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UBC Theses and Dissertations

The influence of abiotic and biotic factors on the geographic distribution of spider sociality : insights from a factor exclusion and transplant experiment Hoffman, Catherine


Species ranges, which are manifestations of species ecological niches in space, are generally determined by gradients of abiotic and biotic factors. In group-living organisms, not only the properties of individuals, but also those of their groups, should interact with environmental challenges and opportunities to determine a species range. Social and subsocial spiders are well known for having distinct geographical distributions. Intriguingly, subsocial species in the genus Anelosimus are absent from the lowland tropical rainforest where social congeners thrive. Previous studies have attributed this absence to increasing rain intensity and predation, particularly by ants, closer to the rainforest. After confirming that these factors do indeed increase in intensity approaching the lowland tropical rainforest, I test these hypotheses by transplanting nests of the subsocial Anelosimus elegans from its native lower montane cloud forest (1000m) to the lowland tropical rainforest (400m). At both locations I performed a fully factorial ant and rain exclusion experiment and monitored colony survival over time. I found that survival was lower in the lowlands, but improved by the exclusion of rain and ants. At the native higher elevation habitat, in contrast, colony survival did not differ between treatments and controls, confirming that neither intense rains nor predation are factors that negatively impact colony survival in the native habitat. At both locations, large colonies were able to build more webbing, suggesting that larger groups with limited dispersal may benefit from reduced per capita web maintenance in addition to increased predator protection. These findings would explain why subsocial Anelosimus, with small single-family groups and dense webs, have been unable to colonize the lowland tropical rainforest where their social congeners thrive.

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