Description
In southern California, the Least Bell's Vireo (Vireo belli pusillus) is an endangered songbird that utilizes dense vegetation of riparian habitat. In the 1980s and 1990s, several riparian restoration projects were undertaken in San Diego County to expand its suitable habitat. This study compared arthropod and vegetation communities in 2014 to those in the past in two former restoration sites along the San Diego River in Mission Trails Regional Park (MTRP). Additionally, the arthropod and vegetation communities of three riparian restoration sites along the San Luis Rey River (SLR) in Oceanside were compared to the MTRP sites in 2014. Arthropods were collected in 1991-1996 and 2014 from four riparian vegetation types at MTRP: willow (Salix spp), cottonwood (Populus fremontii), mule fat (Baccharis salicifolia), and annual and perennial herbaceous vegetation. Two additional species were sampled in 2014: Western sycamore (Platanus racemosa) and San Diego povertyweed (Iva hayesiana). Vegetation was categorized by volume and species composition along the same transects as arthropod sampling locations. A Bayesian information criterion (BIC) framework was constructed to determine the relative explanatory power of different factors in the system for predicting arthropod abundance across years and between sites. Final models were selected based on comparisons of R_ or __, complexity of each model, and _BIC. Differences among years explained the largest proportion of variation in arthropod abundance. Abundance of all taxa decreased at MTRP from 1991 to 1996; however, arthropod abundances in 2014 were similar to those recorded in 1996, despite considerable vegetation community changes. Within each year, abundances varied seasonally for all taxa and the majority of taxa had relatively similar abundances between sites for most or all years of the studies. Riparian vegetation cover decreased throughout both MTRP sites, and loss of cottonwood and willow was more severe at the restored site than the reference site. In 2014, the vegetation community included non-riparian species, including the widespread colonization of San Diego povertyweed. Although future trends of community dynamics cannot be predicted from a single year of sampling, this study provided valuable insights into the dynamics of the site nearly twenty-five years after replanting.
