The spread of invasive alien species Garden lupine (Lupinus polyphyllus Lindl.) in Swedish road verges - a double-folded threat to species diversity?

Sammanfattning

Invasive alien species are a rising threat to biodiversity. Natural and semi-natural grasslands are habitats holding high diversity for many groups of organisms and have seen heavy decline in Europe during the last two centuries. Road verges have the capacity to hold high biodiversity of vascular plants typically found in grassland habitats but are also vectors for spreading and harboring invasive alien species like the Garden lupine (Lupinus polyphyllus Lindley). L. polyphyllus is an expansive nitrogen-fixing legume that establishes in open and nutrient-poor habitats prone to harbor species with lower competitive abilities and low demands for nitrogen. Based on surveys performed in species-rich road verges located in the southwestern part of central Sweden, species richness and diversity of vascular plants were compared between plots without L. polyphyllus and plots dominated by L. polyphyllus. Surveyed species were classified into four functional groups (very poor competitors, poor competitors, moderate competitors and strong competitors) based on their relation to nitrogen and overall competitive ability. For the analysis, very poor and poor competitors were combined into one group called ‘poor competitors’. The study aimed to determine if species with different relations to nitrogen and ability to compete are affected differently by the presence of L. polyphyllus. Strong competitors made up only about 20% of the total species richness and almost half of the species in this group were graminoids. The other functional groups contained a larger ratio of forbs to graminoids. Poor competitors and moderate competitors had significantly lower species richness and diversity in plots dominated by L. polyphyllus, while strong competitors were unaffected by the presence of the invasive. This confirmed that species with strong competitive abilities are less affected by L. polyphyllus. The results suggests that plant communities will be shifting towards more homogenous communities with a higher proportion of graminoids following the invasion of L. polyphyllus. The spread of Garden lupine therefore counteracts the value of road verges as tools for grassland conservation. Considering the species composition in and around species-rich road verges threatened by invasion of L. polyphyllus is an important part of prioritizing conservation and restoration projects.