Understanding the importance of diversity in generating stability is a central goal in ecology. Several recent studies have highlighted the importance of intraspecies diversity among populations in fostering the stability of population complexes. This work has revealed that diversity of phenotypes and dynamics among constituent stocks results in a variance-buffering effect, whereby the relative variance in aggregate of stocks (“the portfolio”) is less than the relative variance in constituent stocks. Here we focus on California’s recently collapsed fall-run Chinook salmon and ask whether portfolio-effect-induced-buffering is observed at any level across the complexity hierarchy from individual stocks, to stocks within river basins, to the Central Valley system as a whole.  

Results/Conclusions

We found that some variance buffering was observed, particularly when comparing the coefficient of variation in adult returns between river basins (Sacramento or San Joaquin) to their constituent stocks. However, a lack of variance buffering was apparent when comparing adult returns to the Central Valley as a whole to the returns to the Sacramento River Basin, and this is a consequence of the significant difference in returns to the two river basins (returns to the Sacramento River Basin far exceed those to the San Joaquin River Basin). The San Joaquin River stocks contribute little to the overall production in this system and, consequently, the addition of these stocks does little to buffer returns to the Central Valley as a whole. We also found that correlations in population dynamics between rivers increased significantly with distance between them, suggesting evidence of some biocomplexity in this collapsed stock complex. Taken together, these results suggest that the greatest potential for strengthening the portfolio effect in Central Valley fall-run Chinook would likely come through restoration of San Joaquin River stocks, which currently contribute little to the overall buffering in adult returns to the Central Valley.