The invasive herb Lupinus polyphyllus can reduce plant species richness independently of local invasion age

The ecological impacts of invasive species may change or accumulate with time since local invasion, potentially inducing further changes in communities and the abiotic environment. Yet, time since invasion is rarely considered when investigating the ecological impacts of invasive non-native species. To examine the effect of time since invasion on the ecological impacts of Lupinus polyphyllus, a perennial nitrogen-fixing herb, we surveyed vascular plant communities in the presence and absence of L. polyphyllus in young, intermediate, and old semi-natural grassland sites (ca. 5, 10, 15 years representing both time since lupine invasion and plant community age). We analyzed vascular plant community composition, vascular plant species richness, and the cover of various ecological plant groups and L. polyphyllus. In contrast to our hypotheses, we found no change in the mean cover of L. polyphyllus (about 35%) with time since local invasion, and an ordination did not suggest marked changes in plant community composition. L. polyphyllus was associated with lower species richness in invaded plant communities but this effect did not change with time since invasion. Invaded plant communities were also associated with lower occurrence of generalist, oligotrophic (low-nutrient-adapted) and copiotrophic (nutrient-demanding) species but no temporal dynamics were detected. We conclude that even the intermediate cover of L. polyphyllus can reduce plant species richness, but the ecological impact caused by this invader might not dramatically change or accumulate with time since invasion.