These findings provide, to our knowledge, the first direct empirical support of longstanding theoretical predictions that life-history diversity improves productivity over long time scales, and buffers population fluctuations. We also note the strong negative correlation between life-history diversity and RPS at short time scales. These seemingly contradictory results can be resolved if certain life-history types are favoured by natural selection each year, but the types that are favoured change among years (Seamons et al. 2007
). These selective effects might result from broad-scale physical processes such as changes in precipitation, ice, and lake levels that differentially affect large components of each population complex. Thus, higher population growth rates could occur with low levels of diversity at short time periods, but high diversity would be needed to maintain large growth rates over long time periods. Meanwhile, life-history diversity should always dampen variation in population growth rate owing to the spreading of juveniles across time and space, but this would be particularly important at longer time periods as rarer but more severe impacts on survival occurred.
The main component of life-history diversity in this analysis—variable duration of freshwater and ocean residency—is based on a complex set of interactions of organisms with their environment. At evolutionary time scales, bet-hedging strategies that support phenotypic plasticity have probably been favoured by fluctuating natural selection (Seamons et al. 2007
; Simons 2009
). For example, population-specific norms of reaction have evolved, such that similar growth rates result in different durations of freshwater residence, and similar sizes at sea water entry and growth rates at sea result in different ages at maturity among populations (Quinn et al. 2009
). At more immediate time scales, the environment in each lake (temperature, zooplankton production, competition) affects growth (Edmundson & Mazumder 2001
), and hence age and size at sea water entry. The outcome of these processes produces some patterns of residency that are not necessarily favourable every year, but are effective at buffering environmental variation at longer time scales.
The sockeye salmon populations we studied are large (104
adults) and live in almost pristine freshwater habitat, but they help inform recovery efforts for populations at risk of anthropogenic disturbance. Our study indicates that long-term monitoring of life-history variation is needed to reveal its positive population consequences. Further, our findings suggest that simplifying a population's life-history portfolio through artificial propagation (Araki et al. 2007
), habitat alteration (Watters et al. 2003
) and size- or age-selective harvest (Berkeley et al. 2004
) will have negative consequences on long-term population viability. Conversely, practices that help diversify population structure, such as maintaining large population sizes, and habitat protection and restoration, may help populations flourish. Such efforts may be critical for populations subject to disturbance regimes altered by habitat modification, harvest and long-term climate change (Finney et al. 2000
; Battin et al. 2007