Positive diversity–invasibility relationships across multiple scales in Finnish agricultural habitats

Abstract

The diversity of native species is assumed to limit invasion by alien species, but there is a negative relationship between native and alien species diversity at fine spatial scales and a positive one at broad scales. This contradiction has been termed the invasion paradox, and it ensues from various processes operating at different spatial scales, such as species interactions and environmental heterogeneity. We investigated the relationship between native and alien plant species diversity components (α-, β- and γ-diversity) and their response to environmental factors at three spatial scales (1, 50 m2, 0.25 km2) in semi-natural agricultural habitats in Finland. Native and alien species diversity components were positively correlated across spatial scales, and the beta diversity contributed most to the total observed alien and native species richness (γ-diversity). The diversities of native and alien species were positively associated with productivity at the 1 m2 scale. At broader scales, alien and native species diversity responded similarly to geographical location, but differently to the productivity, disturbance and landscape diversity. Alien species diversity was positively correlated with disturbance regime, whereas native species were more strongly related to habitat type, and decreasing land-use intensity. Native and alien diversities were affected by both average and variability in local habitat conditions. Thus, both favourable conditions and heterogeneity in environmental conditions may contribute to the diversity–invasibility relationships. Disturbance regime typical of agricultural habitats may create open niches for both native and alien species, limit species competition even at fine spatial scales and lead to a positive diversity–invasibility relationship.