Novel environmental conditions alter subsidy and engineering effects by introduced Pacific salmon

Abstract

We demonstrate that the major ecological functions of Pacific salmon (Oncorhynchus spp.) can be altered or re-ordered in non-native habitats where environmental conditions differ from native ranges. We compared subsidy and engineering effects of spawning Pacific salmon in six streams within their introduced range (Laurentian Great Lakes) with responses reported in their native range (northern Pacific Rim). Streamwater nutrient responses (i.e., subsidy effects) in Great Lakes streams were generally weak compared with those reported in native streams, whereas disturbance (i.e., engineering effects) was often strong where salmon were abundant. We attribute the relatively weak nutrient response to high background nutrient concentrations and low salmon biomass. In contrast, in Great Lakes streams with high salmon biomass, sediment routing was intense and pervasive and, consequently, benthic biofilm and macroinvertebrate abundance often declined by over 90% during the salmon run. These strong disturbance effects were likely facilitated by the small sediments that typified the Great Lakes streams. Our study provides evidence that salmon effects are context-dependent at much broader spatial scales than has been reported previously.