Changes in California Odonata communities since 1914

Overall biodiversity of freshwater ecosystems is declining at particularly high rates throughout the world as a result of land use, water use, and climate change. However, few studies have quantified changes in aquatic insect communities. In 2011 and 2012, we re-surveyed 43 sites that C.H. Kennedy originally sampled in 1914 for Odonata species to examine changes in species richness and community composition. Species richness across all sites declined from 81 to 69 species (~15%). Although average richness at individual sites increased slightly (~0.86, SD=6.4), 20 sites experienced either significant increases or declines in the number of species present. Community similarity of Odonata communities increased at ecoregion, county and site-specific scales. We expect that the highest degree of community change is associated with increases in urbanization and temperature. Using single-season occupancy models for each time period, we also explored changes site occupancy probabilities for individual species. We found that many habitat generalists with high dispersal abilities (e.g. Libellula luctuosa, Pantala hymenaea, and Tramea lacerata) now occur more frequently, while habitat specialists have declined in occupancy rates (e.g. Macromia magnifica and Ophiogomphus morrisoni). This study indicates that Odonata communities in California have become more homogenized over the past century, which is driven by the expansion of certain cosmopolitan species and the decline of habitat specialists. Such resurveys over long time periods will help identify threatened species and locations of particular conservation concern.