A systematic review of ecological thresholds for salmon watersheds

Salmon watersheds are being impacted by multiple cumulative stressors. To avoid and mitigate undesirable outcomes of these cumulative effects, we require understanding of how these stressors impact watersheds and the species within. The concept of ecological thresholds describes the point at which a small environmental change triggers a disproportionately large and often irreversible shift in an ecosystem. Identifying these tipping points is critical for watershed stewardship as they can help anticipate and prevent major ecosystem degradation and loss. For Pacific salmon, cumulative effects present a particular challenge as their migratory life histories expose them to many different stressors throughout their life cycle. We conducted a systematic reviewto identify ecological and land use thresholds relevant for informing the management of salmon watersheds in North America. We extracted and summarized directly observed or cited thresholds for various biotic and abiotic features of salmon habitat, and salmon directly. We also conducted a meta-analysis of diverse responses to watershed scale land cover change. Our review provides multiple sources of support for a threshold effect around 20-30% watershed land cover change. That is, when greater than 20-30% of a watershed is deforested or converted to other land use activities, impacts to physical and biological variables related to salmon habitat integrity become consistently detectable through empirical studies. This result reflects many of the directly observed thresholds we extracted, and supports previous recommendations for implementing watershed scale land cover policies. In this context, our findings suggest that 30% conservation alone is unlikely to be sufficient for thriving salmon watersheds, and that broader land- and water-use safeguards are needed to maintain functional watershed condition before cumulative land cover change crosses the 20-30% range.