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 key for our understanding of social evolution. 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 800km transects and returned to assess colony success in the field after two months. Specifically, we noted colony extinction events after the two-month duration and collected environmental data on the correlates of those extinction events (e.g., evidence of ant attacks, # prey captured). We found that colony extinction events at desert sites were more frequently associated with attacks by predatory ants as compared to 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.