PARENT SESSION
Organized Oral Session 43: Complex consequences of spatial subsidies to food webs
Organizer(s): JM Kraus and WB Anderson
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal

Tangled webs: Characteristics and consequences of disrupting reciprocal subsidies that link streams and riparian forests.

Baxter, Colden^*,1, Murakami, Masashi², Fausch, Kurt³, Nakano, Shigeru⁴, ¹ Idaho State University, Pocatello, Idaho² Tomakomai Research Station, Hokkaido University, Tomakomai, Hokkaido, Japan³ Colorado State University, Fort Collins, Colorado⁴ Kyoto University, Kyoto, Japan

ABSTRACT- Studies conducted in Hokkaido, Japan have demonstrated that reciprocal flows of invertebrate prey link streams and riparian forests, and that altering these prey subsidies triggers direct and indirect effects that may propagate across the land-water interface. Intensive year-round measurements showed aquatic insect emergence peaked in spring, when terrestrial invertebrate biomass was low. In contrast, terrestrial invertebrate input to the stream occurred primarily during summer, when aquatic invertebrate biomass was low. Allochthonous prey alternately subsidized both forest birds and stream fishes, accounting for 26% and 44% of the annual total energy budget of the bird and fish assemblages, respectively. A series of large-scale field experiments demonstrated the effects of direct alteration of these prey subsidies, such as may accompany aquatic or terrestrial habitat degradation, but have also shown they may be disrupted by species invasion. When these prey subsidies were manipulated in field experiments using a greenhouse-type cover over the stream, the loss of terrestrial invertebrate prey caused stream fish to grow less or emigrate, and to shift their foraging to insects that graze algae from the stream bottom. This indirectly increased algal biomass, but also decreased adult aquatic insects emerging from the stream. In turn, this led to reduced density of predators like horizontal orb-weaver spiders that specialize on these prey. Similarly, our experiment showed that nonnative rainbow trout usurped terrestrial prey from native Dolly Varden charr, which precipitated the same foraging shift by the native fish, trophic cascade, decreased insect emergence, and reduction of forest spiders. Further work suggests changing the subsidy to forest predators may have indirect effects that cascade through forest food webs. The Hokkaido studies have shown how fluxes of prey can couple aquatic and terrestrial systems in their vulnerability to human disturbance.

Key words: food webs, resource subsidies, stream ecology, riparian ecology