Evaluating impacts of plant invasions in mountains

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Abstract
Mountain regions worldwide harbour key and unique biodiversity, yet they are increasingly threatened by multiple human pressures projected to intensify throughout the 21 st century. While invasive species are widely recognized as one of the main drivers of global biodiversity loss, until recently, their pressure in alpine and montane systems remained relatively low. But recent evidence suggests that this situation is changing. Climate warming, land-use change, pollution, and increasing rates of alien species introductions are enabling invasive plants to expand into previously less disturbed mountain habitats. Understanding the mechanisms behind this process is essential for predicting future invasions and preventing their impacts. In this study, we assess the magnitude and mechanisms of the negative environmental impacts caused by alien plant species in mountain ecosystems worldwide. Specifically, we obtained a list of 564 species recorded as naturalized across 18 mountain regions, from standardized road surveys along elevational gradients conducted by the Mountain Invasion Research Network (MIREN). We evaluated these species using the Environmental Impact Classification for Alien Taxa (EICAT), an objective and transparent method recognized by the IUCN. EICAT classifies alien species according to the severity of their impacts on native species populations and allows for the classification of the mechanisms driving these impacts. Documented impacts occurring in both mountain and non-mountain ecosystems were included in the assessment. Our preliminary results suggest that empirical evidence on the environmental impacts of most alien plant species remains limited or entirely lacking, particularly in mountain environments. For the species with available information, the most frequently reported impact mechanisms include competition with native species as well as indirect effects mediated through interactions with other organisms (such as disruption of pollinator networks) and allelopathic effects. Overall, this study provides a global overview of the potential impacts of alien plants in mountain ecosystems and establishes a robust scientific foundation for prioritization and the development of targeted management recommendations.