Is purple loosestrife (Lythrum salicaria) an invasive threat to freshwaterwetlands? Conflicting evidence from several ecological metrics

Conflicting interpretations of the negative impacts of invasive species can result if inconsistent measures are used among studies or sites in definin g the dominance of these species relative to the communities they invade. S uch conflicts surround the case of Lythrum salicaria (purple loosestrife). a widespread exotic wetland perennial. We describe here a 1999 study in whi ch we quantified stand characteristics of L. salicaria and associated veget ation in arrays of 30 1-m(2) plots in each of five wet meadows in Connectic ut, USA. We explored linear and non-linear relationships of above-ground pl ant biomass. stem density, and indices of species richness, diversity, and composition to gradients of L. salicaria dominance, including stem density, percent cover, and biomass. Species richness, other diversity metrics, and stem density of other species were not significantly correlated with the d ensity or percent cover of L. salicaria stems. The relative importance valu es (number of quadrats in which they were found) of co-occurring species in low-density L. salicaria quadrats were significantly correlated with their relative importance in high-density L. salicaria quadrats, indicating that only modest shifts in abundance occurred as L. salicaria increased in dens ity. No individual species were consistently associated with or repelled by the presence of L. salicaria across sites. In contrast to density and dive rsity features, however, the total biomass of species other than L. salicar ia was significantly, negatively correlated with the total biomass of L. sa licaria at each site surveyed. Lythrum salicaria in pure, dense stands main tained a greater above-ground standing biomass on invaded sites than uninva ded vegetation of similar physiognomy. This study demonstrates that hypothe ses about L. salicaria effects can vary depending upon the ecological metri c that is examined. Where one-time, correlative studies are the most feasib le option, data taken on a range of metrics-especially biomass-should be ta ken to inform us about mechanisms by which L. salicaria invades and predomi nates in wetlands.