Estimating demographic contributions to effective population size in an age-structured wild population experiencing environmental and demographic stochasticity

Amanda E. Trask (Corresponding Author), Eric M. Bignal, Davy I. McCracken, Stuart B. Piertney, Jane M. Reid

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Abstract

1.A population's effective size (Ne ) is a key parameter that shapes rates of inbreeding and loss of genetic diversity, thereby influencing evolutionary processes and population viability. However estimating Ne , and identifying key demographic mechanisms that underlie the Ne to census population size (N) ratio, remains challenging, especially for small populations with overlapping generations and substantial environmental and demographic stochasticity and hence dynamic age-structure. 2.A sophisticated demographic method of estimating Ne /N, which uses Fisher's reproductive value to account for dynamic age-structure, has been formulated. However this method requires detailed individual- and population-level data on sex- and age-specific reproduction and survival, and has rarely been implemented. 3.Here we use the reproductive value method and detailed demographic data to estimate Ne /N for a small and apparently isolated red-billed chough (Pyrrhocorax pyrrhocorax) population of high conservation concern. We additionally calculated two single-sample molecular genetic estimates of Ne to corroborate the demographic estimate and examine evidence for unobserved immigration and gene flow. 4.The demographic estimate of Ne /N was 0.21, reflecting a high total demographic variance (σ(2)dg ) of 0.71. Females and males made similar overall contributions to σ(2)dg . However, contributions varied among sex-age classes, with greater contributions from 3 year-old females than males, but greater contributions from ≥5 year-old males than females. 5.The demographic estimate of Ne was ~30, suggesting that rates of increase of inbreeding and loss of genetic variation per generation will be relatively high. Molecular genetic estimates of Ne computed from linkage disequilibrium and approximate Bayesian computation were approximately 50 and 30 respectively, providing no evidence of substantial unobserved immigration which could bias demographic estimates of Ne . 6. Our analyses identify key sex-age classes contributing to demographic variance and thus decreasing Ne /N in a small age-structured population inhabiting a variable environment. They thereby demonstrate how assessments of Ne can incorporate stochastic sex- and age-specific demography and elucidate key demographic processes affecting a population's evolutionary trajectory and viability. Furthermore, our analyses show that Ne for the focal chough population is critically small, implying that management to re-establish genetic connectivity may be required to ensure population viability. This article is protected by copyright. All rights reserved.
Original languageEnglish
Pages (from-to)1082-1093
Number of pages12
JournalJournal of Animal Ecology
Volume86
Issue number5
Early online date3 Jul 2017
DOIs
Publication statusPublished - Sep 2017

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effective population size
stochasticity
wild population
viability
population size
demographic statistics
inbreeding
age class
age structure
immigration
demographic method
demography
disequilibrium
gene flow
genetic variation
connectivity
census
trajectory
gender
molecular genetics

Keywords

  • conservation genetics
  • evolutionary potential
  • iteroparity
  • life-history variation
  • population connectivity
  • population management
  • reproductive skew

Cite this

Estimating demographic contributions to effective population size in an age-structured wild population experiencing environmental and demographic stochasticity. / Trask, Amanda E. (Corresponding Author); Bignal, Eric M.; McCracken, Davy I.; Piertney, Stuart B.; Reid, Jane M.

In: Journal of Animal Ecology, Vol. 86, No. 5, 09.2017, p. 1082-1093.

Research output: Contribution to journalArticle

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note = "We thank everyone who helped with fieldwork on Islay, in particular Sue Bignal and Pat Monaghan, as well as all land-owners and farmers who allowed access to nest sites. We thank Bernt-Erik Sӕther, Steinar Engen and Henrik Jensen for their generous help and discussions. AET was funded by the Natural Environment Research Council and Scottish Natural Heritage. JMR was supported by the European Research Council.",
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AU - Reid, Jane M.

