New publication: IPM and tortoise populations!

Segura A., Rotger, A. and Rodriguez-Caro, R., 2025. Hidden Threats to Persistence: Changes in Population Structure Can Affect Well-Preserved Spur-Thighed Tortoise Populations. Herpetologica. https://doi.org/10.1655/Herpetologica-D-23-00066

In a shell: This study on Spur-thighed Tortoises highlights a shift towards a female-biased population, with high juvenile mortality from raven predation, and emphasizes the importance of female survival and sex-ratio especially in the context of climate change.

Abstract:  Population structure and survival are key components of wildlife management. Long-term monitoring of long-lived species, particularly those with indeterminate growth, is crucial when studying demographic processes. Here, we examined a population of Spur-thighed Tortoises, Testudo graeca, over a 7-yr period (17% of its life span), including changes in population structure, causes of mortality, and growth patterns. We found a change in population structure, as evidenced by lower young adult density (both males and females) and a more female-biased population compared to the start of the study. Juvenile mortality was high, and the main cause was predation by common ravens. 

Photo from iNaturalist

For adults, mortality was relatively low and was mostly observed in winter or due to anthropogenic reasons (forestry or road mortality). We also modeled adult size-dependent survival and juvenile threshold survival (minimum number of juveniles needed to reach the adult stage to maintain population viability) using a Bayesian framework and matrix projection models, respectively. Adult survival was high (0.97), but with variation between the sexes. Female survival was not size dependent, but male survival decreased when size exceeded 150 mm carapace length. In this population, longer female life spans and climate change effects seemed to be the most likely reasons for our female-biased population. This study particularly pinpoints the importance of high survival in older females, which contributes to species credit, and stresses the negative potential of low juvenile and male densities in the population. Indeed, the annual juvenile threshold survival range was estimated between 0.32 and 0.49, not accounting for the predation exerted by common ravens in subadults. Therefore, if predation reduces juvenile survival rates below this threshold, population viability can be affected in the future. The study contributes to this species’ conservation by anticipating time-lagged demographic responses based on current climate trends (less annual rainfall and more days over 40°C) and predation.

New Publication on population dynamics of Audouin's Gull !

Genovart, M., Oro, D. and Tenan, S. 2018. Immature survival, fertility, and density dependence drive global population dynamics in a long‐lived species. Ecology, https://doi.org/10.1002/ecy.2515

Abstract: Disentangling the influence of demographic parameters and the role of density dependence on species’ population dynamics is a challenge, especially when fractions of the population are unobservable. Additionally, due to the difficulty of gathering data at large spatial scales, most studies ignore the global dynamic of a species, which would integrate local heterogeneity dynamics and remove the noise of dispersal. We developed an integrated population model (IPM) at a global scale to disentangle the main demographic drivers of population dynamics in a long‐lived species.

We used 28 yr of Audouin's Gull demographic data encompassing 69 local patches (comprising 90% of the world population). Importantly, we took into account the unobservable fraction of non‐breeders and also assessed the strength of density dependence for this fraction of the population. As predicted by life histories of long‐lived organisms, temporal random variation in survival was highest for immature individuals (1.326, 95% credible interval [CRI] 1.290–1.940) and lowest for adults (0.499, 95% CRI 0.487–0.720). Large temporal fluctuations in the probability of taking a reproductive sabbatical would partly explain the consistency in adult survival, with individuals most likely refraining from breeding when environmental conditions were harsh. Immature survival and fertility were the main drivers of population dynamics during the study period (r² = 0.83, 0.77–0.87 and 0.73, 0.63–0.79, respectively). We found strong evidence of density dependence, not only due to the number of breeders (r² = −0.34, −0.43 to −0.24) but also due to individuals on sabbatical (r² = −0.18, −0.33 to −0.01). From a conservation point of view, the species shows a 5% annual global decrease during the last 10 years, and we propose an update of its conservation status. Even though population dynamics of long‐lived organisms are very sensitive to changes in adult survival, we show here that, in the absence of strong environmental perturbations affecting this vital rate, fluctuations in population density are mainly driven by variations in survival of immature individuals and fertility. Integrated models based on long‐term monitoring at a global scale may enhance our ecological and evolutionary understanding of how demographic drivers influence population dynamics.