Recovering in isolation: dynamics of the Alpine brown bear population over two decades post-reintroduction

Large carnivores have been extirpated from much of their historical range, but targeted conservation actions have recently enabled their recovery in several regions. The reintroduction of the brown bear in the central Alps represents a notable example, yet the long-term persistence of this population remains uncertain. Its small size, geographic isolation, and growing conflicts with humans pose significant challenges, highlighting the need for comprehensive assessments of population status and trends to inform management and evaluate its responses to environmental and anthropogenic changes. Using individual genotypes collected non-invasively between 2003 and 2023 within a long-term monitoring program, we analysed population dynamics of the Alpine bear population through spatial capture–recapture, and survival modelling. By combining systematic and opportunistic sampling, we aimed at: obtaining spatially explicit estimates of population density and assessing its spatial drivers; assessing age- and sex-specific survival probabilities and site fidelity; and quantifying temporal variations in female range. In 2023, population size was estimated at 105.6 individuals (95% CI = 91.6 - 121.8). Density has increased at an average annual rate of 7.7 %, with a mean density of 1.61 bears/100 km2 in the whole study area, and 2.50 within the area permanently occupied by females. Bear density also varied in space, being higher in proximity to the original release site and on steeper slopes. Survival probability varied by sex and age, and was lower for males and younger individuals. Site fidelity was highest for females, with no permanent emigration from the study area detected throughout the study period. Despite the high site fidelity, the area occupied by females has expanded substantially over time, increasing by 137% over two decades. However, this expansion remains confined to the western side of the Adige Valley, which represents a major ecological barrier and hinders connectivity with the nearest Dinaric-Pindos bear population. Overall, our results indicate positive trends in both abundance and spatial expansion, but also emphasize persistent vulnerabilities linked to the relatively small size and isolation of this bear population. Female site fidelity emerged as a key determinant of population space use, as all density hotspots were located within areas consistently occupied by females. Observed spatial patterns seem to reflect the interplay between intrinsic biological traits and anthropogenic factors, including artificial barriers and human disturbance. Importantly, the integration of opportunistic genetic detections proved essential for reliable inference: excluding these data would have led to a 33% underestimation of population density in 2023 and a halved estimated growth rate. Sustained genetic monitoring, improved connectivity, and science-based management will thus be crucial to ensure the long-term persistence of this population in a human-dominated landscape and increasingly hostile social context.