Résumé
Common and rare species differ in their level of dispersal ability and habitat specialization, and thus should respond to distinct assembly rules. Rare species are much more spatially restricted and usually show higher habitat specialization, so they can be much more vulnerable to landscape changes than common species. We simulated three species-loss scenarios to assess how different groups of stream fish species respond to environmental and spatial variables and how potential extinctions affected regional functional richness and the average functional distinctiveness. Rare and common species contribute similarly to the global functional richness, but the loss of common species led to a marked decrease in local redundancy. Whilst intermediate and rare species are regulated by dispersal, dominant species respond to niche processes and are more affected by land-use changes. Therefore, common and rare species are under distinct assembly rules and may exhibit distinct responses in the face of environmental anthropization. Addressing rare species is often overlooked, despite constant claims regarding their importance and their frequently concerning conservation status. Accounting for their rarity can offer a more refined look at the responses of species to human modifications, providing valuable information on how to tackle biodiversity loss.