One of the most consistent impacts of bird loss is increased spider abundance. This trend has been observed in the Mariana Islands with spider webs being more abundant and lacking seasonality on Guam, the southernmost island of the chain where birds have been functionally extirpated as a result of the introduction of the brown tree snake, when compared to webs found on other islands in the chain where intact bird communities are present. The follow-on ecological impacts of these observed trends, however, remains unclear. We hypothesize that an important impact of spiders is alteration of the arthropod community which then influences forest dynamics as arthropod-plant interactions play key roles in shaping forest communities. To begin to explore these dynamics, we set up a spider manipulation experiment in which we assigned forest plots to one of three treatments of varying web-building spider densities: low, ambient, and high. In these plots, we set up sticky and pitfall traps and beat vegetation to sample arthropods.
Results/Conclusions
We find that arthropod abundance and diversity decreases as spider abundance increases. Preliminary community composition analysis of arthropods identified to order revealed that community composition is generally similar given varying abundances of web-building spiders; however, some taxa are only found when spider abundance is low. Such results imply that a spider-dominated system increases predation pressure on certain taxa and can shift the arthropod community.