From crisis to partial recovery: Demographic changes in female Alpine ibex following a pneumonia outbreak

Epizootics can profoundly impact population dynamics, affecting both survival and reproduction over relatively short periods. However, the longer-term demographic patterns that may follow an outbreak, as well as the underlying mechanisms, remain poorly documented. To investigate these dynamics, we compared the demographic parameters of females in an Alpine ibex population before (10 years), during (2 years) and after (two subsequent 7-year periods) a severe pneumonia outbreak. Using an integrated population model combining female capture-mark-recapture and census data, and accounting for individual heterogeneity, we estimated the age-specific breeding probability, survival and female population growth rate. We observed an increase in breeding probability after the epizootic, rising from pre-epizootic values of 0.63 to over 0.79 in high-quality females and from 0.03 to over 0.24 in low-quality females that had not bred the previous year. The age of primiparity decreased for most females, and while high-quality females that had previously reproduced were less likely to reproduce again compared to non-reproductive females before the epizootic, the opposite pattern was observed after the epizootic. These findings are consistent with the expected reduced impact of density-dependent processes following the 62% decline in female population size during the epizootic, which affected female survival across all age categories, except for prime-age non-reproductive individuals. This greater reproductive investment contrasted with a decline in survival, which was 6.7% lower for two-thirds of the adult female population (and 2% lower for the entire adult female population) during all post-epizootic periods. The persistent presence of the pathogen in the population, potentially indicating chronic or latent infection, combined with recent environmental changes (e.g. warmer conditions), may have prevented adult survival from returning to pre-epizootic levels. This sustained shortfall in adult survival compared to pre-epizootic levels resulted in a weak post-epizootic recovery of the population (+2.4% year^-1) driven only by high-quality females. These findings illustrate that long-term monitoring populations during adverse environmental conditions, such as disease outbreaks, can provide insights into how long-lived iteroparous females navigate survival-reproduction trade-offs in response to pathogen exposure and access to resources. This can help anticipate demographic responses to emerging infectious diseases and climate change.