PhD Thesis Title:
Why populations rise and fall: global demography of amphibians and reptiles
Supervision:
Co-supervisors: Hugo Cayuela (University of Rennes, FR), Aurélien Besnard (CEFE-EPHE, FR)
Additional mentors: Michael Schaub (Swiss Ornithological Institute, SW), Marlène Gamelon (University of Lyon 1), Daniel Noble (Australian National University, AU)
1. General background
Over the past five decades, population studies on birds and large mammals have generated fundamental insights into vertebrate demography and the drivers of population dynamics (Conde et al. 2019). This body of work has highlighted the central role of spatio-temporal environmental variation and population density in shaping vital rates—such as survival and reproduction—and regulating population dynamics, as well as the importance of age structure for population persistence (Hanski et al. 1990, Gaillard & Yoccoz 2003, Hilde et al. 2020).
However, the strong historical focus on birds and mammals has introduced a major taxonomic bias in population ecology, leading to three key limitations. First, demographic information remains extremely fragmented for entire and highly diverse vertebrate groups, with less than 2% of species studied in taxa such as amphibians and reptiles (Conde et al. 2019). Second, our current understanding of vertebrate demography encompasses only a narrow portion of functional diversity (Conde et al. 2019), largely restricted to endothermic species with relatively simple life cycles. Third, this demographic data gap disproportionately affects taxonomic groups experiencing severe biodiversity crises—such as amphibians and chelonians—driven by global environmental change (Rhodin et al. 2018, Luedtke et al. 2023).
In this context, this PhD project will investigate key demographic processes underlying population dynamics and viability in two major groups of ectothermic vertebrates: amphibians and reptiles. The project will rely on a global demographic database and state-of-the-art modelling frameworks to address fundamental questions on temporal variability, population regulation, and size-structured demography.
2. Aim 1 — Temporal variability of vital rates and the demographic buffering hypothesis
The first component of the thesis will quantify patterns of temporal variability in adult survival, adult recruitment, and population growth rate at both intra- and interspecific levels across more than 200 amphibian and reptile species. This aim will explicitly test the demographic buffering hypothesis (Hilde et al. 2020), which predicts that the temporal variance of demographic rates decreases with their elasticity to population growth rate (λ). In addition, the project will investigate how species traits (e.g. reproductive mode, physical and chemical defences) and environmental factors (e.g. interannual climatic variability) influence temporal variation in demographic parameters. The analyses will rely on capture–recapture data from the ECTOLIFE database, which includes individual encounter histories for more than 400,000 individuals across over 700 populations worldwide. Methodologically, this component will combine Bayesian capture–recapture models (Schaub & Kéry 2021), matrix population models (Caswell 2001), and phylogenetic meta-analyses (Hadfield 2010) to quantify and compare patterns of demographic variability across taxa.
3. Aim 2 — Density dependence of vital rates and population regulation
The second component of the thesis will examine the role of density dependence (Fowler 1981, Hanski et al. 1990) in regulating natural populations of amphibians and reptiles. While density-dependent processes are known to be key drivers of population regulation in birds and mammals (e.g., Paradis et al. 2002, Festa‐Bianchet et al. 2003, Bonenfant et al. 2009), their prevalence, strength, and direction remain poorly documented in ectothermic vertebrates. This aim will quantify the strength and direction (positive or negative) of density dependence in adult survival, adult recruitment, and population growth rate at both intra- and interspecific levels across more than 200 amphibian and reptile species. The project will further assess whether and how density dependence varies along the slow–fast life-history continuum, and how it is modulated by species traits (e.g. reproductive mode, parental care) and habitat characteristics (e.g. terrestrial versus aquatic systems). Analyses will again rely on the ECTOLIFE database and will be conducted using density-dependent Bayesian capture–recapture models combined with phylogenetic meta-analytical approaches.
4. Aim 3 — Size-dependent survival, reproduction, and consequences for population viability
The third component of the thesis will investigate size-dependent patterns of survival and fecundity and their consequences for population viability in ectothermic vertebrates. While populations of birds and mammals are predominantly structured by age (Gaillard et al. 1989, Gaillard & Yoccoz 2003), body size may represent an even more structuring axis in ectotherms due to indeterminate growth—that is, continued growth well beyond the onset of reproduction (Kirkpatrick 1984, Sebens 1987). This aim will characterise the diversity of size-specific vital rate patterns in amphibians and reptiles and test the hypothesis that the elasticity of population growth rate (λ) increases with body size. Growth trajectories and size-specific survival will be quantified for several hundred amphibian and reptile populations using the ECTOLIFE database, while size-specific fecundity data will be extracted from the published literature for hundreds of species. This component will combine hierarchical Bayesian capture–recapture models with matrix population models to assess how size-structured demography influences population viability across ectothermic taxa.
