Phylogeography of Phragmites australis in North America: A historical perspective on a cryptic invasion

Invasive species are considered one of the primary threats to species declines and habitat degradation worldwide. Although research regarding the ability of introduced species to become invasive and their impacts on native communities has become a priority of conservation biologists, little is understood about the role that genetic diversity may play in facilitating invasions.;Over the past century, the distribution and abundance of Phragmites australis has dramatically increased in freshwater and brackish wetlands throughout North America. This study tested the hypothesis that the increased competitive ability of Phragmites could be the result of the introduction of a more aggressive genotype. Two non-coding chloroplast DNA regions were sequenced for samples collected worldwide, throughout the range of Phragmites. North American populations were found to differ from populations in the rest of the world and share five mutations not found elsewhere. To look for changes in the genetic structure of North American populations which could result from an introduction, modern samples were compared with historical ones from herbarium collections. These data show that in the 1800's, native haplotypes were found across North America and structuring distinguishing samples from the Atlantic Coast, Midwest, and Western parts of the continent was evident. However, today one genetically distinct haplotype dominates the Atlantic coast and is also found across the continent in lower frequencies; this type is the most common haplotype found across Eurasia. In the Midwest, native populations are still common but invasive populations are found along roadsides. Gulf Coast populations represent yet another genetically distinct population group. These results are also supported by patterns of genetic variation found across 12 microsatellite loci.;This research suggests that a non-native strain of Phragmites has been introduced to North America and is responsible for the widespread invasions seen over the past century. Little evidence for hybridization between Native and Introduced populations was found. These results can now provide the foundation for other studies, including how native and non-native populations interact with and differ in their impacts on the communities in which they live and how non-natives outcompete native populations. They are also of use to the management community, which can now better target management interventions.