PARENT SESSION
Oral Session # 91: Biogeography III: Aquatic Communities.
Presiding: B Taylor
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 103.
Nonnative stream fish invasion interrupts reciprocal subsidies that shape stream and forest food webs.
Baxter, Colden*,1, Fausch, Kurt1, Murakami, Masashi2, 1 Colorado State University, Fort Collins, CO2 Hokkaido University Forests, Tomakomai, Takaoka, Japan
ABSTRACT- Results of a large-scale field experiment demonstrated the importance of reciprocal subsidies to stream and forest communities, and showed that rainbow trout Oncorhynchus mykiss invasion can lead to significant changes in both of these connected systems. During summer 2002, the effects of invading rainbow trout, and a mesh greenhouse cover that excluded terrestrial prey, were manipulated in a replicated (n=4 each) factorial design in 16 fenced reaches of a small stream in Hokkaido, Japan. In control reaches, native Dolly Varden charr Salvelinus malma foraged substantially on terrestrial as well as aquatic insects. Adding competitively dominant rainbow trout caused charr to shift to foraging on herbivorous benthic insects, thereby increasing stream periphyton biomass in an archetypal trophic cascade. Furthermore, decreased biomass of aquatic insects emerging to the forest resulted in fewer riparian-specialist spiders. Adding the greenhouse reduced input of terrestrial insects, which led to a similar foraging shift by charr, and subsequent increased periphyton biomass, decreased emerging insect biomass, and fewer riparian-specialist spiders. Finally, adding both the greenhouse and rainbow trout produced the same results. These results indicate that rainbow trout had as large an effect as excluding terrestrial prey, and that the effects occurred rapidly, reaching a similar threshold of low herbivore and high periphyton biomass within about 6 weeks. The results also suggest that rainbow trout can cause an 'interaction modification indirect effect', by changing native charr foraging behavior, that cascades to stream periphyton, emerging insects, and riparian consumers.
Key words: biological invasion, salmonids, food web subsidies, stream-forest linkages