Relationship between the abundance of Lythrum salicaria (purple loosestrife) and plant species richness along the Bar River, Canada

Lythrum salicaria (purple loosestrife) is a perennial herbaceous plant that was introduced in the 1800s into North America. Its geographical expansion has generated much concern, in part because its spread may lead to a reduction in the diversity of plant species in wetlands. We tested this hypothesis by examining the association between the abundance of Lythrum salicaria and vascular plant species richness in forty-one 2m x 2m plots located along the Bar River, Ontario, Canada. No significant differences in mean species richness were found between plots with and without L. salicaria. For those plots containing L. salicaria, species richness was not related to the percentage cover of L. salicaria. Furthermore, there were no significant differences in the number of introduced plant species between plots with and without L. salicaria, nor were there differences with increasing percent cover of L. salicaria. Plant species such as Nuphar variegatum, Scirpus atrovirens, Scirpus cyperinus, and Scirpus microcarpus were more likely to be found in plots with Lythrum salicaria than without. However, no plant species were significantly mon likely to be found in plots without Lythrum salicaria than with it. Collectively, these results provide no support for the hypothesis that the number of species in wetlands is decreasing in association with the invasion of Lythrum salicaria in Ontario.