Invasion of the redlegged frogs

Submitted by Ned Rozell
Phone: 907-474-7468
03/06/09

In the early 1980s, a schoolteacher in a Southeast Alaska logging camp ordered a few clusters of frog eggs from a biological supply company. The teacher and his students succeeded in rearing those eggs into tadpoles, and then frogs. They enjoyed the frogs until school ended, when they released the frogs into a nearby pond.

From that act of seeming compassion came an invasion of Chichagof Island by redlegged frogs, a species Alaska hadn’t hosted before. The frogs have spread over the southeast portion of the island, and seem to be doing quite well.

Though they appear harmless as they hop through the island’s lowlands, the frogs could push out (by gobbling up) a native toad that makes its home on Chichagof Island. And that is the danger of exotics, said biologist Lance Lerum of the U.S. Forest Service in Klamath Falls, Oregon. Lerum spent more than a decade in Southeast Alaska before transferring to Oregon.

"My fear was that this introduced population will be another stressor to western toad viability," Lerum said over the phone from Klamath Falls. "In two years of field work, we found thousands of redlegged frogs, but we didn’t find a single western toad in our study area."

During a more recent trip, though, Lerum and his coworkers found 150 adult male western toads at one breeding site on the island. They still exist, but the newfound success of the redlegged frog is perhaps a threat to young western toads. Frogs will eat almost anything smaller than themselves.

Even though its Latin name, Rana aurora, seems to fit its new niche in the state, the redlegged frog is a visitor biologists wish had never come riding up in a cardboard box. And the redlegged frog’s spread over Chichagof Island may foreshadow a leap to other places in Southeast, facilitated not by their legs, but by people, especially little ones, who sometimes make temporary pets of frogs, take them home, then release them. Biologists can envision some frogs reaching different locales in Southeast via rides on the ferry.

Once a species is established and thriving, as is the redlegged frog, there is usually little we humans can do to reverse the process. In addition to out-competing or eating other animals, exotics can spread diseases new to an area. Introductions of non-native fish and game species are now illegal in Alaska, but prevention is the key, Lerum said.

"When (the release of the redlegged frogs) happened, it’s what people did," he said. "But now there’s a lot more awareness of the implications of ’bucket biology.’"

The redlegged frog, now enjoying life in Southeast Alaska, has suffered a bit due to habitat loss on different parts of its natural range, which extends throughout the Pacific Northwest to central British Columbia. This led Lerum and his colleagues Greg Pauly and Santiago Ron to conclude in a recent paper in the Journal of Herpetology that the redlegged frog’s introduction to Alaska may be a good thing for the species in the long run.

"As long as there is no evidence of significant ecological impacts or high likelihood of subsequent introductions, we suggest allowing this population to persist," the authors wrote. "This strategy may benefit future conservation of R. aurora. Given the global decline of amphibians and declines of all western North American (frog) species, it may be useful to allow these disjunct populations to persist as an insurance against future extinctions."