In areas of its natural range, floating pennywort colonies are often considered to be desirable components of aquatic ecosystems, and are commonly sold as ornamental pond plants (3). Although the floating pennywort is considered to be endangered in some north-eastern states of the USA, in many countries where this species has become established it is now considered to be invasive, including in the UK, France, Germany and Belgium (2). In Australia, this species is capable of doubling its biomass in just three days (4), and in southern and western regions of the country it is a government-listed noxious weed (3).
Once established, the floating pennywort can cause major problems in nature reserves, recreation areas and managed waterways (2). This invasive plant competes with native species, which can become shaded out by the floating pennywort’s extensive mats of vegetation (2) (5). It has been reported that the species richness of native aquatic plants may be reduced by a worrying 50 percent due to the presence of floating pennywort, and submerged species may be extirpated entirely (2).
In addition to blocking out light which plants need for photosynthesis, the floating pennywort also reduces the amount of oxygen present in the water (2) (5), which in turn can lead to fish mortality (2). Other environmental impacts as a result of the spread of the floating pennywort include the disruption of the movement of animals, an increase in the prevalence of mosquito breeding areas, and increased nutrient loads in the water (5). Due to causing die-off of other aquatic plants as well as animals, the floating pennywort can facilitate the infilling of shallow waters, as decomposing material gathers on the bottom of the water body and alters the substrate composition. This can affect the drainage of flowing waters (2).
Once introduced, the floating pennywort is capable of thriving in a wide range of conditions, and is able to spread rapidly (2). Its creeping nature has led to problems in irrigation and drainage ditches (3), with reduced drainage having a negative knock-on effect on agricultural lands within the floodplain (2). Additionally, floating pennywort has a negative impact on tourism, fishing and water sports (5), with some areas potentially becoming unsafe for recreational activities (2).
A major concern with the floating pennywort is the ease with which the species can be spread. Any activities which lead to the fragmentation of the species, such as boating or water sports, can facilitate the expansion of the floating pennywort’s range (2). Small fragments can be carried downstream along waterways, or upstream attached to boats (5), while flooding enables the species to become established widely in river valleys (2).
It is feared that, with its proven ability to re-grow from small fragments and rapidly establish itself in a variety of aquatic habitats, the floating pennywort is likely to spread further around the UK, posing a major threat to the ecology and use of water bodies (4). There are also concerns that increasing temperatures in Europe will enable this species to thrive and become more invasive in the central and northern parts of the continent (2).