BRUNSWICK, Maine — It resembled something out of a science fiction movie, but the science behind a massive die-off of tadpoles in a local pond is shockingly real, according to a Bowdoin College biology professor.
An estimated 200,000 wood frog tadpoles died in a pond in his backyard over a 21-hour period in mid-June of 2013, according to Nathaniel Wheelwright, a population biologist who teaches at Bowdoin. The cause, according to a paper recently published in Herpetological Review that Wheelwright co-authored with researchers from the University of Tennessee, is a ranavirus, which is believed to be a significant factor in the global decline of amphibian species.
Wheelwright said there are no other known die-offs in which so many tadpoles died in such a short period of time. Wood frogs are an abundant species in Maine, and there has not been a repeat of the die-off this year — though there haven’t been any tadpoles, either — but Wheelwright is concerned about whether ranaviruses may spread in Maine and possibly put the amphibians at risk.
“It was very surprising,” Wheelwright said Monday. “It is a warning that you want to pay attention to.”
The dead tadpoles had signs of hemorrhaging in their legs and around their throats, and many had skin that was sloughing off their bodies, he said.
In a video about the die-off produced by Bowdoin College, Wheelwright says if that many people died all at once, it would be the equivalent of the population of “Hartford, Connecticut going belly-up overnight.”
Wheelwright stressed, however, the ranavirus does not pose a threat to human health. He said he went swimming last summer in the pond, unaware the ranavirus was killing the tadpoles, and has suffered no ill effects.
Wheelwright has been keeping tabs on frogs in the pond, which is about a half-acre in size and is more than 100 feet into the woods behind his house, for nearly 30 years. He estimated that the pond usually produces about 50,000 to 100,000 tadpoles each spring, of which he guessed maybe 1,000 live to adulthood and migrate into the nearby woods. No fish live in the pond, he said.
“It’s common to have high mortality [among tadpoles], but not 100 percent mortality,” Wheelwright said.
The estimated crop of 200,000 tadpoles in the pond last year was abnormally high, he said, possibly because no leeches were found in the pond. Wheelwright said he tries to bring a leech in to class each year, but couldn’t find any in the pond in 2013. Leeches, insects, other frogs and turtles are among species found in the pond that eat tadpoles.
The high concentration of tadpoles in the pond last year may have been a cause of stress to the tadpoles that contributed to the die-off, he said. But because the ranavirus killed the entire population so quickly, he added, it likely did not spread from one tadpole to the next but was introduced to all of them at roughly the same time.
Other species are known to carry ranaviruses without being significantly affected by them, according to Wheelwright. Green frogs, bullfrogs and painted turtles all can carry the disease and are known to frequent the pond, he said. Spotted salamanders, which also are found in the pond, showed signs of suffering from the ranavirus that killed the tadpoles, he said.
Wheelwright said he is not a disease specialist, but he said it is likely there have been other large-scale tadpole die-offs caused by ranaviruses in Maine that have gone undocumented. The spread of ranaviruses in Maine should be monitored somehow and, at the same time, the general public should avoid activities that could spread the disease, such as transporting plants, animals or other types of debris from one pond to another.
“My hope is that this will not be a perennial problem,” the professor said.
If the population of wood frogs started to decline in Maine, he said, it probably would not create a cascading chain of events that would result in any dire environmental consequences. But in a general sense, he said, it is important to keep track of why some species decline and others thrive, then to try to maintain some sort of balance if people hope to learn how to protect the environment as a whole.
Plus, Wheelwright added, he likes frogs. He would like to see tadpoles grow into frogs in his pond every spring.
“A world without frogs, for me, is a world that’s quieter and less interesting,” he said.