Lonely males at the front: A new open SCR model reveals the spatial sex-age structure of an expanding brown bear population

Abstract

Range expansion is a common feature from invasive to reintroduced or recovering populations. This process is mainly driven by population growth and dispersal and, consequently, different species’ intrinsic characteristics and dispersal mechanisms will result in contrasting population structures in space. How individuals of different sex and age classes are spatially distributed is key to understand and forecast range expansions, but remains largely unexplored. Here, we developed an age-structured open population spatial capture recapture (OPSCR) model to understand how spatial sex and age structures can shape population dynamics, as well as recovery processes. We use the expanding and endangered Pyrenean brown bear population as case study, taking advantage of a comprehensive non-invasive monitoring dataset collected between 2017 and 2021. The expansion front was dominated by adult (>4 years) and subadult males (2-4 years), while females and juveniles (<2 years) prevailed at the core. Overall, density declined with distance to the population core (area where last bears remained in the ‘90s and where translocated females established). Bear expansion was affected by the locations of the remnant range (i.e., the core areas) and constrained by female philopatry. Future projections suggest that increasing reproduction events at the periphery could speed up future population spread, and that this population will continue growing even under some level of demographic stochasticity. Our novel OPSCR model opens new pathways to understand complex spatiotemporal patterns of expanding populations and, in the case of large carnivores, could inform conservation action, for example, anticipating the adoption of damage prevention measures and raising awareness campaigns on how to coexist with bears in future expansion areas.