A re-evaluation of the alien status of Pistia stratiotes L. (Araceae) in Japan based on literature and specimen records

Abstract

Pistia stratiotes, a free-floating aquatic plant of the Araceae, is regulated under the Invasive Alien Species Act of Japan as a highly invasive species. However, the species has been historically recorded from the Ryukyu Archipelago, where it is a doubtful alien species. Here, I re-evaluated the alien status of P. stratiotes through a review of literature and specimen records. The oldest distribution records of P. stratiotes in Japan were based on specimens and an 1854 manuscript by C. Wright, who recorded it as common in paddy fields. Prior to the 1950s, several researchers recorded that the species was distributed from Okinawa Island southward to the Yaeyama Islands, and grew primarily in and around paddy fields, coexisting with many native aquatic plants. Furthermore, several authors treated P. stratiotes as native, and none treated it as exotic between 1890 and 1950. The earliest opinion of P. stratiotes as invasive was in a specimen label described by E. Walker et al. in 1951. Since the 1970s, the treatment of P. stratiotes as exotic has become common without any scientific evidence. Horticultural use of the species began in the 1930s and became commonplace by the 1950s. By the 1970s, the species had begun to naturalise on the Japanese mainland. All Japanese populations of P. stratiotes may have been determined to be exotic without sufficient evidence because older information, such as unpublished manuscripts, was difficult to access until recent digitisation made it open to the public; and researchers’ judgment may have been biased because of several pieces of information, such as the hypothesis of the species’ African origin. Based on the above, the populations of P. stratiotes that have been found in the Ryukyu Archipelago since before the 1850s cannot be considered exotic based on available scientific evidence, and recent molecular phylogeographical findings suggest that they may comprise a natural distribution. These populations may be threatened by shrinking habitat and by competition and hybridisation with the introduced lineage of P. stratiotes. For appropriate regulation under the Invasive Alien Species Act, taxonomic studies and methods to distinguish these P. stratiotes lineages, as well as an understanding of their current habitat conditions, are needed.