Tracking restoration of population diversity via the portfolio effect

Abstract

Abstract Declines in diversity among populations managed together have diminished aggregate stability through a decreased portfolio effect. Although the portfolio effect has been quantified in a variety of ways, management recommendations for the recovery of lost diversity rarely specify the stability benefits possible through such improvements. We introduce a metric, the Diversity Deficit (DD), that relates past losses and potential gains in aggregate stability to the changes in population diversity (i.e. covariability among population time series). We illustrate the use of this metric in retrospective analyses of the aggregate Sacramento River Fall‐run Chinook salmon stock (Oncorhynchus tshawytscha), and project potential future improvements in stability through population diversity. In the retrospective analysis, we removed individual time series from the stability calculations to determine their effects on times and locations of past losses in diversity and stability. We found an early threefold loss in stock stability resulting from the presence of a single tributary, the Sacramento River mainstem. Other shifts in stability resulted from an increase in variability of a single population, and from the synchronizing effects of low ocean survival that led to the 2008–2009 fishery closure. Only one, smaller increase in the DD (i.e. in lost stability) was due to portfolio‐wide increases in covariabilities among tributary abundances. In a prospective analysis using the DD applied to California salmon, we found that increasing biodiversity to the point of population independence and to its early high value would have reduced the probability of triggering a fishery closure. Synthesis and applications. Analyses with the Diversity Deficit (DD) metric illustrate a way to identify the times and locations of losses in population diversity, and to quantify how much restoration of population diversity could increase stability, and thus benefit resource services. In our research, the benefit was a reduction in the probability of falling below a critical management threshold leading to fishery closure, but other tangible benefits (e.g. reduction in probability of extinction) would also be possible.