The current extinction and climate change crises pressure us to predict population dynamics with ever-greater accuracy. Although predictions rest on the well-advanced theory of age-structured populations, two key issues remain poorly explored. Specifically, how the age-dependency in demographic rates and the year-to-year interactions between survival and fecundity affect stochastic population growth rates. We use inference, simulations and mathematical derivations to explore how environmental perturbations determine population growth rates for populations with different age-specific demographic rates and when ages are reduced to stages. We find that stage- vs. age-based models can produce markedly divergent stochastic population growth rates. The differences are most pronounced when there are survival-fecundity-trade-offs, which reduce the variance in the population growth rate. Finally, the expected value and variance of the stochastic growth rates of populations with different age-specific demographic rates can diverge to the extent that, while some populations may thrive, others will inevitably go extinct.
Bibliographical noteFunding Information:
We thank Jeanne Altmann for access to the baboon data and Josephine Pemberton, Tim Clutton-Brock and Loeske Kruuk for access to the red deer data, and D.H. Nussey for comments. We acknowledge funding from the Max Planck Society, Marie Curie Fellowship (PIEF-GA-2008-220322), ERC grant number 249872, the German Research Foundation (Be 916/3 to 9), the Swiss National Science Foundation (SNF PP00P3-133641), the National Science Foundation (BCS-1232349, BCS-1219368, BCS-0621020, IOS 0919200), National Institute of Aging (P30 AG-012836, P01 AG031719 and R01 AG034513), Ramon y Cajal Research Grant RYC-2016-21114, Wenner-Gren Foundation, Leakey Foundation, National Geographic Society, Zoological Society of San Diego, University of Pennsylvania and Argentinean National Council of Research. We are very grateful to the three reviewers for their constructive comments on this manuscript.
- Age-structured population models
- Bayesian inference