Effects of Grouping Behavior and Predators on the Spatial Distribution of a Forest Floor Arthropod

Spatial aggregations arising from social behavior or habitat patchiness are common in nature and have important implications for population dynamics, community stability, and conservation. Distinguishing between these behavioral and environmental causes of pattern is of general interest to spatial ecologists and continues to be a key unresolved issue. Despite the importance of this problem, systematic approaches for resolving the underlying mechanisms are not well developed. We demonstrate here the value of a three-tiered systematic approach involving descriptive spatial sampling, individualbased observation and diffusion modeling, and manipulative field experiments. We used this approach to test socialvs. habitat-driven hypotheses explaining spatial aggregation in the collembolan Orchesella hexfasciata. Our results show that aggregation is a gregarious behavior triggered by seasonal increases in soil moisture. Initial field observations suggested that aggregation was habitat driven and associated with soil moisture, but individual-based observations and modeling revealed that moisture was only a triggering mechanism for socially driven aggregation. This was corroborated in field experiments by testing hypotheses that were motivated by the individual-based analyses. Thus, the three-tiered approach led to a more complete understanding of aggregation than would any single technique.