Drivers of individual plant species contributions to β-diversity are scale-dependent

Species introductions and native extirpations are driving biotic homogenisation in plant communities by reducing {beta}-diversity. Individual species vary in their contributions to {beta}-diversity (Species Contribution to {beta}-Diversity; species-{beta}), yet our understanding of how species characteristics shape these contributions remains limited. Additionally, although the ecological processes influencing {beta}-diversity are known to vary with spatial scale, we lack understanding of how species contributions, or their underlying determinants, change across scales. Here, we modelled how plant functional traits, phylogenetic relatedness, and introduction status influence their contributions to {beta}-diversity using plant community data from 429 plots surveyed from 2017-2023 across three nested spatial scales (up to 1 km2) in nine sites spanning four countries. We extended the analysis to broader spatial extents (100 km2 to the entire UK) using GBIF occurrence data. We found that functional traits associated with competitive ability influenced species-{beta}, but the direction and strength of their effects varied with scale. Likewise, phylogenetic novelty increased species-{beta} at small scales but reduced it at larger ones. After accounting for traits and phylogeny, introduced species consistently contributed less to {beta}-diversity than native species--especially at broader spatial extents. These results demonstrate that species ecological and evolutionary characteristics shape their contributions to {beta}-diversity, but that these effects are scale-dependent. Our findings highlight the importance of scale-explicit approaches in understanding both the determinants of {beta}-diversity, and how we can combat its loss to mitigate biotic homogenisation.