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Stochastic models reveal conditions for cyclic dominance in sockeye salmon populations
Author(s) -
Wilson White J.,
Botsford Louis W.,
Hastings Alan,
Holland Matthew D.
Publication year - 2014
Publication title -
ecological monographs
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.254
H-Index - 156
eISSN - 1557-7015
pISSN - 0012-9615
DOI - 10.1890/12-1796.1
Subject(s) - dominance (genetics) , semelparity and iteroparity , population , biology , ecology , population cycle , cohort , demography , predation , reproduction , medicine , sociology , gene , biochemistry
The cause of population cycles is a common question in ecology, and one especially puzzling case is the cycles over the past century in populations of sockeye salmon, Oncorhyncus nerka . Some populations of this semelparous species in British Columbia, Canada, exhibit a phenomenon termed cyclic dominance : every four years there is a dominant cohort of primarily four‐year‐old spawners, orders of magnitude more abundant than other cohorts, producing a distinctive four‐year cycle. In some populations, these cycles stop, start, or shift phase. We used a stochastic age‐structured model to investigate the conditions allowing cycles and the events that could cause them to move in and out of cyclic dominance. We first defined cyclic dominance as high values of cyclicity , the fraction of time the population is cyclic, and dominance , the difference in abundance between the dominant cohort and the other three cohorts. We then used simulations to determine the values of (1) relative population persistence (i.e., proximity to collapse), (2) variability in survival, (3) variability in growth, and (4) spread in spawning age distribution that led to the observed levels of cyclic dominance in 18 stocks from the Fraser River, British Columbia, and nine stocks from Bristol Bay, Alaska, USA. Our simulations produced a range of dynamics similar to those observed in real stocks, from noncyclic to intermittent cycles to extremely consistent cyclic‐dominant cycles. We found that cyclicity and dominance were most likely to be high under conditions of low population persistence, high variability in survival, and narrow age structure. Populations could be driven into cyclic‐dominant behavior by unusually large perturbations in survival, but not in individual growth rate. Because this triggering mechanism is stochastic, populations may exist under conditions enabling cycles for a substantial time without displaying cyclic dominance, which is consistent with observations from real stocks. The association between cyclic dominance and low population persistence is of some concern for management. Also, the dependence of cyclic variability on intrinsic population dynamics (albeit stochastically driven) should be taken into account when assessing whether statistically independent fluctuations of stocks produce a biodiversity portfolio effect.