The ecology of spider sociality: A spatial model

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Zsóka Vásárhelyi and István Scheuring, Centre for Ecological Research, Institute of Evolution, Budapest, Hungary, István Scheuring, MTA-ELTE Theoretical Biology and Evolutionary Ecology Research Group, Hungarian Academy of Sciences, Budapest, Hungary, Leticia Avilés, Zoology, University of British Columbia, Vancouver, BC, Canada

Background/Question/Methods
The emergence of animal societies offers unsolved problems for both evolutionary and ecological studies. Both abiotic and biotic factors have been postulated to favour group living, often with multiple factors playing a role in particular systems. Cases where closely related species that differ in their social systems exhibit a strong signal in their geographic distribution can shed light on the factors that favour or hinder group living. Such is the case of the social spiders, which have arisen independently multiple times in eight genera and five spider families. Based on 30+ years of research on the spider genus Anelosimus, which contains the largest number of known social species, we developed a spatially-explicit simulation model that recreates observed macroecological patterns in the distribution of its social systems—social species, whose colonies contain hundreds to thousands of individuals, concentrated in lowland tropical areas, and subsocial species, which form single-family groups, absent from these areas, but present at higher elevations and latitudes. With this model, parameterized with data on the Anelosimus system and the Andes elevational gradient, we tested the hypothesis that two correlated gradients —one of insect size, the other of disturbance (strong rain, predators)— are necessary and sufficient to explain observed macroecological patterns.
Results/Conclusions
With the full insect size and disturbance spectra, we recreated the spatial distribution of the social systems. With the full disturbance spectrum, but small prey everywhere, in contrast, social species disappeared from the grid, whereas subsocial species occupied only intermediate and higher elevation areas; the lowest elevations were empty. With the full insect size spectrum, but high disturbance everywhere, on the other hand, only areas close to the lowland tropical rainforest were occupied as here insects were large enough to support colonies large enough to withstand high disturbances. Higher elevations were empty, as large colonies could not form because of a lack of large prey, whereas small colonies were wiped out due to disturbance. The model thus demonstrates the role of disturbance in creating conditions that require group living while tempering the dynamics of large social groups. We review the robustness and limitations of the model and conclude by discussing implications of the model to other social systems and group living and sociality, in general.