After years of failed attempts, a cohort of Oregon Zoo-reared northern leopard frogs has finally survived through the winter at the Columbia National Wildlife Refuge in eastern Washington.
“We’ve tried this for a few years and have not had any success in overwintering before,” said Patricia Valcarcel, an amphibian and reptile biologist with the Washington Department of Fish and Wildlife.
What changed this time?
The bullfrogs.
Invasive American bullfrogs, which are native to the eastern United States, have driven northern leopard frogs (and other native aquatic species) to the brink of extinction. The American bullfrog is three-times the size of a northern leopard frog and can eat the smaller frogs with ease.
So, before releasing the northern leopard frogs, biologists worked to control the bullfrog population by trapping their tadpoles and hunting adult bullfrogs at night. Most of the ponds at the Columbia National Wildlife Refuge are managed to control water levels, and the biologists were able to temporarily dry out the pond in order to kill bullfrog tadpoles.
Last year, more than 100 northern leopard frogs were released into a pond in the wildlife refuge. The frogs had been hatched from eggs collected by the Washington Department of Fish and Wildlife and were raised through their tadpole stage at the Oregon Zoo before being released as adult frogs with a better chance of survival.
In February and March, Fish and Wildlife biologists returned to the pond and counted 16 healthy frogs, identifying them by their spot patterns and color tags. It’s possible more frogs survived than were counted during surveys.
While 16 frogs may not sound like many, it’s a significant improvement for the species, which has been listed as endangered in Washington since 1999. Only one known wild population remains in the state.
“These first documented survivors represent hope for the recovery of the species,” Valcarcel said.
Historically, the northern leopard frog was found throughout most of Canada and the northern half of the U.S., but its numbers have been dwindling for a variety of reasons: habitat loss, disease, climate changes and — the big one — threats from invasive predators such as bullfrogs.
In areas where the bullfrog population has been culled, Valcarcel said biologists also have seen greater numbers of tiger salamanders and chorus frogs.
“This is great news,” said Jen Osburn Eliot, who oversees the Oregon Zoo’s frog-rearing efforts. “We care for these frogs at the zoo until they’re big enough to return to the wild, but without efforts to control the invasive bullfrog population they wouldn’t survive. This is a strong indicator that those efforts are working.”
Whether the program will continue is uncertain, Valcarcel said. Last year’s northern leopard frog reintroduction program was funded by a U.S. Fish and Wildlife Service competitive state wildlife grant, and President Trump’s proposed budget includes significant cuts to the federal agency’s conservation programs.
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