Species that are considered high risk have a well-documented history of invasiveness in at least one location globally, and a high or medium climate match to the contiguous United States.
Eurasian watermilfoil (Myriophyllum spicatum) is an aquatic plant native to Eurasia that can grow in a wide variety of conditions and may hybridize with native species. The history of invasiveness is high. M. spicatum has been introduced throughout much of the world. It was introduced in the United States through ship ballast water and/or the aquarium and aquatic nursery trade. M. spicatum spreads by attachment to boats and ships. This plant can outcompete and extirpate native plant species creating monocultures that can alter hydrology, change nutrient dynamics and alter food webs. At high densities, abundance and diversity of invertebrates that provide food to fish and foraging space for predator fish are reduced. Heavy infestations limit recreational activities and provide habitat for mosquitoes and parasites that cause swimmer’s itch. Substantial financial investments have been made to limit the impact of this species in aquatic environments. Climate matching indicated the contiguous United States has a high climate match for M. spicatum. Certainty of this assessment is high. The overall risk assessment category is high.
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