Range expansion and reconnection of historical populations in the Eurasian otter (Lutra lutra) in France: Insights from heterogeneous data and integrated species distribution modelling

Abstract

Many small carnivore populations are declining and urgently require evidence-based conservation. Despite their ecological importance, the spatial dynamics of small carnivores remain understudied compared to large carnivores. The Eurasian otter (Lutra lutra) represents a rare example of a recovering small carnivore population in Europe, following nearly a century of decline. However, recovery remains slow in some regions, with populations still vulnerable across much of the species' range. In France, spatial and temporal heterogeneity in data availability has hindered the development of accurate, high-resolution distribution maps, limiting insights into the drivers of this recovery and the factors constraining populations. Here, we mapped the evolution of otter distribution in France from 2009 to 2023 using an integrated species distribution model. This approach combined opportunistic occurrence data and standardized detection/non-detection data from diverse monitoring programs while accounting for non-detection and environmental variability. Our results revealed an expanding distribution, including the reconnection of historical population cores and the emergence of active colonization fronts spanning multiple regions. While occurrence was slightly higher in salmonid-rich rivers with forested banks at broader scales, habitat preferences varied significantly at regional scales, indicating that otters are now occupying more diverse habitats. This study demonstrates the potential of integrating heterogeneous data sources to study carnivore distributions. Our findings provide valuable insights into the status and trends of Eurasian otters in France and offer a framework for guiding effective national-scale conservation efforts.