The story of European frog-bit’s introduction and spread in North America follows a pattern common to many invasive species: intended for cultivation in a controlled setting, they escape and spread rapidly at the expense of native species.
European frog-bit is an invasive aquatic plant that grows in stagnant or slowmoving bodies of water, infesting lakes, marshes, swamps, and streams. It has become a dominant species in many wetland ecosystems in eastern North America.
Found widely in Europe and certain parts of Asia, European frog-bit was first imported into Canada from Switzerland in 1932. It was cultivated at the Arboretum of the Central Experimental Farm in Ottawa. By 1939, the plant escaped the Arboretum and wild populations were spotted on the Rideau Canal. By 1952, it had reached the Montreal area, and was observed on Lake Ontario and Lake Erie by the 1980s.
Invasive species are successful because they possess traits that give them a competitive advantage in a new environment. European frog-bit has submerged roots and horizontal stems called “stolons.” Rather than embed in the sediment at the lake bottom, the roots intertwine underwater, allowing the plants to aggregate in dense, free-floating colonies. This ability for dense growth is the plant’s main advantage. Unfortunately, the thick layer of foliage formed on the water’s surface reduces the amount of light, nutrients, and dissolved gases available to native plants growing under the surface.
Besides monopolizing vital resources, European frog-bit is also able to spread rapidly due to its reproductive strategies. Typically, the species reproduces asexually as new plants grow out of the tips of the stolons. This mechanism allows entire colonies to establish when pieces of stolon— which naturally break off—are carried to new locations by water currents. This kind of reproduction, also known as “vegetative reproduction,” is a characteristic of many invasive species. It offers a competitive advantage, allowing new individuals to produce at a much faster rate compared to sexually reproductive plants.
In the autumn, European frog-bit also uses a variation of asexual reproduction specific to aquatic plants, in which specialized buds known as “turions” are formed. The turions fall off the plant and sink to the sediment below, where they spend the winter in dormancy. Once spring arrives, the buds float back towards the surface and start developing into new plants. Since up to 100 turions can be released from a single adult individual, turion production contributes to the species’ high reproductive rate, helping it quickly take over a particular area.
Currently, there are no known control measures to combat the European frog-bit invasion, and it is extremely difficult to destroy an established colony. The most effective way to stem the spread of the species is to prevent the extent of new populations. Like zebra mussels, European frog-bit can reach new locations by attaching itself to boats and other watercraft. Cleaning watercraft before moving between bodies of water is an important tactic used to control outbreaks. Waterways and wetlands should also be monitored regularly for new population outbreaks, and any frog-bit that appears should be quickly removed and correctly disposed of, ideally before autumn, so that turions do not have the chance to form and disperse.
