Ecological Risk Screening Summary - American Spongeplant (Limnobium spongia) - Uncertain Risk

Species that are considered uncertain risk need a more in-depth assessment beyond the Ecological Risk Screening Summary to better define the species’ risk to U.S. environments.

Hydrocharis spongia, American Spongeplant, is a plant that is native to the southeastern United States. It is sometimes a nuisance species within the native range, forming large mats that interfere with navigation and recreational usage. New Jersey lists the species as endangered. Although some sources report introductions of Hydrocharis spongia to Connecticut, New York, Kentucky, and Alabama, these disjunct populations failed to establish, and it is uncertain if they were the result of natural dispersal or anthropogenic introductions. Hydrocharis spongia is available in trade as an ornamental plant but there are limited data to characterize the volume and duration of this trade. Hydrocharis spongia is regulated in California. The History of Invasiveness for Hydrocharis spongia is classified as No Known Nonnative Population due to records of introductions outside of the native range, but no records of established non-native populations. The climate matching analysis for the contiguous United States indicates establishment concern for this species outside its native range. Locations with the highest climate match are found throughout the Southeast where it is native, southeastern Midwest, and Mid-Atlantic regions. The Certainty of Assessment for this ERSS is classified as Low due to uncertain and conflicting information regarding this species’ native and introduced range and taxonomic standing. Conflicting information on established populations and impacts of introduction is largely the result of some sources attributing introduced populations in California of the congener Hydrocharis laevigata to Hydrocharis spongia. However, most sources treat the two as distinct species. The Overall Risk Assessment Category for Hydrocharis spongia in the contiguous United States is Uncertain.