Biotic and abiotic influences on native and exotic richness relationship across spatial scales: favourable environments for native species are highly invasible

1. Biological invasions threaten biodiversity, and understanding the factors that influence a community’s susceptibility to invasion informs both management of invasive species and conservation efforts towards promoting biodiversity. 2. In this study, we examined the native–exotic richness relationships (NERR) at two spatial scales and asked what variables mediate the relationship between native and exotic species richness in 1-m plots among old fields. We also tested whether the favourable environment and spatial heterogeneity mechanisms alter the relationship between native and exotic richness. Additionally, we identified biotic, abiotic and landscape factors that accounted for patterns of exotic species richness across spatial scales, from 50-m transects (i.e. local scales) to entire old fields (i.e. landscape scales). 3. We found that native and exotic richness were positively related at both local and landscape spatial scales, with the strength of their relationship greater at the landscape scale. In old-field communities with lower foliar cover, native and exotic plant richness were negatively related across 1-m plots, whereas in old fields with higher foliar cover, native and exotic plant richness were positively related across 1-m plots. Overall, predictors of exotic species richness depended on spatial scale: at local scales, native plant richness, light and landscape factors accounted for most of the variation, but biotic factors alone accounted for most of the variation at the landscape scale. 4. Our findings suggest that fields favourable for native species are also suitable for exotic species, and management efforts towards exotic richness should be tailored to spatial scale.