Invasiveness in wetland plants in temperate North America

The spread of invasive taxa, including Lythrum salicaria, Typha x glauca, Myriophyllum spicatum, Phalaris arundinacea, and Phragmites australis, has dramatically changed the vegetation of many wetlands of North America. Three theories have been advanced to explain the nature of plant invasiveness. Aggressive growth during geographic expansion could result because 1) growth is more favorable under new environmental conditions than those of resident locales (environmental constraints hypothesis); 2) herbivores may be absent in the new locale, resulting in selection of genotypes with improved competitive ability and reduced allocation to herbivore defenses (evolution of increased competitive ability hypothesis); and 3) interspecific hybridization occurred between a new taxon and one existing in an area, resulting in novel phenotypes with selective advantages in disturbed sites or phenotypes that can grow under conditions not favorable for either parent (introgression/hybrid speciation hypothesis). A review of published literature found few studies that compare the growth and dynamics of invasive populations in their new range versus those in historic ranges. However, there is evidence that hydrologic alterations could facilitate invasions by Typha x glauca and Phalaris arundinacea and that increased salinity promoted spread of Typha angustifolia (parental taxon) and Phragmites australis. The potential for reduced herbivory causing aggressive growth is greatest for Lythrum salicaria. Introgressive hybridization is potentially a cause of invasiveness for all five species but has been established only for Typha x glauca and Lythrum salicaria.