Arthropod Richness and Abundance in the Reservoirs of the Elwha River, Washington

We evaluated terrestrial arthropod responses to ongoing restoration in former Elwha River reservoirs following recent dam removals. Restoration in the Elwha ecosystem involved the largest and most comprehensively studied dam removals ever conducted, but no information is available regarding terrestrial arthropod species composition, distributions, and responses. Addressing this information gap is important, because arthropods perform influential restoration functions, including herbivory, pollination, organic matter decomposition, and food resources for diverse wildlife. We hypothesized that density of vegetation establishing on the reservoir beds determines arthropod abundance and species richness, each of which is greater in areas with fine sediment deposits than those with coarse sediments. We tested this hypothesis by sampling arthropods using sweep nets along transects traversing vegetation on fine and coarse sediment deposits on both Elwha reservoir beds. We analyzed relationships between vegetation density and arthropod abundance and species richness using linear and nonlinear regression. Similar to other ecosystem components in Elwha restoration, our work demonstrates that early arthropod responses have been rapid, but full restoration will occur over longer time scales. Our results have implications for restoration involving dam removal in other river basins, including defining restoration goals, revegetation planning, and understanding restoration time scales