Invading Bullfrogs Pose New Threat to Native Frogs

The sonorous night-time bellowing of North American bullfrogs is sounding an alarm to two University of Victoria biologists, who fear that the invasive giants may be spreading a deadly disease to native frog species.

A recent study published in the international journal Biology Letters has found that non-native bullfrogs are frequent carriers of the fungus Batrachochytrium dendrobatidis, which is lethal to some amphibians. The fungus, endemic to Africa, was introduced by the worldwide distribution in the mid-20th century of African clawed frogs, which were used for pregnancy tests. Although the bullfrogs are unaffected by the fungus, it appears to be rapidly wiping out many other amphibians in parts of the world.

“Serious declines have been documented in North, South, and Central America, Europe, and Australia and it’s happening fast,” says Dr. Purnima Govindarajulu, a co-author of the paper. She’s been researching the bullfrog invaders on Vancouver Island since 1997 for her PhD and post-doctoral studies at UVic. She and UVic biologist Dr. Brad Anholt have applied for funding to study the causes of the disease and develop measures for preventing its spread.

“We urgently need to find out what’s happening so that we can try and do something before it’s too late,” says Govindarajulu. “Vancouver Island has large populations of bullfrogs that are showing significantly high levels of the fungal infection. Some of the native frogs are also testing positive for the fungus but we’re not yet aware of any catastrophic declines. The time to understand the disease and prevent a crisis, if possible, is now.”

Govindarajulu has been mapping the spread of bullfrogs and testing them for the fungus in collaboration with provincial and federal biologists, and with an international team at the London Zoological Society and the Imperial College London. Some of the findings in the Biology Letters paper are based on data from Vancouver Island.

Bullfrogs were originally brought to B.C. in the 1930s and 1940s to be farmed for frogs’ legs. They’re now well-established in ponds and lakes in southwestern B.C.

Govindarajulu hopes that public awareness will support efforts to contain the bullfrogs and the spread of the fungus. People who see or hear bullfrogs can report their location to Frogwatch at www.env.gov.bc.ca/wld/frogwatch/index.htm, a provincial program that is monitoring the bullfrog situation.

“Amphibians are a crucial link between aquatic and terrestrial food webs,” says Govindarajulu. “Many of them are already high on endangered species lists. This [disease] adds another twist to their sad story.”

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