Before treatment with a 3 mg/l Noxfish (0.15 mg/l active ingredient; rotenone) for 4 8 hours. benthic invertebrate communities were quantitatively sampled with a modified Surber net. Then spring, summer. and fall post-rotenone samples were taken monthly a t each of four Strawberry River stations for five years. Statistical analyses of the data indicated that the application of rotenone had a significant effect on the following species density: Cinygrnula sp., Pteronarcella badia , Hesperoperla pucflca, Hydropsyche sp.. and Brachycentrus americanus. Thirty-three percent of the benthic invertebrate taxa a t the four stations showed resistance to rotenone. Up to 100% of Ephemeroptera, Plecoptera and Trichoptera species were missing after the second rotenone application. Forty-six percent of the taxa recovered within one year, but 2 1% of the taxa were still missing after five years. Of the 19 taxa still missing, 47% were Trichoptera. 21% were Ephemeroptera. 16% were Plecoptera, 11% were Coleoptera, and 5% were Megaloptera.

Abstract. The abundance of exotic plants is thought to be limited by competition with resident species (including plants and generalist herbivores). In contrast, observations in semiarid Chile suggest that a native generalist rodent, the degu (Octodon degus), may be facilitating the expansion of exotic annual plants. We tested this hypothesis with a 20-year data set from a World Biosphere Reserve in mediterranean Chile. In this semiarid environment, rainfall varies annually and dramatically influences cover by both native and exotic annual plants; degu population density affects the composition and cover of exotic and native annual plants. In low-rainfall years, cover of both native and exotic herbs is extremely low. Higher levels of precipitation result in proportional increases in cover of all annual plants (exotic and native species), leading in turn to increases in degu population densities, at which point they impact native herbs in proportion to their greater cover, indirectly favoring the expansion of exotic plants. We propose that bottom-up control of consumers at our site results in top-down indirect facilitation of invasive annual herbs, and that this pattern may be general to other semiarid ecosystems.

No abstract