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International audience; Rewilding has become a new paradigm to promote the natural recovery of many flagship species, including large carnivores. A better understanding of the factors that drive rewilding processes would allow predicting these in order to support them where desirable. Yet large-scale assessments of the factors that favor or limit mammal populations of conservation concern remain rare in Europe. This study sought to monitor changes in Eurasian otter (Lutra lutra) occupation over the years to identify the factors limiting its recolonization. To this end, we monitored otter signs and spraints over 1238 sampling sites in southwest France from 2003 to 2005 and then over 615 sites in 2011-2012. We modeled this dataset in a multi-season occupancy framework using biotic and abiotic covariates identified as potentially affecting otter detection, distribution and/or colonization rates. As otters are a cryptic and mobile mammal, we integrated detection probability and spatial autocorrelation (contagion) into the modeling design. Otter occurrence was detected at 31 % of the sites surveyed in 2003-2005 and 37 % of the sites surveyed in 2011-2012. Occupancy and colonization probabilities were strongly negatively correlated to the proportion of cultivated areas. Contagion covariates interacted strongly with other variables. These results suggest that, despite its recent colonization, the otter population in southwest France is still constrained in its expansion, notably by intensive agriculture. Natura 2000 schemes should incorporate rewilding approaches to further potentiate the colonization of this flagship species. Actions should focus on the conservation of natural riparian habitats and promote organic agriculture.

The increases in mute swan (Cygnus olor Gmelin) population size have caused concern among stakeholders, who sometimes consider it as a pest species. Here, we aim to review existing studies on the ecological effects that mute swans have on wetlands. Claim that mute swans threaten other waterbirds were partly supported: mute swans sometimes behave territorially towards conspecifics and other waterbird species, but this does not systematically occur. A second common claim, that mute swans damage aquatic plant beds, was upheld in that the species did indeed affect aquatic plant communities in several studies. However, grazing by mute swans does not systematically have negative effects on aquatic plants. Habitat patch size, distance between habitat patches, resource availability and water velocity affect habitat selection process by mute swans, with varying effects depending on season and mute swan breeding status. Scientific knowledge does not support the idea that mute swan population increase can be considered as a biological invasion in Europe. Conversely, there is a genuine risk of biological invasion in North America. In light of the literature review, we discuss the relevance of mute swan population management in Europe and in North America, and propose future research avenues.