Phragmites australis (common reed) is one of the most widely distributed angiosperms in the world. Today it is found across North America where, in many locations, it has become the dominant plant species in wetland communities and an indicator of disturbance. Three distinct lineages are found in North America, with the endemic native subspecies P. australis americanus found across much of the continent, the Gulf Coast subspecies P. australis berlandieri found distributed across the southernmost states, and the introduced (European) subspecies found throughout the United States and southern Canada. Although classified as the same species, these three lineages are genetically and ecologically distinct, and few hybrid crosses between the lineages have been identified. In the Southwest, the native lineage still predominates (unlike in the East), but the introduced lineage is quickly spreading from urban centers. While hybridization between native and introduced Phragmites australis remains an uncommon event, it poses an ongoing threat to native P. australis across its range. This is especially true for native populations in the biologically rich, but sparsely distributed wetlands of the Southwest, which are among the most imperiled systems in North America.
Results/Conclusions
We identified widespread hybridization between native and introduced P. australis in the Las Vegas Wash watershed, NV, a key regional link to the Colorado River basin. Should hybrids between native and invasive genotypes become common, the genetic integrity of native populations may become compromised by interbreeding, or hybrids could display increased vigor and outcompete remaining native populations, which are typically given high conservation priority. Downstream escape of hybrid plants to Lake Mead and wetlands throughout the lower Colorado River basin is of management concern as these hybrids appear vigorous and could spread rapidly. Additional changes in floral composition are occurring along the lower Colorado basin where native species, including the native Phragmites subspecies, are being replaced by the Gulf Coast genotype. Ultimately, invasion and hybridization may lead to shifts in wetland composition from a high diversity of native species to low diversity systems dominated by Phragmites.