Avalanches create unique habitats for birds in the European Alps

Mountain regions harbour dynamic ecosystems that have been historically shaped by interactions between natural abiotic processes and human activities, but nowadays they are threatened by climate change. Avalanches are one of the main sources of natural disturbance in mountain areas, creating habitat mosaics with a high vegetation heterogeneity that can be exploited by a suite of animal species. Since snow precipitation regimes in mountain regions will be altered by climate change, it is fundamental to study the interactions between avalanches and biodiversity. We carried out surveys on 240 points across horizontal and vertical transects in the Western Italian Alps to assess the diferences in habitat, bird assemblage composition and ecological traits between avalanche tracks and control points. Diferences in both habitat and bird assemblages were more pronounced at lower elevations and became less obvious towards alpine grasslands. Avalanche tracks were characterised by a higher structural vegetation diversity with a greater cover of rocks, shrubs and a higher number of small trees. Bird assemblages difered between avalanche tracks and adjacent control points, with the former being more diverse and characterised by a higher proportion of species typical of open habitats and migrants. Importantly, the avalanche track bird assemblage difered from both forest and treeline ecotone assemblages, showing that these heterogeneous areas are unique, harbouring a mixture of bird species from diferent habitats. Avalanche frequency may be afected by climate change in the future, with consequences for mountain biodiversity at a broader scale, and hence should be a priority for alpine ecological research.