an adult gray tree frog
An adult gray tree frog, the species of frog that colonized the artificial ponds that were kept in the dark.

Shining a light on amphibian decline

New research links excessive artificial light to developmental issues in toads; possible connection to worldwide amphibian crisis

A pair of researchers at Case Western Reserve University are suggesting that excessive, human-caused artificial light at night may be contributing to a worldwide amphibian population decline.

Experiments in 2015 by Case Western Reserve University associate professor of biology Michael Benard and then-PhD student Kacey Dananay revealed a dramatic decline in growth among juvenile toads exposed to increased light at night. The results of those experiments were published online July 4 in the science journal Proceedings of the Royal Society B.

Some researchers have reported that nearly one-third of amphibians—a unique group of vertebrates containing over 7,000 known species such as frogs, toads and salamanders—are threatened worldwide and that more than 40 percent are in decline. Further, more than 150 species are believed to have gone extinct in the last few decades.

Much of the research into declining amphibian populations has focused on climate change generally, chemical pollution or, most prominently, an ancient skin-eating fungus called Batrachochytrium dendrobatidis.

The research by Dananay and Benard brings something new to that conversation: Artificial Light At Night (ALAN), a “widespread pollutant that has the potential to disrupt organisms’ behavior, growth and development,” Benard said.

“Until now, the negative effects of ALAN on amphibians have not been recognized,” said Dananay, lead researcher in the project. “The main finding of this paper is that we are seeing that this artificial light alters the behavior of the toads, particularly in the next life stage, the juvenile life stage.”

photo of the artificial ponds
Some of the artificial ponds, made from a metal tank for feeding livestock, used by the researchers.

Benard added that even the local amphibian population preferred darker ponds.

“Our study found the presence of ALAN near artificial ponds altered how tree frogs colonized those ponds: tree frogs preferred to colonize dark ponds, especially when other amphibians were present,” he said. “We also found the presence of ALAN in terrestrial habitats caused young toads to be more active at night, and to grow more slowly.

“So we’re getting smaller frogs, and as a frog, you don’t want to be small. You want to be a bigger adult in the long run to increase your reproductive success.”

The experiments on the American toad species (Anaxyrus americanus) were conducted both at Case Western Reserve’s University Farm east of campus, then in a lab on campus in Cleveland once the tadpoles had developed into juveniles.

photo of a tadpole
An American toad tadpole.

“There was a 15-percent decrease in mass for the toads who metamorphosed from tadpoles in a pond with artificial light,” said Dananay, who just completed her PhD and is about to start a position as an assistant professor at West Virginia Wesleyan College.

“We think toads exposed to ALAN are burning energy by  being more active at night,” Dananay said. “Under the darkness of night, we see the toads go under the leaf litter and rest.”

The experiments began with 50 tadpoles placed in 20 artificial ponds made from cattle troughs, half left in the natural dark of nighttime and the others exposed to a relatively small amount of artificial light.

After about a month under those conditions, once the tadpoles had grown into juvenile toads, eight animals from each tank were brought to the university labs and monitored further as they developed.

“This is the first-ever post metamorphic study juvenile toads exposed to ALAN,” Dananay said. “Other studies have looked only at tadpoles or the effect of other stressors such as predators on the juvenile animals.”

For more information, contact Mike Scott at

This article was originally published July 6, 2018.