Water lettuce, Pista stratiotes: Native…Invasive…Or does it really matter?
By Dr. Jason Ferrell, UF/IFAS Center for Aquatic and Invasive Plants Director
Water lettuce is an extremely common floating plant in Florida. This leafy plant can be observed from north to south and has the tendency to grow and reproduce rapidly, form very dense mats and cause numerous problems. Because of these issues, water lettuce is one of the most commonly targeted plants for management in state waters. The guiding principle behind aquatic plant management is to reduce the population of invasive plants to low levels and promote the growth of native plants. The controversy lies in the fact that experts disagree on whether water lettuce is native or invasive. Some excellent genetic data seems to imply it originated from the near east region many millions of years ago and ancient fossil records show the plant occurring in India, Germany, and other European locations. Conversely, both John and William Bartram clearly documented this plant in Florida in the 1700’s and fossil records have also been found in Wyoming. So, which is it? Native? Invasive? Or does it really matter? To discuss this, let us start with an understanding of what invasive plants really are.
Invasive plants are not just troublesome, they are the most damaging plants in any ecosystem. By definition, an invasive plant originates from a foreign habitat and is known, or likely to cause, environmental or economic harm or harm to human health. Researchers have suggested said that second to habitat loss, invasive plants are the greatest threat to natural environments and biodiversity. This sentiment is believed because invasives have the tendency to take over and dominate ecosystems (e.g. environmental harm). Conversely, native plants originated in a given ecosystem and are often observed to coexist with a great diversity of other species and provide many ecosystem services.
Based on these definitions and the desire to promote biodiversity, biologists and ecologists have successfully promoted the concept that invasive plants are bad and native plants are good. It is a simple, clear-cut message. Although this message is generally true, reality is often more nuanced than this. Are all invasive plants 100% bad with no redeeming qualities? Well, not necessarily. Certain invasive plants can be used for bioenergy production while others are able to support native pollinators. Can some native plants be weedy and troublesome?
Sure. Crabgrass seems to be present in lawns and sidewalks in every neighborhood and poison ivy is a common problem for most all nature lovers. Rather than just seeing plants in terms of native = good and invasive = bad, an additional concept should be considered: the nuisance species. A nuisance plant is one that causes management issues, possesses a threat to public safety or is an annoyance. Nuisance plants often require management and some native plants fall into this category. These also include Carolina willow and cattail which often require management in aquatic systems.
So back to water lettuce. Is it native or invasive? Honestly, this fact is unknown and may never be known. A better question is this: is water lettuce a nuisance species? To me, this is much clearer and easier to answer. Let us circle back to Bartram, where he describes the plant like this:
“It is remarkable that at the entrance of the river into the great lake there floats prodigious quantities of the pistia, which grows in great plenty most of the way from hence to the head of the river, and is continually driving down with the current, and great quantities lodged all along the extensive shores of this river and its islands, where it is entangled with a large species of water-numularia, persicaria, water-grass, and saxifrage…growing all matted together in such a manner as to stop up the mouth of a large creek, so that a boat can hardly be pushed through them, though in 4 foot water; these by storms are broke from their natural beds and float down the river in great patches…”
Based on Batram’s description, an infestation of this size can easily harbor millions of mosquito larvae, impede water flow and increase the potential for flooding, while also shading out acres of desirable submersed plants – thus reducing biodiversity. So, is water lettuce native or invasive? We don’t know. Is it a nuisance species? Clearly. Should it be managed to reduce the impact on the environment? For a plant capable of such massive proliferation, management is essential.
This naturally sets up the next question. If water lettuce has been in Florida since Bartram’s days when no management was conducted, why does it need to be managed now? Should nature take care of itself? These are excellent questions, but the answers will have to wait for the next article.
Dr. Jason Ferrell, UF/IFAS CAIP Director, wrote this piece. Any questions should be directed to Shelby Oesterreicher at firstname.lastname@example.org. For more information about the UF/IFAS Center for Aquatic and Invasive Plants, please visit http://plants.ifas.ufl.edu. Be sure to follow us on social @UFIFASCAIP.
UF/IFAS CAIP, Turning Science Into Solutions.
 Susanne S Renner, Li-Bing Zhang, Biogeography of the Pistia Clade (Araceae): Based on Chloroplast and Mitochondrial DNA Sequences and Bayesian Divergence Time Inference, Systematic Biology, Volume 53, Issue 3, June 2004, Pages 422–432, https://doi.org/10.1080/10635150490445904
 Diary of a Journey through the Carolinas, Georgia, and Florida from July 1, 1765, to April 10, 1766. – John Bartram and Francis Harper. P 39. Transactions of the American Philosophical Society, Vol. 33, No. 1 (Dec., 1942).