UBC Theses and Dissertations
Local and global dynamics in social spider metapopulations Sharpe, Ruth Victoria
The relative costs and benefits of group living change with group size. In the social spider Anelosimus eximius, as colonies grow, the number of insects captured per capita decreases, but the size of insects increases, causing biomass captured per capita to peak at intermediate colony sizes. One aspect of group living that changes with group size is competition for resources. Whether intraspecific competition occurs via scramble vs. contest competition can affect the stability and survival of the group or population. By feeding large and small prey to artificial colonies of the social spider Anelosimus eximius, we investigated whether prey size could alter the type of competition that takes place and, thus, potentially, influence colony population dynamics. We found that large prey were shared more evenly, and that individuals in poor condition were more likely to feed when prey were large. Next, we investigated whether the condition of individuals vary as a function of colony size by measuring condition of individual spiders in a wide range of nest sizes. We found that, due to reducing per capita food supply as colonies grow, individuals have lower condition in larger colonies. We also found that condition variance decreases with colony size, further suggesting that scramble competition predominates in large colonies. Although dispersing females were larger and in better condition than philopatric ones, nests established by dispersing females had low survival rate, suggesting that dispersal is costly. Dispersers, therefore, likely face multiple constraints. They have to be large enough to stand a chance of survival following dispersal, but, due to dispersal costs and benefits of group living, should not disperse except from large colonies. Diminishing resources in large colonies, however, coupled with scramble competition, should make it hard for individuals to accumulate sufficient resources to disperse. This combination of factors may contribute to the observed sudden extinction of large colonies that fail to disperse representing a paradox of how social spider metapopulations persist. Using an individual-based simulation model, we demonstrate that rare increases in food supply due to environmental stochasticity may precipitate occasional mass dispersal from large colonies, allowing the metapopulation to persist.
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