Limited but positive influence of Plant Invasion on the diversity of Mediterranean Mountain Ecosystems of Southern Iberian Peninsula

Mediterranean mountain ecosystems host a unique and highly endemic flora, yet they are increasingly threatened by global change, including the spread of invasive plant species. While elevational gradients are known to structure biodiversity and invasion patterns, the combined role of anthropogenic disturbance and environmental heterogeneity across spatial scales remains insufficiently understood. In this study, we assessed patterns of plant invasion and its association with total species richness, J’Pielou evenness and plant diversity (Shannon-Wiener and Simpson indices) across six mountain roads spanning broad environmental gradients in the southern Iberian Peninsula. We surveyed plant communities both along roadsides and in adjacent natural vegetation and evaluated the relative importance of broad scale environmental gradients (captured by elevation), spatial variation among roads, soil microhabitat conditions and anthropogenic influence, in shaping plant invasive species occurrence, and plant species richness, evenness and diversity indices. Our results show limited plant invasive species occurrence, with invasive species largely confined to roadsides. Plant invasion occurrence was positively associated with Shannon diversity, whereas no significant effects were detected for dominance or evenness, a pattern consistent with an early stage of invasion. Elevation emerged as a key driver of both invasion occurrence and species richness, while soil microhabitat conditions, particularly soil moisture and temperature, played a central role in shaping evenness and diversity. Road proximity significantly increased both invasion probability and species richness. These findings highlight the need for complementary assessments of diversity and prioritize roads for monitoring and early management to safeguard plant biodiversity in Mediterranean mountain landscapes.