Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plantspecies diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicumDesf. ex Fisch.-invaded and noninvaded plots from 12 locations in northern Norway. There was lower nativecover, evenness, taxonomic diversity, nativebiomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two nativespecies fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the nativecover, evenness, and nativebiomass, and a positive association with the height of the native plants. Plant communitiescontaining only nativespecies appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant communityby H. persicum exerts consistent negative pressure on vascular plantdiversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant communityto further invasion, it severely reduces the abundance of nativespecies and makes them more vulnerable to competitive exclusion.