Ecological Risk Screening Summary - Alligator Weed (Alternanthera philoxeroides) - High Risk

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.

Alternanthera philoxeroides, alligator weed, is a wetland plant native to South America. It has been transported around the world as an aquarium and ornamental plant and as a contaminant in other plant materials and ballast water. The History of Invasiveness for A. philoxeroides is classified as High. This aquatic plant has been established in the southeastern United States since the early 1900’s and has also been recorded as introduced and established in Europe, Asia, and Oceania. A. philoxeroides grows in dense mats, clogging waterways and outcompeting native shoreline plants. It is also a weed in agricultural fields and pastures and has caused economic damage as a result. Once established, control of A. philoxeroides is expensive and difficult. Numerous U.S. States have placed regulations on the importation, possession, or trade of A. philoxeroides. The climate matching analysis for the contiguous United States indicated establishment concern for this species. High climate matches extended into the Midwest, Pacific Northwest, and Southwest regions, beyond the current range of the species in the southeastern United States and California. The Certainty of Assessment is High because of the abundant negative impacts reported for this species where introduced. The Overall Risk Assessment Category for A. philoxeroides in the contiguous United States is High.