Passengers, not drivers of change: Rethinking alien impacts in a human-shaped world

Human activities have introduced alien plant species far beyond their native ranges around the globe. Although only a fraction of these alien plants becomes harmful to ecosystems, alien species are often treated as a uniform group, irrespective of their actual ecological effects. Using over half a million vegetation plots across Europe, we conducted the first large-scale analysis of how alien vascular plants impact local plant species richness, disentangling the role of species origin, dominance and environmental context. To ensure a fair assessment, we compared the impacts of alien species with those of natives and distinguished between intracontinental (European) and extracontinental (non-European) introductions.

Our results show that both European and non-European alien species tend to be more dominant than natives, albeit less frequent. However, alien plants were only associated with lower species richness compared to natives at low cover, with no significant difference in impact between biogeographic origins at higher cover levels. Furthermore, non-European aliens exhibited stronger average impacts and higher likelihood of reaching dominance when present than European aliens, underscoring the potential influence of shared evolutionary history to limit ecological impacts. We further observed that alien plant richness is primarily associated with anthropogenic disturbances and proxies for eutrophication and acidification. Hence, the reduced species richness in plots with low alien cover likely reflects environmental degradation preceding alien establishment, rather than competitive exclusion, suggesting that most alien species act as passengers and not drivers of ecological change. Indeed, plot-level species richness was mostly reduced by human pressures, while alien plant prevalence explained relatively little variation in native species richness – respectively 0.9% and 1.9% for European and non-European aliens.

Overall, our study challenges normative classifications of invasiveness, since many species labelled as invasive exhibited neutral or even positive overall effects on plant species richness. These findings highlight the need to manage plant species based on demonstrated ecological impact rather than on biogeographic origin or expert-based assessments and emphasize the central role of anthropogenic stressors in shaping European plant communities.