Genetic Variation in the Common Reed, Phragmites australis, in the Mississippi River Delta Marshes: Evidence for Multiple Introductions

Abstract

Multiple introductions are believed to play an important role in increasing genetic diversity and adaptability of invasive species, but there are few well-documented examples. The common reed, Phragmites australis, has dramatically increased in tidal wetlands throughout the USA in the past century due primarily to the introduction of a Eurasian lineage. In the Mississippi River "Balize" delta, P. australis is the dominant vegetation where monotypic stands of an introduced form blanket the outer marshes. The delta's interior marshes, on the other hand, are more vegetatively diverse, serving as important waterfowl foraging habitat. Recent encroachment by various phenotypic forms of P. australis into the interior marshes led to this study examining genetic variation in these stands. Our results revealed four chloroplast DNA haplotypes that also segregate based on microsatellite variation. Three of these are closely related and introduced, but differ relative to time and likely mode of introduction. The "Delta" type (haplotype M1), which is unique to the region and the most common lineage, displays considerable microsatellite diversity. The Eurasian introduced lineage of P. australis (haplotype M), which is invasive elsewhere in North America, is increasing its distribution in the delta. A novel haplotype, AD, was also identified which is phenotypically and genetically similar to haplotype M. Despite the close relatedness, we found no evidence for inter-haplotype gene exchange at the nuclear level, suggesting that intraspecific hybridization is not a contributing factor to these invasions. The site provides a unique opportunity for researchers to understand the dynamics of multiple P. australis invasions.

Coastal Habitats of the Gulf of Mexico

“…Historically, Phragmites was only a minor component of tidal marsh vegetation assemblages and was confined to higher elevations at the upland boarders of marshes and areas along creek banks (Niering et al 1977;Warren et al 2001). However, recent expansion of Phragmites into more diverse habitats has been occurring along the U.S. Atlantic and Gulf coasts (Chambers et al 1999;Peterson and Partyka 2006;Hauber et al 2011). Reasons for the Phragmites expansion vary by location but can include tidal restrictions, habitat modification, disturbance, and invasion by a more competitive European population (Chambers et al 1999;Burdick et al 2001;Howard et al 2008;Hauber et al 2011).…”

“…However, recent expansion of Phragmites into more diverse habitats has been occurring along the U.S. Atlantic and Gulf coasts (Chambers et al 1999;Peterson and Partyka 2006;Hauber et al 2011). Reasons for the Phragmites expansion vary by location but can include tidal restrictions, habitat modification, disturbance, and invasion by a more competitive European population (Chambers et al 1999;Burdick et al 2001;Howard et al 2008;Hauber et al 2011). For example, in coastal Mississippi Peterson and Partyka (2006) found that Phragmites was widespread along creek banks in up-estuary/low salinity environments where there was little anthropogenic disturbance, but Phragmites also occurred in high elevation/high salinity areas that were heavily modified by man.…”

“…The situation in Louisiana is somewhat unique, particularly in the Mississippi River's Birdfoot Delta, where distinct genotypes of Phragmites can occupy different areas within the same marsh (White et al 2004a;Hauber et al 2011;Lambertini et al 2012) (Figure 6.133).…”

“…Phragmites is the dominant vegetation type in the outer two-thirds of the Birdfoot Delta, yet within these Phragmites areas are patches of the reed that are both phenotypically and genetically distinct (Hauber et al 1991(Hauber et al , 2011White et al 2004a) (Figure 6.133). However, the occurrence of Phragmites on the outer portions of the delta is a mixed blessing.…”

“…However, the occurrence of Phragmites on the outer portions of the delta is a mixed blessing. On one hand, Phragmites populations stabilize an easily erodible landscape (Rooth and Stevenson 2000) and protect the more diverse and fragile interior marshes of the delta (Hauber et al 2011). On the other hand, Phragmites populations provide little in the way of habitat and food for migrating waterfowl that overwinter in the Mississippi River delta (Hauber et al 1991).…”