Use of Terrestrial Arthropods to Evaluate Restored Riparian Woodlands

Abstract The goal of this study was to use terrestrial arthropods to help evaluate the progress of a riparian restoration effort along the San Luis Rey River in California by comparing characteristics of the arthropod community at the reconstructed system to those of a naturally occurring riparian woodland used as a reference site. Insects and other arthropods were sampled throughout 1989–1991 using pan trapping and sweep sampling of dominant plants. Assemblages of taxa were monitored as indicators of functional groups that influence ecosystem processes: pollinators, herbivores, predators, parasites, and detritivores. Relative abundances of indicator assemblages were compared between sites to evaluate the establishment and maintenance of processes critical for the natural function of the reconstructed riparian ecosystem. A major objective of this project was to create habitat for the Least Bell's Vireo, so a group of potential prey items was designated to indicate vireo food resources. Over 230,000 arthropods were identified to order or family and by size. Insect communities developed rapidly at the restored habitat. Although the abundance of all arthropods was lower at the reconstructed site than at the reference site, the same orders were present after three years in similar proportions at both locations, and mean abundances were within an order of magnitude of each other. Abundance of certain groups, such as detritivores, suggested that arthropods had propagated rapidly at the restoration site, a possible indication of resiliency. Relatively low numbers of other arthropods, such as predators and parasites, at the reconstructed site indicated the need for continued monitoring. The decline of pollinators and herbivores by 1991 at the reconstructed site suggested that they may have immigrated or been introduced with transplanted vegetation in 1989, but have had difficulty colonizing the site. Although the Least Bell's Vireos were seen foraging at the restored site after three years, no nests were found. Nesting is anticipated, however, as the site matures.