Do Riparian Habitat Disturbances that Alter Cross-habitat Resource Subsidies Magnify Effects of Nonnative Fish Invasions on Stream Food Webs?

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DEB-0108222FauschHabitat destruction and biotic invasions are the two leading agents of global environmental change causing loss of species worldwide. An estimated 40-50% of the land on Earth has been degraded by human actions, and more than 90% of riparian zones adjacent to streams have been altered. Nonnative species invasions cost $137 billion annually worldwide to control. Habitat degradation and species invasions occur simultaneously in most ecosystems and are believed to interact strongly, but there is little experimental evidence to help predict what kinds of habitat disturbances will facilitate invasions or increase their effects. The field experiments proposed here test the idea that riparian habitat disturbances that reduce inputs of terrestrial invertebrate prey from forests to headwater streams during summer magnify the direct and indirect effects of invading fish on stream food webs. In addition, effects of the fish invaders on stream invertebrates are predicted to reduce reciprocal subsidies of emerging aquatic insects from stream to forest during fall through spring. If so, this will demonstrate the potential for habitat loss and fish invasions to have negative indirect effects on terrestrial consumers like birds and spiders. Experiments by Japanese ecologists showed that excluding terrestrial insects from a stream in northern Japan using a canopy (to mimic the usual result of riparian habitat disturbance) caused native trout (charr) to deplete herbivorous stream invertebrates, producing a 'top-down' trophic cascade that increased algal biomass. Further research showed that inputs of forest terrestrial insects (TI) to the stream during summer supplied about 50% of the annual energy budget of fishes, and that aquatic insects emerging from the stream supplied about 25% of the annual energy budget of forest birds. Rainbow trout, a globally-important nonnative stream fish, are rapidly invading streams in northern Japan, apparently exclude native charr, and specialize on TI. Therefore, the current experiments will compare controls with native charr alone in replicate stream reaches to two treatments, one where rainbow trout are added, and another with rainbow trout added but TI excluded using stream canopies. Direct effects of rainbow trout are predicted to reduce native charr growth, abundance, and ultimately to exclude native species via emigration and mortality. Exclusion of TI as well is predicted to intensify effects of rainbow trout on native charr, reduce benthic invertebrate biomass and species richness, and reduce aquatic insect emergence that supplies forest consumers. Predicted indirect effects include foraging shifts by charr that further reduce benthic invertebrate biomass, increase algal biomass via a trophic cascade, and reduce cross-habitat flux of emerging aquatic insects that subsidize forest consumers. This will be the first large-scale experiment to test the idea that human disturbance in one habitat can exacerbate effects of an invasion in another. A richer understanding of the interactions between habitat disturbances and biotic invasions in complex real-world food webs is needed to foster more complete theory in both fields, and to aid natural resource managers attempting to conserve native fish, invertebrate, and bird diversity.