Abstract
A survey of the bees in semi-natural habitat along the American and Cosumnes rivers in Sacramento County, California, was conducted during the flower season of 2007. Although the highly modified landscapes surrounding the two rivers is distinctly different, with urban and suburban development dominant along the American River, and agriculture along the Cosumnes River, there is no difference in the proportion of modified landscape between the two rivers. The proportion of semi-natural habitat is also similar between rivers. Sixty four species of plants provided floral resources for bees, dominated by nonnative species. Over half of the bee diversity were associated with 3 nonnative plants—Hirschfeldia incana, Centaurea solstitialis, and Cichorium intybus—indicating the importance of nonnative plants in providing floral resources A total of 122 bee species were identified in five families from 7910 specimens collected or observed. Bee abundance was dominated by the Halictidae family, with 50% coming from 4 species. Apidae was the most specious family, and Andrenidae and Colletidae accounted for less than 5% of bee abundance. A surprising 17% of bee diversity included specialist bees, with the Cosumnes river accounting for higher richness, abundance, and number of unique species. Five species of nonnative bee species were identified, but there were no indications of nonnative bees exhibiting preferences for nonnative plants. Similarity measurements reveal that bee communities are generally associated by river, with the exception of one site on the American river at the confluence with the Sacramento river, indicating the possibility of river systems providing uniquely similar bee communities.