5. Candidate profile
The ideal candidate will demonstrate:
• Strong quantitative skills and excellent command of R for data analysis and modelling.
• A clear interest in evolutionary ecology, demography, and large-scale comparative approaches.
• Willingness to engage with advanced statistical methods, including Bayesian models, phylogenetic comparative analyses and matrix population models.
• Ability to manage and analyse large and complex datasets.
• Autonomy, scientific rigour, organisation and critical thinking.
• Excellent written and oral communication skills in English is required.
• Ability to work within international research networks and collaborate with multiple institutions.
• High motivation, scientific curiosity and the ambition to develop an international profile in quantitative evolutionary ecology.
5. Eligibility and application
Administrative eligibility:
• Applicants must hold a Master’s degree (or equivalent) in ecology, evolutionary biology, biostatistics, or a related field at the time the PhD starts.
Application details:
• Application deadline: 30/05/2026
• Interview: 06/2026
• Expected start date: 01/09/2026
To apply, please submit:
• A curriculum vitae (CV),
• A cover letter describing your motivation, relevant experience, and research interests,
• Contact details for three academic referees.
Applications and inquiries should be sent to: hugo.cayuela@univ-rennes.fr
5. Literature cited
Bonenfant, C., Gaillard, J. M., Coulson, T., Festa‐Bianchet, M., Loison, A., Garel, M., … & Duncan, P. (2009). Empirical evidence of density‐dependence in populations of large herbivores. Advances in ecological research, 41, 313-357.
Caswell, H. (2001). Population matrix models: Construction, analysis and interpretation. Sinauer, Sunderland, Massachusetts.
Conde, D. A., Staerk, J., Colchero, F., da Silva, R., Schöley, J., Baden, H. M., … & Vaupel, J. W. (2019). Data gaps and opportunities for comparative and conservation biology. Proceedings of the National Academy of Sciences, 116(19), 9658-9664.
Festa‐Bianchet, M., Gaillard, J. M., & Côté, S. D. (2003). Variable age structure and apparent density dependence in survival of adult ungulates. Journal of animal ecology, 72(4), 640-649.
Fowler, C. W. (1981). Density dependence as related to life history strategy. Ecology, 62(3), 602-610.
Gaillard, J. M., & Yoccoz, N. G. (2003). Temporal variation in survival of mammals: a case of environmental canalization?. Ecology, 84(12), 3294-3306.
Hadfield, J. D. (2010). MCMC methods for multi-response generalized linear mixed models: the MCMCglmm R package. Journal of Statistical Software, 33, 1-22.
Hanski, I. A. (1990). Density dependence, regulation and variability in animal populations. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 330(1257), 141-150.
Hilde, C. H., Gamelon, M., Sæther, B. E., Gaillard, J. M., Yoccoz, N. G., & Pélabon, C. (2020). The demographic buffering hypothesis: evidence and challenges. Trends in Ecology & Evolution, 35(6), 523-538.
Kirkpatrick, M. (1984). Demographic models based on size, not age, for organisms with indeterminate growth. Ecology, 65(6), 1874-1884.
Luedtke, J. A., Chanson, J., Neam, K., Hobin, L., Maciel, A. O., Catenazzi, A., … & Stuart, S. N. (2023). Ongoing declines for the world’s amphibians in the face of emerging threats. Nature, 622(7982), 308-314.
Paradis, E., Baillie, S. R., Sutherland, W. J., & Gregory, R. D. (2002). Exploring density‐dependent relationships in demographic parameters in populations of birds at a large spatial scale. Oikos, 97(2), 293-307.
Rhodin, A. G., Stanford, C. B., Van Dijk, P. P., Eisemberg, C., Luiselli, L., Mittermeier, R. A., … & Vogt, R. C. (2018). Global conservation status of turtles and tortoises (order Testudines). Chelonian Conservation and Biology: Celebrating 25 Years as the World’s Turtle and Tortoise Journal, 17(2), 135-161.
Gaillard, J. M., Pontier, D., Allainé, D., Lebreton, J. D., Trouvilliez, J., & Clobert, J. (1989). An analysis of demographic tactics in birds and mammals. Oikos, 59-76.
Schaub, M., & Kéry, M. (2021). Integrated population models: Theory and ecological applications with R and JAGS. Academic Press.
Sebens, K. P. (1987). The ecology of indeterminate growth in animals. Annual review of ecology and systematics, 371-407.
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