Predictors of colony extinction vary by habitat type in social spiders

Abstract

Many animal societies are susceptible to mass mortality events and collapse. Elucidating how environmental pressures determine patterns of collapse is important for understanding how such societies function and evolve. Using the social spider Stegodyphus dumicola, we investigated the environmental drivers of colony extinction along two precipitation gradients across southern Africa, using the Namib and Kalahari deserts versus wetter savanna habitats to the north and east. We deployed experimental colonies (n = 242) along two ~ 800-km transects and returned to assess colony success in the field after 2 months. Specifically, we noted colony extinction events after the 2-month duration and collected environmental data on the correlates of those extinction events (e.g., evidence of ant attacks, no. of prey captured). We found that colony extinction events at desert sites were more frequently associated with attacks by predatory ants as compared with savanna sites, while colony extinctions in wetter savannas sites were more tightly associated with fungal outbreaks. Our findings support the hypothesis that environments vary in the selection pressures that they impose on social organisms, which may explain why different social phenotypes are often favored in each habitat.

Exploring changes in social spider DNA methylation profiles when succumbing to infection in CpG, CHG, and CHH contexts

“…They cooperate during reproduction and foraging, including transferring digestive fluids during communal external digestion of prey (typically flying insects) leading to highly similar microbiomes among nest mates (Busck et al ., 2020; Rose et al ., 2023). Such close associations also create the ideal conditions for the transmission of pathogens, and indeed it is thought both bacterial and fungal infections are a key source of mortality (McEwen et al ., 2020; Busck et al ., 2022). As part of a larger study, we located five nests on farmland in northern Namibia (Busck et al ., 2022).…”

The Ecology of Spider Sociality: A Spatial Model

“…Species in the genus Stegodyphus (three social species), however, occupy dry thornbush savanna habitats where strong precipitation clearly does not play a role (Majer et al 2013). Strong predation, however, may be the main source of disturbance for these spiders (Henschel 1998;McEwen et al 2020), whereas the high productivity of their habitats likely contributes to sufficiently high prey biomass to explain the geographical range of the social species (Majer et al 2013). Interestingly, whereas social Stegodyphus species are excluded from less productive temperate habitats, it is unclear whether subsocial species in this genus are excluded from areas where social species are common.…”

“…Avilés and collaborators refer to this as the Predation/Rain Intensity hypothesis . We refer to it here as the Disturbance Hypothesis, to allow for other external factors that may disrupt the growth and survival of colonies, such as drought, wind, fire, disease, etc., which, in addition to predation, could affect species in other genera (e.g., Stegodyphus (Lubin and Crouch 2003;McEwen et al 2020)).…”

Parasites of spiders: Their impacts on host behavior and ecology

“…Dense aggregations and high levels of inbreeding within colonies may increase susceptibility to disease outbreaks. A recent survey found that populations in wetter climates are more likely to succumb to fungus-driven extinctions (McEwen et al 2020), though the fungus has not yet been identified and it remains unknown whether it is pathogenic, opportunistic, or saprophytic (i.e., growing only after idiopathic colony death). The genus Stegodyphus Simon, 1873, in which sociality has evolved three times independently, has become a powerful test system for questions regarding the microbial communities of social spiders and the nests in which they reside.…”