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UBC Theses and Dissertations

Ecological influences and the biogeographic distribution of sociality in Anelosimus spiders Purcell, Jessica

Abstract

The puzzle of how complex and costly social behaviours have evolved in so many diverse organisms has challenged many generations of biologists. This thesis focuses on interactions between sociality and ecology. My empirical work investigates South American Anelosimus spiders. This genus provides an ideal system for investigating the ecology of social evolution because the species are easy to manipulate and possess social behaviours ranging from nearly solitary to highly social. Social species cooperate to build communal nests, capture prey, and raise young, and groups may persist for many generations. Most Anelosimus species exhibit subsocial behaviours, in which siblings cooperate for a portion of their life cycle, but disperse each generation prior to sexual maturity. I investigate four distinct questions regarding the role of ecology in spider sociality and more generally. First, I ask whether sociality varies between populations of a social species along an altitudinal gradient. I then experimentally transplant small subsocial groups across this altitudinal gradient to investigate the ecological factors that may contribute to this pattern. Third, I examine how sociality may shape community structure in an area where social and subsocial Anelosimus species coexist. Finally, I explore the co-evolutionary dynamics between different social behaviours and dispersal in an individual-based simulation model. I document an intraspecific gradient of decreasing sociality with increasing elevation within the social spider Anelosimus eximius in Ecuador. Through a transplant experiment, I demonstrate that ecological factors including intense rainfall and predator abundance likely contribute to the absence of small groups or solitary Anelosimus spiders from the lowland tropical rainforest. In one area containing at least five sympatric Anelosimus species, I find that social and subsocial species utilize different local habitats. Within those habitats, co-occurring species show different phenologies and construct nests in different positions on a common plant substrate. My modelling study shows that less costly social traits are less sensitive to selection on dispersal than more costly ones, thus extending previous research emphasizing the interplay between dispersal and costly altruistic behaviours. Overall, this thesis shows that ecological factors can influence the origin and maintenance of sociality, both in current communities and over evolutionary time.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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