Myriophyllum aquaticum has a high probability of introduction to the Great Lakes (Confidence level: High).
Potential pathways of introduction: Natural dispersal through waterbody connections; Hitch-hiking or fouling of recreational gear, boat structures, fauna, or other objects entering the Great Lakes from surrounding region; Unauthorized intentional release from live trade; Accidental introduction to Great Lakes by escapees With established nonindigenous populations in states adjacent to the Great Lakes, parrot feather has potential to be introduced to the Great Lakes from nearby water bodies. The closest parrot feather population to the Great Lakes has been recorded from Meserve Lake, Indiana, which drains though the Pigeon River into the St. Joseph River, a tributary of Lake Michigan (Wersal 2011). Fragments of this plant are capable of transport by river currents and could also become attached to or entangled with recreational boats (e.g., propellers, trailer tires) or fishing gear. Its rhizomes are very tough and can be transported long distances on boat trailers, surviving for up to a year when kept moist and cool (Washington State Department of Ecology 2003, in Mabulu 2005).
Parrot feather has been an ornamental favorite in hanging baskets, fountains, and aquaria for more than a century due to its blue-green color, feather-like leaves, oxygenating properties, and cascading pattern of growth (Les 2002; Les and Mehrhoff 1999). Often sold under incorrect names, introductions of this species are usually attributed to the water garden and aquarium trades (Davis 1996; Center for Aquatic and Invasive Plants, UF/IFAS 2010; Les 2002; Les and Mehroff 1999). It has escaped cultivation through mechanical fragmentation and unintentional plantings, readily taking root. In a Great Lakes regional study, this aquatic plant was found in 25% of the stores surveyed in Michigan and Ontario, near Lake Erie, between 2002 and 2003 (Rixon et al. 2005). Moreover, water garden plants are often left outside to overwinter, which can lead to unintentional escape during spring flooding. The locations of Ontario water gardens indentified by 2006 survey respondents suggests that many of these gardens are within the coastal regions of four of the five Great Lakes, though if these were also flood-prone areas was not determined (Marson et al. 2009b).
Parrot feather is of growing interest for environmental remediation of soil and water contaminated with chlorinated solvents, trinitrotoluene (TNT), and other nitrogenated explosive/aromatic compounds, but this is currently a technology in limited, experimental use (Medina et al. 2000; Nwoko 2010).
Among the Great Lakes states and provinces, M. aquaticum is prohibited in Illinois, Michigan, and Wisconsin and regulated in Minnesota. Furthermore, it is listed as a noxious weed by nine non-Great Lakes states (Alabama, Connecticut, Idaho, Maine, Massachusetts, Maryland, New Hampshire, Vermont, and Washington) (IISG 2008; GLPNS 2008; WIDNR 2011). Without more stringent laws regulating sale and disposal throughout the entire region, introduction could occur through disposal of aquarium fragments, unintentional escape from culture, or intentional unauthorized planting to support live trade.
Not established in the Great Lakes.
Myriophyllum aquaticum has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: Moderate).
Myriophyllum aquaticum is a hardy species with broad environmental tolerances (see Ecology above). It occurs as a floating plant in the deep water of nutrient-enriched lakes like the Great Lakes (Washington State Department of Ecology 2011). It is known to tolerate freezing temperatures in California’s Bay area winters (Aiken 1999). However, this plant can be killed by extended periods of frost (WNDR 2011) and so may benefit from warmer winters predicted to result from climate change.
Parrot feather grows vigorously and quickly following invasion in new habitats, forming dense canopies that occupy large amounts of space and block sunlight and oxygen exchange. As a result, this species outcompetes and replaces native flora that might be of more value to fish and wildlife (Stiers et al 2010; WNDR 2011).
Reproduction and dispersal of M. aquaticum in North America occurs by vegetative fragmentation, which is an effective method for short-range, but not long-range, dispersal (Les and Mehrhoff 1999). Although parrot feather’s natural dispersal potential is limited, this species is widespread outside its native range (Moody and Les 2010). Myriophyllum aquaticum has expanded its range mainly in the southern United States and may be relatively innocuous in the northeast due to a smaller number of occurrences (Hoyer et al. 1996). Nonetheless, this species has survived in southern New England and caused serious local infestations (WIDNR 2011). The rapid spread of M. aquaticum is correlated with its widespread cultivation and the transport of fragments by waterfowl or vehicles. When transport agents are not present, the threat of its escape and establishment depends more on the number of localities where it is grown. Unfortunately, M. aquaticum remains widely available from sources of cultivated water plants and dealers occasionally plant it intentionally to propagate a local supply (Aiken 1981; Les and Mehrhoff 1999).
Nonindigenous M. aquaticum specimens collected from geographically diverse locations in North America have been found to have identical ITS genotypes and are all female. Seed production has not been recorded (Moody and Les 2010).
Myriophyllum aquaticum has the potential for high environmental impact if introduced to the Great Lakes.
Potential:
The U.S. EPA (2008) predicted that M. aquaticum could have a high impact and spread rate in the Great Lakes, as it is adaptive to a variety of environments. According to Les and Mehrhoff (1999), rapid spread has been relatively common in this macrophyte’s North American invasion history (Les and 1999). Outside the U.S, a risk assessment prepared for Australia in 1995 by Pacific Island Ecosystems at Risk recommended rejection of the plant for import on mainland due to its likelihood of becoming a pest (Pheloung 1995, in Mabulu 2005). By 2002, parrot feather was assessed as one of the top 200 invasive naturalized plants in Southeast Queensland, Australia (ranked #69 of 200) (Queensland Herbarium 2002).
