SCOTT F. COLLINS
Illinois Natural History Survey
The Ecological Importance of Salmon Subsidies to Stream-Riparian Ecosystems:
An Experimental Test and Implications for Approaches to Nutrient Mitigation
The annual delivery of nutrients and energy by salmon can greatly influence the productivity of receiving habitats. Unfortunately, dams have blocked salmon migrations, effectively decoupling an important subsidy of nutrients and energy in inland streams and lakes. Natural resource managers are turning to nutrient additions of different physical and nutrient forms to increase productivity in freshwater streams, however terrestrial environments are often overlooked. To test the effectiveness of certain mitigation tools, annual additions of salmon carcasses and analog pellets were experimentally added to streams of central Idaho over four years. Responses of aquatic and terrestrial organisms were tracked to identify how salmon subsidies directly and indirectly affect the structure and function of stream-riparian food webs. Salmon carcasses were frequently removed (typically by black bears) from streams to the wetted margins and riparian zones, whereas salmon analog pellets were not. Over the duration of the experiment, subsidies increased fish production, which increased top-down regulation of benthic insect larvae and reduced the emergence of adult aquatic insects to the riparian zone. In turn, there was a reduction in the abundance and activity of spiders and bats that are reliant these aquatic insects as prey. Translocation of salmon carcasses to riparian habitats attracted terrestrial Diptera and increased their productivity, resulting in increased activity of select bats. In a pattern consistent with apparent competition, these bats also consumed select riparian spider taxa, reducing spider abundance in streams treated with salmon subsidies. Our experiment demonstrated that food web responses to salmon subsidies are highly complex and not always positive. Measures to mitigate in the most realistic fashion should consider the form of the subsidy and its importance to both aquatic and terrestrial environments.