Assessing the level of plant invasion: A multi-scale approach based on vegetation plots

Recent studies highlight the importance of selecting the appropriate scale and indices of invasion level for evaluating the abundance and impact of alien plants. Our survey considers the use of vegetation plot databases compared with floristic checklists to address invasion patterns regarding alien–native relationships across vegetation types by means of a multi-scale approach. We analysed the alien–native richness relationship in 1077 vegetation plots from the Basque Country (N. Spain) at ecosystem level and phytosociological class and alliance levels. According to our results, the alien species richness (Alo)–native species richness (Nat) relationship is variable and depends not only on the scale but also on the vegetation type. In contrast with other multi-scale approaches, no negative correlation has been detected at any studied level. The strong correlation existing between plot number and cumulative Alo and cumulative Nat highlights the constraints of using checklists to generalize invasion patterns. Our results demonstrate that the combined use of both relative alien species richness and relative alien species cover facilitates the understanding of invasion patterns across plant communities at different scales. In addition to climate, disturbance and propagule pressure, habitat type proved to be an important filter for alien species, capable of explaining such patterns.