The investigators propose a foodweb-based procedure for analysis of biodiversity problems. To test the validity of this approach, they will apply it to a river food web at a site within an old-growth conifer forest in northern California. At present, this site is relatively pristine, but invasion by exotic predators and herbivores (squawfish and bullfrogs) is anticipated within a few years. In the pre-invasion community, the researchers will investigate how macroalgae, which dominate primary producer assemblages in the river, mediate predator-prey interactions involving the dominant vertebrate and invertebrate grazers. The effects of macroalgae on habitat selection, feeding rates, growth, survival, and emergence of 1) frog tadpoles (Hyla regilla and Rana boylei) in the presence and absence of predatory aquatic garter snakes (Thamnophis couchi); and 2) algivorous midge larvae (Pseudochironomus richardsoni) in the presence and absence of predatory fishes (roach, Hesperoleucas symmetricus, and juvenile steelhead, Oncorhynchus mykiss) will be analyzed in manipulative field experiments. Results from these studies will allow the investigators to evaluate the relative importance of macroalgae in providing food, refuges, or thermally favorable microhabitats to these dominant vertebrate and invertebrate grazers, or to their predators. Predictions of how changes in algal or predator assemblages following invasions will alter critical food web linkages will be developed, then tested in similar river reaches where predatory squawfish and/or bullfrogs have already arrived. These predictions will also be tested at the primary study site of the principle investigators following the establishment of invaders.
