Ecological Risk Screening Summary - Water Lettuce (Pistia stratiotes) - High Risk

Species that are considered high risk have a well-documented history of invasiveness in at least one location globally and are of establishment concern for the contiguous United States based on a climate matching analysis.

Pistia stratiotes, Water Lettuce, is a free-floating, fast growing, aquatic plant that is widely distributed in tropical and subtropical environments throughout the world, although the location of its native range is under debate. It is also found in temperate regions, reproducing via seed or persisting year-round in thermal waters. It is a widely popular aquarium and pond plant; escape or improper disposal of plants from ornamental trade are likely responsible for many recent introductions, although contamination of boats and fishing equipment may contribute to secondary spread. Possession, transport, or trade of P. stratiotes is prohibited or restricted in the following U.S. States: Alabama, California, Connecticut, Florida, North Carolina, South Carolina, Texas, and Wisconsin. Introductions of P. stratiotes have led to the decline of native species, decreased dissolved oxygen levels, and habitat loss. Floating mats of P. stratiotes have been known to clog waterways and impede fishing, swimming, and water navigation. This species has caused the loss of water for paddy crops, irrigation, and drainage, and provides favorable habitat for the transmission of water-borne diseases. The History of Invasiveness for Pistia stratiotes is classified as High due to its record of nonnative establishment and consistency of negative impacts. The climate matching analysis for the contiguous United States indicates establishment concern for this species; nearly the entire contiguous United States had a medium or high climate match. The Certainty of Assessment for this ERSS is classified as High due tothe strength of evidence for the History of Invasiveness and Establishment Concern, despite cryptogenicity and introductions in thermal waters. The Overall Risk Assessment Category for Pistia stratiotes in the contiguous United States is High.