The American bullfrog is considered an invasive alien species in Europe. To speak of an invasive alien species, we mainly look at two things: 1) is the species able to establish itself and multiply here and 2) is the species responsible for nuisance or damage.
The American bullfrog was imported to Europe from America as a live fish stowaway for fish stocking, but was also commonly imported for aquaculture, landscaping, or even angling. In the early 1990s, bullfrog larvae were commonly found for sale in many pet stores and garden centers for keeping in ponds or aquariums. Tens of thousands of tadpoles have ended up in our regions in this way. But tadpoles develop legs, and the young bullfrogs have colonized the surrounding areas from the gardens where they ended up as tadpoles. This often involves fish ponds, recreational ponds and other nutrient-rich waters that are typically heavily disturbed by human activity. Some of the frogs and larvae were probably also moved by private individuals from their pond or aquarium to the wild. Since then, bull frogs have proven that they are more than capable of reproducing in Flanders, causing their range to have expanded considerably since their original introduction.
In its native range, the American bullfrog has natural enemies such as alligators, snapping turtles, American otters and raccoons. However, comparable enemies do not exist in our regions. Hence, bullfrogs that find their way into Flemish nature encounter little resistance and can reproduce unhindered. In addition, different stages of the bullfrog's life are apparently distasteful to many of our natural predators of frogs, although the tadpoles are occasionally eaten by predatory fish and waterfowl. Surprisingly, the only real bullfrog predator in our region is the bullfrog itself, as the species is highly cannibalistic. However, the impact of cannibalism on bullfrog populations is very limited, as a single female can lay up to 20,000 eggs.
Bullfrogs are a very large and voracious frogs. The species is known for having an insatiable appetite and their uncanny ability to eat just anout anything that fits into their mouths. Insects, but also frogs, toads, salamanders, fish, chicks of waterfowl such as moorhen and various duck species, mice and other small mammals are on the menu. When bullfrogs arrive in our regions they tend to occupy ponds where, due to their voracious nature, they quickly begin to dominate those ponds. The introduced bullfrogs swiftly start eating our native amphibians such as toads, green and brown frogs and various salamander species. In addition, young bullfrogs eat the same insects as our native amphibians, which creates further competition.
A bullfrog attempts to eat a fish it caught.
On top of eating our native fauna, the bullfrog is also a carrier of several diseases, including the chytrid fungus which is responsible for much of the worldwide decline in amphibian species. The species can also carries the rana virus which, much like chytrid, can spread rapidly and cause high mortality in amphibian populations. Bullfrogs themselves appear mostly resistant to by these pathogens, but our native species, such as the rare midwife toad and fire salamander, become infected through contact with infected water bodies and die from the diseases. In this way, the introduction of even a single bullfrog into a pond can cause a rapid decline in the local native frog populations.
For these reasons, the bullfrog has been included in the international and European list of '100 Worst Invasive Species'.
Top left: a microscopic section of frog skin infected with chytrid; Top right: a microscopic slide of frog tissue containing lots of rana virus; Bottom: a victim of chrytrid infection. New infectious spores emerge from deceased animals, infecting other amphibians through contact with infected water and wreaking havoc on amphibian populations.
Several large populations are known in Flanders, particularly in the Province of Antwerp. Especially in the valley of the Grote Nete, where the largest population of Flanders is located, the presence of bullfrogs is causing declines in native frogs and salamander populations. This large metapopulation expands their range upstream and downstream in the valley of the Grote Nete every year. Traditional control with existing techniques, such as placing fykes and draining ponds, are therefore no longer considered sufficient.