Identifying Limits to Variability Reduction with the Portfolio Effect: Application to Central Valley Chinook Salmon

Biodiversity is diminishing, raising concerns regarding the lost ecological services.  In general, empirical studies link increased diversity with reduced aggregate community or ecosystem variability.  One proposed mechanism - the portfolio effect – suggests that the pattern may be statistical, resulting from lower variability with more independent subpopulations.  However, as currently applied in multi-population salmon stocks, portfolio effect calculations have limited utility because of spatial averaging, low temporal resolution, and no consideration of the maximum potential for independence (diversity) to reduce variability.

Here we address the question, “how much could diversity stabilize aggregate variability through the portfolio effect?”  We develop a new metric quantifying the maximum aggregate variability reduction possible through increased diversity.  We apply this metric to Sacramento River Fall Chinook (SRFC) salmon, a population complex where, arguably, increased subpopulation diversity could have prevented the fishery’s closure during the late 2000s.  We then retrospectively identify the spatial and temporal sources of changes in the SRFC portfolio effect.  We found that one population in the mid-1980s was largely responsible for the overall rise in covariability, and two other periods of increased covariance coincided with observed shifts in ocean dynamics and marine species survivals.