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Population diversity and the portfolio effect in an exploited species



One of the most pervasive themes in ecology is that biological diversity stabilizes ecosystem processes and the services they provide to society1,2,3,4, a concept that has become a common argument for biodiversity conservation5. Species-rich communities are thought to produce more temporally stable ecosystem services because of the complementary or independent dynamics among species that perform similar ecosystem functions6. Such variance dampening within communities is referred to as a portfolio effect7 and is analogous to the effects of asset diversity on the stability of financial portfolios8. In ecology, these arguments have focused on the effects of species diversity on ecosystem stability but have not considered the importance of biologically relevant diversity within individual species9. Current rates of population extirpation are probably at least three orders of magnitude higher than species extinction rates10, so there is a pressing need to clarify how population and life history diversity affect the performance of individual species in providing important ecosystem services. Here we use five decades of data from Oncorhynchus nerka (sockeye salmon) in Bristol Bay, Alaska, to provide the first quantification of portfolio effects that derive from population and life history diversity in an important and heavily exploited species. Variability in annual Bristol Bay salmon returns is 2.2 times lower than it would be if the system consisted of a single homogenous population rather than the several hundred discrete populations it currently consists of. Furthermore, if it were a single homogeneous population, such increased variability would lead to ten times more frequent fisheries closures. Portfolio effects are also evident in watershed food webs, where they stabilize and extend predator access to salmon resources. Our results demonstrate the critical importance of maintaining population diversity for stabilizing ecosystem services and securing the economies and livelihoods that depend on them. The reliability of ecosystem services will erode faster than indicated by species loss alone.

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Figure 1: Bristol Bay sockeye habitat and associated change in variability of returns at different spatial scales and levels of life history aggregation.
Figure 2: Effect of interannual variability on the probability of fishery closures or capacity-swamping returns.
Figure 3: Annual run timing to fishing districts and streams.


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We thank the Gordon and Betty Moore Foundation, the US National Science Foundation, the University of Washington, the Alaska salmon processing industry and the H. Mason Keeler Professorship for support for this work. N. Baron, L. Neeley and S. Sethi provided feedback and comments on the manuscript, and P. Lisi and G. Holtgrieve helped prepare the figures.

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D.E.S. designed and coordinated the project; R.H., B.C. and L.A.R contributed to the analyses; M.S.W. helped design the project; and all authors contributed to the writing.

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Correspondence to Daniel E. Schindler.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Information comprising: Value of sockeye salmon resources in Bristol Bay and Variance scaling in data, Supplementary Figures 1-3 with legends and References. (PDF 273 kb)

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Schindler, D., Hilborn, R., Chasco, B. et al. Population diversity and the portfolio effect in an exploited species. Nature 465, 609–612 (2010).

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