Alien Bullfrog

The bullfrog is the largest frog in North America. It has made a negative impact on native frogs.


Amphibians like frogs, toads, newts, salamanders and the limbless worm-like caecilians will live in the water but can survive on land. They are cold-blooded vertebrate animals of which there are about 2,500 known species in the world.

At least 53 species of birds have frogs, toads and salamanders as their favourite delicacy. Long-legged wading birds like herons and egrets feast on these amphibians. In fact, 34 frogs were found in the stomach of one little blue heron. The common merganser will ingest frogs and 5% of the belted kingfisher’s diet is frogs. Crows seems to be a voracious eater of frogs with a one-time consumption of 24 frogs. They also love salamanders.

The belted kingfisher consumes the large tadpoles of the bullfrog when it dives into the shallows of small ponds. Unfortunately, the kingfisher does not eat enough tadpoles to make a difference in the population of the Alien bullfrog, which not exists on Vancouver Island.

Bullfrogs are native to North America from southern Quebec and Ontario to the Gulf of Mexico. Farming of bullfrogs for their hind legs as food and the subsequent failure of these ventures have seen their introduction into western North American from California to southern British Columbia.

The Alien bullfrog (Rana catesbeiana) is successful breeders and can live up to 15 years. They begin breeding at the age of 4, with females lying up to 20,000 eggs in single spawn. These tadpoles grow to about 15 cm. over a two-year period before they turn into a frog. They breed in early May in permanent ponds and lakes in sub-urban, rural, disturbed and wooded habitats. Sometimes they move into stream habitats either as tadpoles or young frogs.

The bullfrog is the largest frog in North America growing 8 - 20 cm. long and weighing up to 600 grams. They are green to brownish green in colour with the breeding males having yellow under their chin. They have a conspicuous skin fold around the ear membrane down to the shoulder. The breeding males call the females with a very low-pitched drone. Their diet mainly includes insects, snakes, small mammals, birds, as well as smaller native frogs. They have been seen catching and swallowing birds up to the size of a woodcock.

This species of frog has made a negative impact on native frogs. Both the adults and tadpole were known to prey upon the tadpoles of smaller frogs and caused them to relocate into unfavourable habitats.

A monitoring study has been underway for the past few years to determine the location of the Alien Bullfrog and to prevent their further expansion. It is apparent their introduction has displaced native frogs in the lower Fraser Valley and southeastern Vancouver Island. Evidence shows they have made their way past Nanaimo.

To prevent their spread into new areas, the public must not move frogs, tadpoles or frog eggs from one wetland to another. This would mean young boys and girls may have to curb bringing home frog eggs in an ice cream pail from ditches, ponds or streams.

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