Pistia stratiotes L. in the Florida Peninsula: Biogeographic Evidence and Conservation Implications of Native Tenure for an ′Invasive′ Aquatic Plant

Pistia stratiotes L. (water lettuce) is a floating aquatic plant with wide pantropical distribution and the sole extant species of the Pistia genus. Fragmented knowledge about pre-modern Holocene dispersal and widespread observations of ecological invasiveness have made Pistia a classic example of a 'cryptogenic' species (i.e., indeterminately native or non-native) in much of its contemporary range. Questions about Pistia biogeography have likely received the most attention in North America's Florida peninsula, where the species is currently listed and aggressively controlled as an invasive non-native. However, emergent conservation concerns have prompted interest in resolving persistent uncertainties about this designation and associated management strategies. Towards this purpose, this paper develops a comprehensive and critical review of scientific literature pertaining to the Florida biogeography of Pistia. Remarkably, all claims in support of the non-native designation can be dismissed as scientifically and/or logically insufficient through hypothetico-deductive analysis. Conversely, a holistic synthesis of paleo-botanical, historical, and ecological evidence overwhelmingly points toward a native Florida presence for Pistia. Observations and associated ecological inferences further suggest that intensive Pistia control programs, which for many years have been implemented on an assumption of non-native status, may pose some conservation concerns for rare native biota in Florida's spring-fed streams. This case study joins several other studies indicating that due caution and research diligence should be employed when managing cryptogenic taxa in this time of global change.