Effective size of density‐dependent populations in fluctuating environments |
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| Authors: | Ane Marlene Myhre Steinar Engen Bernt‐Erik Sæther |
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| Affiliation: | 1. Department of Biology, Centre for Biodiversity Dynamics, Norwegian University of Science and Technology, Trondheim, Norway;2. Department of Mathematical Sciences, Centre for Biodiversity Dynamics, Norwegian University of Science and Technology, Trondheim, Norway |
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| Abstract: | Reliable estimates of effective population size  are of central importance in population genetics and evolutionary biology. For populations that fluctuate in size, harmonic mean population size is commonly used as a proxy for (multi‐) generational effective size. This assumes no effects of density dependence on the ratio between effective and actual population size, which limits its potential application. Here, we introduce density dependence on vital rates in a demographic model of variance effective size. We derive an expression for the ratio  in a density‐regulated population in a fluctuating environment. We show by simulations that yearly genetic drift is accurately predicted by our model, and not proportional to  as assumed by the harmonic mean model, where N is the total population size of mature individuals. We find a negative relationship between  and N. For a given N, the ratio depends on variance in reproductive success and the degree of resource limitation acting on the population growth rate. Finally, our model indicate that environmental stochasticity may affect  not only through fluctuations in N, but also for a given N at a given time. Our results show that estimates of effective population size must include effects of density dependence and environmental stochasticity. |
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| Keywords: | demographic stochasticity density dependence effective population size environmental stochasticity fluctuating populations genetic drift |
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