Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8873
Title: Defining ecosystem thresholds for human activities and environmental pressures in the California Current
Author: J. F. Samhouri, K. S. Andrews, Gavin Fay, C. J. Harvey, Elliott L. Hazen, Shannon Hennessey, K. K. Holsman, Mary E. Hunsicker, Scott I. Large, Kristin Marshall, A. C. Stier, Jamie Tam, Stephani Zador
Publication Year: 2017
Journal: Ecosphere
Keywords: ecosystem threshold,nonlinear,ecosystem indicator,reference points,ecosystem-based management

The oceans are changing more rapidly than ever before. Unprecedented climatic variability is interacting with unmistakable long-term trends, all against a backdrop of intensifying human activities. What remains unclear, however, is how to evaluate whether conditions have changed enough to provoke major responses of species, habitats, and communities. We developed a framework based on multimodel inference to define ecosystem-based thresholds for human and environmental pressures in the California Current marine ecosystem. To demonstrate how to apply the framework, we used gradient forest and generalized additive model (GAM) analyses to screen for nonlinearities and potential threshold responses of ecosystem states (n=9) across environmental (n=6) and human (n=10) pressures. These analyses identified the existence of threshold responses of five ecosystem states to four environmental and two human pressures. Gradient forest analyses revealed that mean trophic level of the groundfish community, northern (boreal) copepod biomass for both summer and winter, and scavenger biomass had significant thresholds explained by indices of environmental pressures, namely the Pacific Decadal Oscillation (‘PDO’, during summer and winter months), and a human activity (commercial shipping). The GAMs revealed a significant nonlinearity between a different ecosystem state—California sea lion pup production—and annual variation in summer PDO values. Furthermore, the GAM analysis detected nonlinear relationships between California sea lion pup production and the Northern Oscillation Index during summer; northern copepod biomass during summer and the North Pacific Gyre Oscillation during winter; and, northern copepod biomass during winter and an indicator of human-caused habitat modification. Taken together, our findings provide a new way to interpret changes in the intensities of human and environmental pressures as they relate to the ecological integrity of the California Current ecosystem. These insights can be used to make more informed assessments of when and under what conditions intervention, preparation, and mitigation may enhance progress toward ecosystem-based management goals.

Theme: Ecosystem approach to improve management of marine resources
Foci: Provide scientific support for the implementation of ecosystem-based management
Assess ecosystem status and trends.