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N2 - 1.A population's effective size (Ne ) is a key parameter that shapes rates of inbreeding and loss of genetic diversity, thereby influencing evolutionary processes and population viability. However estimating Ne , and identifying key demographic mechanisms that underlie the Ne to census population size (N) ratio, remains challenging, especially for small populations with overlapping generations and substantial environmental and demographic stochasticity and hence dynamic age-structure. 2.A sophisticated demographic method of estimating Ne /N, which uses Fisher's reproductive value to account for dynamic age-structure, has been formulated. However this method requires detailed individual- and population-level data on sex- and age-specific reproduction and survival, and has rarely been implemented. 3.Here we use the reproductive value method and detailed demographic data to estimate Ne /N for a small and apparently isolated red-billed chough (Pyrrhocorax pyrrhocorax) population of high conservation concern. We additionally calculated two single-sample molecular genetic estimates of Ne to corroborate the demographic estimate and examine evidence for unobserved immigration and gene flow. 4.The demographic estimate of Ne /N was 0.21, reflecting a high total demographic variance (σ(2)dg ) of 0.71. Females and males made similar overall contributions to σ(2)dg . However, contributions varied among sex-age classes, with greater contributions from 3 year-old females than males, but greater contributions from ≥5 year-old males than females. 5.The demographic estimate of Ne was ~30, suggesting that rates of increase of inbreeding and loss of genetic variation per generation will be relatively high. Molecular genetic estimates of Ne computed from linkage disequilibrium and approximate Bayesian computation were approximately 50 and 30 respectively, providing no evidence of substantial unobserved immigration which could bias demographic estimates of Ne . 6. Our analyses identify key sex-age classes contributing to demographic variance and thus decreasing Ne /N in a small age-structured population inhabiting a variable environment. They thereby demonstrate how assessments of Ne can incorporate stochastic sex- and age-specific demography and elucidate key demographic processes affecting a population's evolutionary trajectory and viability. Furthermore, our analyses show that Ne for the focal chough population is critically small, implying that management to re-establish genetic connectivity may be required to ensure population viability. This article is protected by copyright. All rights reserved.

AB - 1.A population's effective size (Ne ) is a key parameter that shapes rates of inbreeding and loss of genetic diversity, thereby influencing evolutionary processes and population viability. However estimating Ne , and identifying key demographic mechanisms that underlie the Ne to census population size (N) ratio, remains challenging, especially for small populations with overlapping generations and substantial environmental and demographic stochasticity and hence dynamic age-structure. 2.A sophisticated demographic method of estimating Ne /N, which uses Fisher's reproductive value to account for dynamic age-structure, has been formulated. However this method requires detailed individual- and population-level data on sex- and age-specific reproduction and survival, and has rarely been implemented. 3.Here we use the reproductive value method and detailed demographic data to estimate Ne /N for a small and apparently isolated red-billed chough (Pyrrhocorax pyrrhocorax) population of high conservation concern. We additionally calculated two single-sample molecular genetic estimates of Ne to corroborate the demographic estimate and examine evidence for unobserved immigration and gene flow. 4.The demographic estimate of Ne /N was 0.21, reflecting a high total demographic variance (σ(2)dg ) of 0.71. Females and males made similar overall contributions to σ(2)dg . However, contributions varied among sex-age classes, with greater contributions from 3 year-old females than males, but greater contributions from ≥5 year-old males than females. 5.The demographic estimate of Ne was ~30, suggesting that rates of increase of inbreeding and loss of genetic variation per generation will be relatively high. Molecular genetic estimates of Ne computed from linkage disequilibrium and approximate Bayesian computation were approximately 50 and 30 respectively, providing no evidence of substantial unobserved immigration which could bias demographic estimates of Ne . 6. Our analyses identify key sex-age classes contributing to demographic variance and thus decreasing Ne /N in a small age-structured population inhabiting a variable environment. They thereby demonstrate how assessments of Ne can incorporate stochastic sex- and age-specific demography and elucidate key demographic processes affecting a population's evolutionary trajectory and viability. Furthermore, our analyses show that Ne for the focal chough population is critically small, implying that management to re-establish genetic connectivity may be required to ensure population viability. This article is protected by copyright. All rights reserved.

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KW - evolutionary potential

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KW - population connectivity

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KW - reproductive skew

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