Monitoring Frog & Toad Populations?

Look on Land and in the Water

Eastern narrow-mouthed toad
Eastern narrow-mouthed toad larvae develop very quickly – within 8 weeks. Photo by Alessandro Catenazzi, courtesy of Wikimedia Commons.

Over the past few decades, scientists have become increasingly concerned about amphibians. “Populations of many frog and toad species have declined,” says U.S. Forest Service research ecologist Katie Greenberg. “The global decline highlights the need to monitor frogs and toads where they live.”

Greenberg has been doing just that for 24 years. Since 1994, Greenberg and her colleagues have monitored frogs and toads at eight ephemeral wetlands in the Ocala National Forest in Florida.

Ephemeral wetlands are fish-free places for frogs and toads to lay eggs. However, even in ephemeral wetlands there are many perils. For example, aquatic insects feed on eggs and tadpoles, and wetlands sometimes dry out before the tadpoles complete their development and transform into juvenile froglets or toadlets.

For most frogs and toads, their relationship to water defines their life. Almost all amphibian species lay their eggs in the water. The eggs hatch in water, and as they develop, the larvae swim, eat, and breathe in water. Eventually, they metamorphose and the juveniles begin a new life, on land. After maturing, they return to the water to mate and lay their eggs.

Listen to a brief audio clip by author Katie Greenberg describing this publication. • Text Transcript

“We sampled adults and juveniles on the land, and tadpoles in water,” says Greenberg. “We compared both methods to see how well they detected species presence.” The scientists also assessed breeding success – did breeding eventually produce healthy young froglets and toadlets?

Greenberg and her colleagues used drift fences with pitfall traps. The traps were positioned around the eight wetlands to capture breeding adults. The traps also captured juvenile amphibians that were leaving after completing development from tadpoles. The drift fence sampling is part of an ongoing, long-term study of population dynamics in relation to climate and wetland hydrology.

For this study, the scientists also used box traps to sample tadpoles. Every three weeks for a two year period, the scientists visited the wetlands to trap tadpoles.

The results were published in the Canadian Journal of Zoology.

Sometimes, a tadpole species would be detected one visit, but not the next, and reappear at a later date. Or, it would be present at one wetland but not another on the same sampling date.

“We suspect tadpole mortality is very high,” says Greenberg. “This would explain why detection or presence of larval amphibians ‘blinks’ on and off.”

High larval mortality would also explain another finding – the number of tadpoles did not predict the number of juvenile frogs or toads. Dragonfly nymphs, diving beetles, and other aquatic insects probably killed most of the eggs or tadpoles.

study site
The ephemeral wetlands are located in the Ocala National Forest, FL. Photo courtesy of Katie Greenberg.

In addition, ephemeral wetlands exist fleetingly and sometimes dry up before the tadpoles have metamorphosed.

Greenberg and her colleagues observed this firsthand. “One of the wetlands dried out right before tadpoles metamorphosed,” says Greenberg. “The entire cohort almost surely died.”

The results suggest that occasional aquatic sampling may detect some species but miss others entirely.  When possible, the scientists recommend sampling frequently and including more than one wetland.

“Terrestrial and aquatic sampling methods are complementary,” says Greenberg. “Terrestrial sampling doesn’t explain changes in tadpole abundance or mortality. Aquatic sampling doesn’t reflect the number of breeding adults – or even if breeding occurred at all.”

Tracking population trends – and the continued presence of amphibians in the places they live – is especially important.

“Effective monitoring programs must use complementary methods,” says Greenberg. “Sampling should also occur often enough to detect species and track them over time.”

The Florida Fish and Wildlife Conservation Commission funded the study.

Read the full text of the study.

For more information, email Katie Greenberg at

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