Invasive Plants

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Native to North America or introduced (or both)?
Information on the Morphological Differences between the Native and Introduced Species

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Fig. 1. Phragmites invasion replacing native wetland plants in Washington State

Peat core analyses suggest that Phragmites australis has been an uncommon member of mixed tidal wetland plant communities in North America for at least 3000 years (Niering et al. 1977, Orson et al. 1987). In the 19th but particularly in the late 20th century, P. australis began invading fresh and brackish wetlands in North America greatly expanding its range and abundance. Mixed wetland plant communities are replaced by near monocultures of P. australis, (Fig. 1) resulting in changed ecosystem processes and associated detrimental impacts on native wildlife (Marks et al. 1994, Meyerson et al. 2000). The population explosion of P. australis is often thought of being facilitated by changes in land use patterns and hydrologic regimes, increased disturbances, urbanization and eutrophication (Marks et al. 1994). However, the very same factors are thought to cause declines of P. australis in Europe (van der Putten 1997).  Alternatively, it has been suggested that the invasiveness of P. australis is attributable to introduction of more aggressive European genotypes (Metzler and Rosza 1987, Tucker 1990, Besitka 1996) but until recently little information was available to support this hypothesis.

Research by Kristin Saltonstall at Yale University  has now confirmed the present-day existence of native North American haplotypes (lineages) and of introduced European haplotypes. (Click on the citation to view the PDF file of Saltonstall's work) Saltonstall, K. 2002. Cryptic invasion by a non-native genotype of Phragmites australis into North America.  Proceedings of the National Academy of Sciences, USA. 99(4): 2445-2449.  A total of 27 haplotypes were identified of which 11 (A-H, S, Z, AA) are native to North America (Saltonstall 2001b). Within the North American populations, a continuum of geographic substructuring exists for the native haplotypes. Types AA, F, Z and S are known historically form the Northeast; types E, G, and H are found throughout the Midwest (Fig. 2); and types A-D are found in the South and Intermountain West only. Two haplotypes show worldwide distribution (I and M) with M as the most common type in North America, Europe and Asia. Type I is found along the Gulf Coast and also occurs in South America and Asia. (for more details see Saltonstall 2001b)

Comparing the genetic structuring of present-day populations with those available in herbarium specimens collected prior to 1910 reveals significant changes in haplotype frequencies in North America. While the herbarium samples show a widespread distribution of native haplotypes across North America, modern populations show a striking range expansion of the M haplotype (for more details see Saltonstall 2001b). Type M has entirely replaced native types in New England and expanded to the southeast where no historic P. australis populations were known to occur. Type M (which is most closely related to other European types) has spread to the West and is also becoming prevalent in the Midwest. It is likely that the introduction of type M material has occurred sometimes in the early part of the 19th century, probably at several Atlantic coast ports. Over the last 150 years, among-population variation in North America has declined significantly and today the genetic structure of North American populations resembles that of Europe.

Fig. 2. Non-invasive native Phragmites in the boundary waters canoe area co-existing with native species

Bernd Blossey