Multiple invading species and ecological consequences for salmonids in the John Day River, Oregon

Abstract

Freshwater ecosystems increasingly face multiple, interacting stressors that threaten the persistence of biodiversity and challenge existing conservation strategies. Among these threats, biological invasions and climate change are becoming more prevalent and can have cumulative effects on imperiled populations of Pacific salmon and steelhead. However, understanding of how invasive species interact with one another and potential responses to climate-driven warming remains limited. My thesis research investigated the ecological impacts of two invasive species, smallmouth bass (Micropterus dolomieu) and rusty crayfish (Faxonius rusticus), within the John Day River Basin, Oregon, USA, with emphasis on species interactions, dynamic ecological processes, and implications for juvenile steelhead (Oncorhynchus mykiss).In Chapter 1, I developed a spatially explicit individual-based model to compare the interactive effects of smallmouth bass and rusty crayfish under past and future climatic conditions. By comparing single-species and multi-species simulations, I identified emergent patterns in abundance, distribution, and rates of spread across the river network. Smallmouth bass exerted strong top-down control on rusty crayfish populations in reaches where bass were established, reducing crayfish abundance, slowing rates of spread, and limiting range expansion. These findings demonstrate that interactions among invaders can generate disparate estimations that alter invasion trajectories and provide significant value to modeling frameworks. In Chapter 2, I evaluated the trophic ecology of rusty crayfish along an invasion gradient that spans critical spawning and rearing habitat for a threatened population of steelhead in the South Fork John Day River. Using stable isotope analysis, fatty acid analysis, and assessments of macroinvertebrate community composition, I identified collectively powerful indicators of the ecological impacts of an invasive crayfish on pre-migratory steelhead. Rusty crayfish exhibited substantial variation in trophic position and resource use through the study area, with individuals occupying lower trophic positions upstream and higher trophic positions downstream. Considerable isotopic niche overlap between crayfish and steelhead smolts suggested potential for resource competition during freshwater rearing. Although crayfish density was not associated with reductions in preferred salmonid prey abundance, fatty acid profiles indicated the incorporation of marine-derived nutrients and provided evidence consistent with opportunistic predation on these vulnerable salmonid life stages. Collectively, this research demonstrates that invasive species can affect freshwater ecosystems through both direct trophic interactions and indirect ecological dynamics mediated by interactions among invaders. The results highlight the importance of accounting for species interactions when predicting invasion outcomes and suggest that the ecological effects of invasive crayfish may be context dependent across space. As freshwater ecosystems continue to experience warmer temperatures and expanding biological invasions, effective conservation and management of threatened steelhead populations will require an integrated understanding of these complex ecological dynamics.