Meeting Abstract
Most spider species are solitary, but in some species individuals facultatively aggregate. In a population of the golden-orb web spider Nephila clavipes near Baru, Costa Rica roughly 50% of spiders form clusters, consisting normally of 2-3 individuals of different size classes. We monitored 400 female spiders for 8 weeks in both 2013 and 2014 to determine whether clustering frequency was related to the trade-offs associated with being in a group, and to test the hypothesis that such trade-offs depend on spider size. We expected that clustering individuals would capture less food than solitary ones, but experience lower predation rates, and that small spiders would more likely experience these trade-offs than larger spiders due to their smaller web size and greater chance of being depredated. Here we report on inter-annual variation in the costs and benefits of clustering and differences between the years that might explain them. Most notably, in the first study year we found no trade-offs associated with clustering regardless of spider size, while in the second year small, but not medium, spiders had reduced prey capture but higher survival if clustered than if solitary. Additionally, small spiders were much less likely to be in clusters in 2013 than in 2014. These differences may be explained by the fact that in 2013 the amount of prey captured in webs was low and predation on the spiders themselves was only 35%, while food was more abundant, and 78% of spiders were depredated during the 2014 study. These results support the idea that fluctuating environmental factors influence the costs and benefits and hence frequency of clustering, and, further, that such trade-offs may not be equal for individuals of all life stages.
