Synchronization of a portfolio of salmon populations

Background/Question/Methods

Losses of biodiversity have inspired massive efforts to understand relationships between diversity and ecosystem function. While a large body of research has documented losses of species diversity and subsequent ecological impacts, there has been little research devoted to losses of population diversity. We used time series of abundance from 21 populations of Chinook salmon (Oncorhynchus tshawytscha) within the Snake River evolutionarily significant unit to quantify changes in population diversity.

Results/Conclusions

Within this portfolio of populations, population productivities have become synchronized over the last 40 years, with a threefold increase in correlation coefficients over that period on average. During this period, these populations have suffered a variety of anthropogenic impacts that may have increased synchronization. In contrast, relatively pristine populations of salmon in Alaska have not demonstrated concurrent synchronization. We propose that this synchronization represents a loss of diversity at the population level and has dramatic implications for conservation. For example, these data were used to parameterize simulations that indicate that this synchronization dramatically increases extinction risk of the entire portfolio of populations. In addition, we applied economic portfolio theory to this dataset to demonstrate that this synchronization has decreased the ratio of expected return to variance, an index of portfolio performance. While the loss of species is receiving the bulk of attention from scientists, preserving diverse populations is likely an important component of maintaining resilience given impending global change.