Ecological Impacts: Why should we be concerned?

Non-native Phragmites can negatively affect the biodiversity and ecological functions of invaded habitats, impair the recreational use of wetlands and shorelines, decrease property values, and increase fire risk. This section covers a variety of concerns brought on by the spread of Phragmites throughout the Great Lakes region.

Phrag-GreenBay

Photo Credit: Great Lakes Commission

Native Phragmites

Native Phragmites has been present in North-America for over 3,000 years. However, the non-native Phragmites is now more widespread, particularly in disturbed environment such as roadsides and drainage ditches (visit the Native vs. invasive? and Where is it found?). The spread of non-native Phragmites into natural wetlands of the Great Lakes basin could have detrimental impacts on the native strain. This further increases the complexity of Phragmites management programs, as managers need to develop control strategies that will successfully reduce non-native Phragmites populations without affecting the native strain.

Inferences made from historical herbarium  data suggested that some populations of native Phragmites  could have been displaced by the more aggressive invasive strain, although this has never been directly observed in the wild. Studies hypothesized that non-native Phragmites tends to have a competitive advantage over the native strain, as it produces more biomass, forms denser colonies and shows a broader tolerance to environmental stressors. Further, recently published studies demonstrated that the invasive strain produces more seeds and has a greater dispersal capacity, enabling it to spread more rapidly through the environment.

It was long thought that the two strains of Phragmites could not hybridize as they bloom and senesce at different times. However, researchers from the University of Rhode Island were able to produce viable hybrid seedlings by cross-pollinating non-native and native Phragmites. Hybrids were recently discovered in two populations of Lake Erie, but hybridization is still believed to be rather uncommon. Potential large-scale hybridization of the two subspecies nevertheless remains a great concern, as this could lead to a genetic pollution of the native strain, or the creation of an aggressive hybrid.

Resources:

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Flora

Non-native Phragmites produce large quantities of seed and can expand rapidly via vegetative growth, which enables it to quickly colonize new habitats and dominate areas in which it is established (see the How does it spread? section for more details on its reproduction mechanisms). As with many other invasive species, Phragmites can flourish in a wide range of environmental conditions, such as pH, salinity,  or soil type. Several studies have demonstrated that Phragmites tends to become the dominant species in a variety of moist habitats, and that increasing Phragmites generally leads to decreases in the overall biomass, richness and abundance of native plant species.

 Phragmites is able to persist and thrive in a broad variety of ecosystems. Other mechanisms such as early emergence, allelopathy, and litter accumulation facilitate the rapid spread of Phragmites. Emerging early in the growing season, Phragmites gains an advantage over other plant species because it can exploit the pool of resources and shade out its competitors. Phragmites  has also been shown to be allelopathic, producing a toxic chemical in its root system that deters the growth of other species. Phragmites also grows quickly and produces large amounts of above- and below-ground biomass that decomposes slowly. Especially above-ground, this standing litter can inhibit the germination of seeds and seedling establishment of other plant species by reducing light and temperature at the soil surface.

Resources

  • Chambers, Meyerson and Saltonstall. 1999. Expansion of Phragmites australis into tidal wetlands of North-America. Aquatic Botany 64(3): 261-273 [visit the Aquatic Botany website to read the abstract and obtain a reprint].
  • Holdredge and Bertness. 2011. Litter legacy increases the competitive advantage of invasive Phragmites in New England wetlands. Biological Invasions 13(2): 423-433 [visit the Biological Invasions website to read the abstract and request a reprint].
  • Resources: Rudrappa et al. (2007) Root-secreted Allelochemical in the Noxious WeedPhragmites australis  Deploys a Reactive Oxygen Species Response and Microtubule Assembly Disruption to Execute Rhizotoxicity. Journal of Chemical Ecology. Volume 33, Number 10 (2007), 1898-1918.
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Fauna

Impacts of non-native Phragmites on animals are less clear and tend to vary between taxa. Phragmites stems and leaves have poor nutritional value and very few organisms seem to be feeding on it in North America.  Replacement of native vegetation by the less nutritious Phragmites could have negative consequences for herbivores. And, while Phragmites colonies may provide habitats for certain species, there are many other species that may be negatively impacted by loss of other habitat types.

Bird surveys conducted in tidal wetlands of Connecticut showed that Phragmites-dominated marshes were characterized by a lower diversity of birds than adjacent mixed marshes. Meanwhile, a study conducted in freshwater wetlands of Quebec showed little difference between Phragmites-dominated marshes and other plant communities in terms of birds’ abundance and diversity.

Fish at their larval and juvenile stages seems to be the most affected by Phragmites. The species produces an abundant litter which can reduce the mobility of juvenile fish in creeks. Effect on mature fish varies among species. Spotkin killifish, mummichog and northern pike were all shown to be less abundant within Phragmites communities.

Currently very little is known about potential impacts of Phragmites on amphibians. Preliminary results of a research conducted in freshwater marshes of Quebec did show no significant impacts on northern leopard frogs, green frogs and spring peeper, while a field experiment conducted in Connecticut found that larval American bullfrogs were positively effected by non-native Phragmites.

Resources:

  • Rogalski and Skelly. 2012. Positive effects of Nonnative Phragmites australis on larval Bullfrogs. PLOS One 7(8).
  • Warren, Fell, Grimsby, Buck, Rilling and Fertik. 2001. Rates, patterns, and impacts of Phragmites australis expansion and effects of experimental Phragmites control on vegetation, macroinvertebrates, and fish within tidelands of the lower Connecticut River. Esturaries 24(1): 90-107 [visit the Estuaries website to read the abstract and request a reprint].
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Ecosystem services

Phragmites is an ecosystem engineer- a species that can successfully modify the form and function of the habitats in which it is found. For example, the rapid growth and slow decomposition of above-ground biomass leads to large accumulations of organic material over several growing seasons. This can lead to greater rates of sediments deposition which can increase the elevation of invaded marshes and affect drainage, leading invaded marshes to slowly become drier, in turn, causing further changes in the remnant plant population at a site.

 

Other resources:

Check out this great fact sheet by the Midwest Invasive Plant Network: Why Should I Care About Invasive Plants?