Mass Die-off of Wood Frog Tadpoles Concerns Scientists

Mass Die-off of Wood Frog Tadpoles Concerns Scientists

BRUNSWICK, Maine (NEWS CENTER) -- Every spring, thousands of wood frogs make their way to Nat Wheelwright's half-acre pond to reproduce.

"These frogs are amazing," stated Wheelwright, a biology professor at Bowdoin College. "Most people don't even know that they are mating right in their backyards in early April if you live in a rural area."

The frogs leave behind hundreds of thousands of eggs which hatch into tadpoles, and over the course of the summer, those tadpoles mature into wood frogs.

"Life is hard for a tadpole," he explained. "Probably only one in a hundred makes it out of the pond."

Last year, he estimates there were hundreds of thousands of wood frog tadpoles in his small pond - which is unique because it has no fish in it.

Then, one day in mid-June, things changed.

"I've never seen a population go just go belly-up so quickly," said Wheelwright.

All of the wood frog tadpoles died.

"Emotionally, I thought, oh no, what else is happening in the world," recalled Wheelwright. "But I also thought, as a scientist, that this is an interesting scientific problem. What could have cause a catastrophic die-off of 200,000 tadpoles in a 21 hour period."

He shipped some samples of the dead tadpoles off to colleagues at the University of Maine and the University of Tennessee to see if they could help uncover what had occurred.

"What they found is that the tadpoles had very, very high, unusually high levels of something called a ranavirus," he explained.

Wheelwright says ranavirus is specific to mostly frogs and salamanders, but can also infect some fish and reptiles. He says there is little risk of it jumping from frogs to humans.

"What we do know is that this is a fairly new disease," he added. "It was first described in the late 1960's and first seen in the wild in the late 1980's."

What Wheelwright finds most interesting is that the virus was lethal to the entire population of wood frog tadpoles at the same time. He says he is not sure what triggered the mass die-off, but suspects some kind of stressor, such as high temperatures, the introduction of pesticides or a lack food is to blame.

He says his pond supports about five or six other species of frogs and toads, and none of them seem to have been as adversely impacted as the wood frog tadpoles.

He says the adult wood frogs returned to the pond to mate again this spring, but he has not seen any of their tadpoles reach maturity this summer.

"If this were to happen for three, four or five years in a row, then we would see a consequence, at least for this population," he said.

He is not certain if this is an isolated event, or if it is happening in other ponds and vernal pools, but is concerned that wood frog populations could be in danger.

"It is not unthinkable that conditions could be such that wood frogs could disappear," said Wheelwright.


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