Functional distinctiveness and rarity highlight climate vulnerability of mountain birds.

The loss of both functionally distinct and geographically restricted (i.e., rare) species can limit the capacity of ecological communities to respond to ongoing environmental changes. Mountains, which harbor high biodiversity and unique species, are particularly vulnerable to rapidly shifting climate conditions and remain understudied compared to lowland areas, creating significant global conservation challenges. In this study, we assessed the inter-relatedness of species' functional distinctiveness, geographic restrictedness, and rarity (i.e., the combination of geographic restrictedness and functional distinctiveness) in the context of thermal niche space for 800 mountain birds breeding in the Holarctic. We demonstrated that mountain bird species in colder thermal niches exhibit distinct trait-based responses that are spatially non-stationary and play unique functional roles in their ecosystems, highlighting the combined roles of environmental filtering and functional turnover in shaping mountain bird communities. When linking geographical restrictedness to functional distinctiveness, we found that most rare species occur in mountain areas experiencing high rates of warming. Together, these results highlight the vulnerability of cold-adapted species or communities of the Holarctic mountains to climate change. Due to the irreplaceable role of functionally distinct species in ecological networks, we anticipate that cold regions within the Holarctic mountains may be particularly susceptible to cascading, climate-driven species loss and community disruptions. Understanding the linkages between species-specific vulnerability to climate change and ecosystem functioning is key to preserving the unique ecological and evolutionary characteristics contained within mountains.