Lythrum salicaria (aquatic plant, herb)

Ecology Distribution Management Info Impact Info References and Links Contacts

General Impact Lythrum salicaria is often reported to outcompete and replace native grasses, sedges, and other flowering plants that provide a higher quality food source and habitat for wildlife. A literature review by Lavoie (2010) of the studies published on purple loosestrife impacts on plants found that 10 out of 11 manipulative studies detected a negative impact on plants. Interestingly all seven observational (field) studies detected no negative impacts. Of the two studies that use both manipulative and observational methods, Yakimowski et al. (2005 in Lavoie, 2010) showed a reduction of the abundance and richness of vascular plant seedlings in purple loosestrife invaded wetlands, while Denoth and Myers (2007 in Lavoie, 2010) concluded that the competitive effect of purple loosestrife on a rare species was not greater than the impact of native plants. Lythrum stands can deleteriously impact wildlife habitat used by birds and furbearers. L. salicaria forms dense homogeneous stands that restrict native wetland plant species, including some endangered plants. L. salicaria can overrun wetlands and almost entirely eliminate open water habitat if left untreated. The recreational and aesthetic value of wetlands and waterways is diminished as dense stands of L. salicaria choke waterways and decrease biodiversity. A review of literature by Lavoie (2010) found that out of fourteen animal species or groups studied only six were negatively affected, while others were either not affected or positively affected by purple loosestrife. The strongest negative effects were on tadpoles and the marsh wren (Cistothorus palustris). The American toad (Bufo americanus is also negatively affected by compounds leached from L. salicaria. There have been no studies published on the impacts on fish, mammals or waterfowl (Lavoie, 2010). In North American freshwater wetlands, L. salicaria alters decomposition rates and nutrient cycling, water chemistry, leads to reductions in wetland plant diversity, reduces pollination and seed output of the native Lythrum alatum, and reduces habitat suitability for specialized wetland bird species such as black terns, least bitterns, pied-billed grebes, and marsh wrens (Blossey et al., 2001; Fickbohm & Zhu, 2006). L. salicaria effects on wetlands, incurring from lost forage and control costs have been estimated to cost US $28 million per year (Barbier & Knowler, 2006). However, according to Lavoie (2010) “their analysis was apparently based on a previous estimation of the damages calculated by Thompson et al. (1987), damages that were only conjecture.”          Location Specific Impacts: Canada   Competition: The formation of dense stands of purple loosestrife results in the loss of wetland habitat for the establishment and spread of native aquatic vegetation. In some sites purple loosestrife is a threat to species designated nationally at risk by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). This is the case with swamp rose mallow, (see Hibiscus moscheutos Species at Risk), a species designated as vulnerable by COSEWIC. Economic/Livelihoods: The spread of purple loosestrife also has a direct economic impact when plants clog irrigation or drainage ditches on farmlands or cause degradation and loss of forage value of lowland pastures. Modification of nutrient regime: The proliferation of purple loosestrife in wetlands also changes the composition and cover values of plants that may serve as food sources for waterfowl and furbearers or as habitat for other aquatic life. United States (USA)   Habitat alteration: Lythrum salicaria reduces habitat suitability for specialized wetland bird species of the United States such as black terns, least bitterns, pied-billed grebes, and marsh wrens (Blossey et al, 2001). Modification of nutrient regime: In North American freshwater wetlands, Lythrum salicaria alters decomposition rates and nutrient cycling, water chemistry (Blossey et al, 2001). Other: The high tannin concentrations of L. salicaria leaves are believed to have the potential to create environments that are directly toxic to B. americanus tadpoles. It is hypothesized that obligate gill breathers such as B. americanus tadpoles are highly sensitive to gill damage caused by high concentrations of phenolics (Maerz et al, 2005). Reduction in native biodiversity: Lythrum salicaria reduced wetland biodiversity by displacing native plants, altering habitats, and changing ecological communities (Blossey et al, 2001). L. salicaria reduces pollination and seed output of the native Lythrum alatum (Blossey et al, 2001). Oregon (United States (USA))   Competition: A study in several wetland locations of Oregon found Lythrum salicaria to exponentially decrease the abundance of native plants species, but there was not a significant pattern of decrease in diversity of native plants (Schooler et al, 2006).