Abstract
Biological invasions are among the most pervasive threats to biodiversity, ecosystem functioning, and human well-being. Despite international policy efforts, the number of introductions continues to rise worldwide. Mountains, once considered resistant to biological invasions due to harsh climates and isolation, are becoming increasingly vulnerable. Human activities-tourism, infrastructure development, and land-use change-combined with climate warming, are creating new pathways and suitable conditions for non-native plants to spread upslope. Global evidence shows a rapid increase in alien species richness in mountain ecosystems, with some taxa shifting elevation by hundreds of meters. The problem of biological invasions becomes critical when considering that mountains harbor nearly a quarter of the planet's total biodiversity. This issue is even more concerning in biodiversity hotspots such as the Mediterranean Basin, where mountains present an exceptionally high rate of endemism and have served as glacial refugia. The Pyrenees exemplify this dynamic: historically shaped by millennia of human activity, they now face growing pressures from tourism and climate change. Recent cataloging efforts reveal 771 alien taxa, surpassing figures for larger ranges like the Alps. These findings challenge long-held assumptions about mountain resilience and underscore the urgent need for coordinated monitoring, early detection, and management strategies-including citizen science initiatives-to mitigate ecological impacts and protect mountain biodiversity under accelerating global change.