of a fertile soil island beneath the nests and by ameliorating local environmental conditions. These modifications benefit a wide variety of invertebrate and vertebrate animal populations, increasing biodiversity, particularly in more the more arid regions where sociable weavers are found [75,76].…”

Understanding anthropogenic impacts on zoogeochemistry is essential for ecological restoration

“…As a nutrient-poor, arid environment (mean annual precipitation is approximately 300 mm/year), wildlife plays a critical role in recycling and redistributing nutrients within the reserve. For example, sociable weavers (Philetairus socius) and brown hyaenas (Parahyaena brunnea) have been shown to create "islands of fertility" at their nest and latrine sites in TKR, respectively (Prayag et al 2020;Abraham et al 2022b), with implications for tree growth and animal activity (Prayag et al 2020;Lowney & Thomson 2022). Consequently, any modification to these or other zoogeochemical processes by TKR wildlife management may impact the attainment of restoration goals.…”