Optimal foraging theory suggests that organisms forage in a way that maximizes energy gain and minimizes energy loss. Spiders may increase their foraging success by placing webs in areas with higher prey abundance. Artificial light sources often attract insects eaten by spiders, and some spiders may seek artificial light sources, experiencing increased catch rates. Further, benefits of artificial lights may be greater for spiders that build webs nocturnally and remove their webs each morning. In our experiment, we hypothesized that a nocturnal orb weaver that removes its web each morning (Neoscona arabesca) would aggregate at artificial lights and would benefit more from those lights than a web-weaving spider species (Kukulcania hibernalis) that maintains a single web for weeks at a time. To assess spider aggregation, we measured the abundance of spiders in 2X5 meter plots along walls of buildings, and measured the mean light intensity in lux in each plot. To assess catch rates of the spiders, we measured light intensities at individual webs and counted exoskeletons in each web, categorizing prey items by size.
Results/Conclusions
Our results showed that neither spider species aggregated significantly in areas with higher-intensity artificial light. Yet we found that both spider species captured more prey near artificial lights. Although the webs of K. hibernalis spiders are out continuously, these spiders caught more prey at night. In addition, members of this species caught more prey items that were large (> 1 cm) in artificially lit areas than did spiders in darker areas. As larger prey items may strongly enhance spider fitness, our results suggest a biologically important benefit of selecting locations close to high-intensity artificial lights. Because the webs of both spider species were swept down and some spiders were killed shortly before we measured aggregation, our results on aggregation may not accurately reflect the tendency of these spiders to aggregate at artificial lights. Thus, these two spider species meet at least some of the predictions of optimal foraging theory. In addition, our results provide further evidence that artificial light can alter predator-prey relationships.