Effects of wolf spider density on decomposer community structure and function in the Arctic

Background/Question/Methods

Understanding how biological communities respond to environmental changes requires detailed knowledge of organismal adaptations and how they fit into a broader ecological context. In the Alaskan Arctic, wolf spiders are among the largest and most abundant invertebrate predators, and they prey primarily on the decomposer community. Because longer growing seasons are associated with increased spider body size, fecundity, and potentially spider density, the rapid rates of warming in the Arctic have the potential to greatly increase predation pressure on decomposers. Increased predation could in turn have important indirect effects on other ecosystem processes such as decomposition rates and nutrient cycling. The goal of this study was to quantify the influence of spider density on the decomposer food web and to measure whether changes in food web structure lead to altered decomposition rates. To this end, we established a fully factorial mesocosm experiment (spider density x temperature) in an area of arctic tundra near Toolik Lake, Alaska to test the hypotheses that 1) spider predation alters the biomass distribution of key functional groups in arctic decomposer communities; 2) spider predation indirectly slows the rates of carbon loss; 3) warming alters the effects of spider predation on community structure and function.

Results/Conclusions

During the first summer of the experiment, we found that wolf spider density did not predict prey densities of collembolans, oribatid mites, or predatory mites. Densities of prey in the high and low predator experimental plots were similar to those in the control plots. However, unlike our expectation, high spider densities marginally increased rates of decomposition. Temperature had no effect on either prey densities or decomposition. Other studies on the indirect effects of predators on decomposers and decomposition have stressed the importance of having longer-term data for understanding the community-level consequences of treatments. Nevertheless, the results from this first season indicate the interesting possibility that higher spider densities may lead to faster decomposition rates in the arctic tundra.