Are Bullfrogs Invasive? American Bullfrogs in Europe and the West

Where bullfrogs are native and where they are invasive

The American bullfrog is native from the Canadian Maritimes and southern Quebec south to Florida, and west across the Great Plains toward the Rocky Mountains. Within that area it is an ordinary, valued part of the wildlife.

Everywhere else it was brought by people. The main reasons were frog farming and the frog-leg trade, plus stocking for food, accidental mixing with fish being stocked, and the sale of tadpoles and frogs for garden ponds and as pets. Today bullfrogs are established in:

• Western North America: California, Oregon, Washington, Nevada, Arizona, Utah, Colorado, New Mexico and other western states, southern British Columbia, and Hawaii.
• Latin America and the Caribbean: most of Mexico, Cuba, Hispaniola, Jamaica, Puerto Rico, and countries such as Brazil, Argentina, Uruguay, Colombia and Venezuela.
• Asia: Japan, China, Taiwan, South Korea and parts of Southeast Asia.
• Europe: Italy, France, Belgium, Germany, the Netherlands, Spain and Greece, with small populations found in the United Kingdom.

American bullfrogs in Europe

Italy has Europe’s oldest and largest bullfrog populations. Frogs were introduced in the 1930s, and they spread through the rivers, lakes and canals of the Po Plain around Mantua and Pavia, with other populations near Rome.

In France, a well-known introduction dates to 1968, when about a dozen frogs were brought from North America to an ornamental pond in the Gironde region near Bordeaux. They escaped and spread through the surrounding wetlands, and a second population was later discovered in the Sologne region of central France.

In Germany, the main population lives in the old Rhine backwaters and gravel-pit lakes of the Upper Rhine Plain north of Karlsruhe. Belgium and the Netherlands have scattered populations that probably started from released pond animals. In the United Kingdom, breeding bullfrogs were found in southeast England from around 1999, and they have been the target of government control efforts.

Because of these invasions, the American bullfrog has been on the European Union’s list of invasive alien species of Union concern since 2016. Under EU law it may not be imported, kept, bred, sold, transported or released anywhere in the EU.

Bullfrogs in the western United States

In the American West, bullfrogs were introduced from the late 1800s and early 1900s, often to supply frog legs. They thrive in warm reservoirs, stock ponds and irrigation canals, habitats that suit them better than the cool streams many native frogs need.

In California, bullfrogs are considered one factor in the decline of the threatened California red-legged frog, and they eat young giant garter snakes and western pond turtles. In Arizona and New Mexico they prey on native leopard frogs and garter snakes, and in the Pacific Northwest they compete with the Oregon spotted frog. Several western states now list bullfrogs as a harmful species and restrict moving or releasing them.

How bullfrogs harm native wildlife

Bullfrogs damage invaded ecosystems in several ways:

• Predation. As explained in What do bullfrogs eat?, adults eat almost any animal they can swallow, including native frogs, newts, small snakes, young turtles and ducklings.
• Competition. Bullfrogs and their tadpoles compete with native amphibians for food and space. Native tadpoles may grow more slowly or hide more when bullfrog tadpoles are present.
• Disease. Bullfrogs often carry the chytrid fungus Batrachochytrium dendrobatidis and ranaviruses without showing serious illness. Moving them around the world has helped carry these pathogens to frogs that have no resistance.
• Breaking food chains. Adult bullfrogs eat many of the animals that would normally eat bullfrog tadpoles, such as dragonfly larvae and water bugs, which makes it easier for the next generation to survive.

Because of this combined impact, the species is included in the list of 100 of the World’s Worst Invasive Alien Species compiled by the IUCN’s Invasive Species Specialist Group.

Why bullfrogs spread so successfully

Several traits make the bullfrog an unusually effective invader. A female can lay up to about 20,000 eggs in one clutch. Its tadpoles taste bad to many fish, so they survive in ponds where native tadpoles are eaten. It tolerates warm, nutrient-rich and disturbed water better than many native frogs. Adults are wary and quick to dive, and they can move overland between ponds on wet nights. Finally, bullfrog tadpoles can live for one to three years, which helps populations survive years when breeding fails.

Are bullfrogs ever a problem in their native range?

Generally not. Within eastern and central North America, bullfrogs evolved alongside predators, competitors and prey that keep their numbers in check, including large fish, herons, otters and snapping turtles. They can still be heavy predators in small ponds, but they are a natural part of those ecosystems.

The difference between native and invasive is about place, not the frog itself. The same species is protected wildlife in one region and a harmful invader in another, which is why moving any animal outside its natural range can have serious consequences.

How people can help

The most important thing anyone can do is avoid spreading bullfrogs further. Do not buy, move or release bullfrogs or their tadpoles, including into garden ponds, and never empty aquarium or pond animals into the wild. In Europe and the western US, wildlife agencies welcome reports of unusual large frogs or deep “jug-o-rum” calls, which are distinctive; see Why do frogs croak? for how frogs call. Managing invasive bullfrogs is a job for trained agency staff and conservation groups.