Dense infestations of parrot feather can rapidly overtake small ponds and sloughs, changing their physical and chemical properties, including impeding water flow, which can result in increased flood duration and intensity. The spread of aquatic nonindigenous plants into a waterbody can also lead to increased rates of evapotranspiration and water loss. One mesocosm experiment found that colonization by M. aquaticum was correlated to an increase in water loss of about 1.5 to 2 times that experienced by an open water surface (Rosa et al. 2009).
Myriophyllum aquaticum can dramatically alter ecosystems by shading out algae, pondweeds, and coontail on which waterfowl feed (Ferreira and Moreira 1994; Washington State Department of Ecology 2011). Floating mats of M. aquaticum have been measured at up to 26 kg of fresh weight in Europe and are capable of reducing the oxygen content of the water below to <1 mg O2L-1, which can be detrimental to fish (Fonseca 1984 cited in Moreira et al. 1999; Hussner 2008 in Hussner 2009). In Germany, the infestation of these mats created anoxic, shaded conditions in shallow waters, and appeared to be correlated with a decline in native macrophyte diversity (Hussner 2008 in Hussner 2009).
In Chinese laboratory experiments, parrot feather outcompeted native species with respect to relative growth rate, with the most significant results on high-nutrient sediment (Xie et al. 2010). A separate mesocosm study by Wersal and Madsen (2011) found that the yield (biomass) of M. aquaticum was positively related to tissue nitrogen content, suggesting that high levels of nitrogen contribute to nuisance levels of growth. However, an inverse relationship existed between M. aquaticum yield and tissue phosphorus content. Wersal and Madsen (2011) proposed that high levels of phosphorus favored the growth of algae (superior competitors in phosphorus uptake) causing shading in the water column and suppressing the growth of M. aquaticum (Wersal and Madsen 2011).
Stiers et al. (2011) compared Belgian lake sites that were heavily invaded (90-100% cover), semi-invaded (~25% cover), and uninvaded by M. aquaticum and found that native species richness was 57% lower in heavily invaded sites relative to uninvaded sites. Parrot feather cover was also negatively correlated with invertebrate species richness and abundance. The authors observed lowered levels of dissolved oxygen at some sites, as well as a dense mat of decomposed plant litter and sediments at the bottom of heavily-invaded sites; they hypothesized that this condition created unsuitable habitat for invertebrate colonization (Stiers et al. 2011). Plant species that are rare (Utricularia vulgaris) and vulnerable (Hydrocharis morsus-ranae) IUCN Red List species in Belgium were absent in heavily invaded sites but present in semi-invaded sites (Steirs et al. 2011). Furthermore, mayflies (Caenis spp.) were present in uninvaded sites, but were not reported in invaded sites (Steirs et al. 2011).
Myriophyllum aquaticum can also alter the cycling of heavy metals in aquatic systems. Cardwell et al. (2002) found that M. aquaticum accumulated the highest overall levels of metals (zinc, cadmium, copper, and lead) in its tissues of all 15 aquatic plants that underwent testing. While this suggests that M. aquaticum could be used as an important indicator species (see below), the consumption of M. aquaticum by grazers could increase the bioaccumulation of heavy metals in the food web.
Myriophyllum aquaticum has the potential for moderate socio-economic impact if introduced to the Great Lakes.
Potential:
Parrot feather infestations have been reported in both natural and man-made water bodies, including lakes, ponds, canals, drainage and irrigation ditches, and lagoons. Plants and floating mats of vegetation are sometimes uprooted, choking waterways, inhibiting navigation, and potentially blocking pumps or drainage (Engineer Research and Development Center 2007; Sheppard et al. 2006). Dense growth can also diminish the recreational value and seriously affect the perceived aesthetic qualities of infested waterways (Banfield 2008; Washington State Department of Ecology 2011).
Myriophylum aquaticum monocultures provide prime mosquito habitat; higher parrot feather density has been correlated with higher mosquito egg and larval abundance (Orr and Resh 1992), which may lead to increased prevalence of mosquito-born diseases.
Myriophyllum spp. have invaded rice paddies could adversely affect wild rice (Zizania palustris) found in the upper Great Lakes (Quayyum et al. 1999). One account by South African farmers also reported that tobacco crops gained a red tint (reducing the sale value of the crop) when irrigated with water from an area colonized by M. aquaticum roots (Cilliers 1999).
Myriophyllum aquaticum has the potential for moderate potential benefits if introduced to the Great Lakes.
Potential:
Assessment protocols have been developed using M. aquaticum as a primary indicator species of sediment toxicity in potentially polluted areas (Feiler et al. 2004; Knauer et al. 2008). It is an important species in the aquarium trade and can be found in shops in both the American and Canadian Great Lakes regions (Marson et al. 2009a; Rixon et al. 2005). It is reportedly sold as an “oxygenating plant” in Europe (Sheppard et al. 2006).
Parrot feather may provide cover for some aquatic organisms (Washington State Department of Ecology 2011). Parker et al. (2007) found that beavers (Castor canadensis) in Georgia fed on M. aquaticum to the extent that invasive populations were reduced, although no strong preference for this plant species over others was documented. Myriophyllum aquaticum could be used for nitrogen and phosphorus remediation (e.g., in a constructed wetland remediating nutrient runoff), but Polomski et al. (2009) found that other invasive macrophytes (Eichhornia crassipes and Pistia stratiotes) had equal or greater uptake efficiency levels relative to M. aquaticum. Parrot feather can also aid in environmental remediation of soil and water contaminated with chlorinated solvents, trinitrotoluene (TNT), and other nitrogenated explosive/aromatic compounds (Medina et al. 2000; Nwoko 2010).