Outstanding in the Field (Ep 4): Amphibian Surveys – Call of the Frog

[Intro music fades out]

Narrator: Welcome, and thanks for listening to another episode of Outstanding in the Field, the U.S. Geological Survey’s podcast series produced by the Ecosystems Mission Area. This series highlights our fun, fascinating, and hoppin fieldwork studying ecosystems across the country. I’m Marisa Lubeck.

Narrator: Today’s story is about USGS’s efforts to track frog populations in the southeast United States.

Have you ever walked outside in the evening and heard something like this?

[sound clip – frog chorus]

Narrator: To the untrained ear, it might sound like a lot of senseless noise or the primal calls of nature at night. But to Hardin Waddle, a research ecologist at the USGS’s Wetland and Aquatic Research Center in Gainesville, Florida, it’s a symphony of frog calls.

Hardin Waddle: “Sometimes it can be deafening. I’ve been out in the swamp before where I was in a huge chorus of squirrel tree frogs, which they’re called that because they sound a little bit like an angry squirrel when they’re calling. And it was so loud, it was just hurting my ears. It was just amazing. And then I’ve been out where you hear bullfrogs calling and they get in a large chorus and you can feel it vibrating in your chest because of that low frequency. So, I love that.”

Narrator: Frogs – like other amphibians - live a dual existence, partly on land and partly in water, so they play an important role in worldwide food webs.

Waddle: “Frogs are also important to society in a lot of ways maybe people don’t recognize. For instance, they eat huge amounts of insects, collectively, put it all together, and the biomass of insects the frogs consume has a big effect so it reduces a lot of those pest insects that might be around your house.”

Narrator: Frogs are also great indicator species, meaning they can offer insight into the health of their environment. Frogs breathe and drink through their skin. This permeable skin means they easily absorb anything in the environment that shouldn’t be there, like pollutants and toxins.

Unfortunately, declines in amphibian populations have occurred worldwide. Habitat destruction from deforestation and development is a large cause for such declines, but there are other potential threats that may play a role. Contaminants, disease, and invasive species are just a few. The USGS Amphibian Research and Monitoring Initiative, or ARMI, is just that, an army of scientists trying to understand and help managers slow or stop amphibian population decline. 

Hardin and his team are part of ARMI and have been studying frog populations in Texas, Oklahoma, Arkansas, Mississippi, and Louisiana since 2007. Identifying frogs by their calls is one way they track the region’s populations.

Waddle: “One of the things that we like about frog vocalizations, frog calls, is that each one is unique to the species. So, we can use frog calls to help identify which species are there without having to walk around and catch each one. We use it in a couple of different ways. One is to look for rare species or species that we’re concerned about getting into an area, for instance, the Cuban treefrog, which is an invasive species that can have a big effect, a negative effect, on the native frogs because it eats them and competes for the same resources, the same food. And then, we also use it to monitor the diversity of species, and also look at the timing of calls, what we call phenology, so when things start calling and stop calling and how that changes with environmental conditions and over time.”

[sound clip – Cuban treefrog]

Narrator: Cuban treefrogs are native to Cuba, the Bahamas, and the Cayman Islands, but are an invasive species in the U.S. The treefrogs were first discovered in Florida in the 1930’s and were later introduced to Puerto Rico and the U.S. Virgin Islands. In 2018, Hardin and fellow USGS biologist Brian Glorioso discovered a population of the invasive treefrog in New Orleans, more than 430 miles from the nearest known population in Florida. Cuban treefrogs outcompete native frogs for food and habitat. They’re also a real nuisance to homeowners, clogging plumbing and causing power outages when they take shelter in utility boxes.

[fade in – Cuban treefrog]

Waddle: “It’s not a particularly pleasant sound. It sounds a lot like a…I think of a rusty hinge, kind of squeaking open or a squeaky door or something like that.”

[fade out – Cuban treefrog]

Narrator: Hardin’s team collects these frog calls by placing special recording devices in areas of interest, like near ponds on Bogue Chitto National Wildlife Refuge in Louisiana. The devices record day and night for a couple months, giving researchers an opportunity to collect continuous, long-term data without having to sit next to these ponds for months on end. The recording devices are collected and brought back to the lab where special software is used to identify what species calls were recorded.

While frog calls may be helpful to scientists, they first serve a biological purpose.

Waddle: “Frog calls, frog vocalizations, are primarily made by males and they’re usually used to advertise to females so they’re trying to attract a mate generally. And the calls are pretty interesting because there’s information in the call that the female can understand. So, the frequency of the call relates to the size of the male and so females actually – generally, in frogs – have female choice for mates and they can select the mate that has the call that represents the bigger body size.”

Narrator: The frog call data can tell us if an invasive species, such as the Cuban treefrog, has moved into an area for the first time. This gives managers an early warning that this invader is present and helps to inform management decisions on how to respond.

Waddle: “We know that amphibians are declining around the world, so any tool that we can use to help make it more efficient to sample amphibians and to understand what’s going on with our local populations is something we want to try.”

Narrator: Citizen scientists can get involved in identifying frog calls too. Like birders who use bird calls to identify species, frog enthusiasts can easily learn how to distinguish between frog species’ unique calls.

Waddle: “I would start by looking up what frog species live in your area and then getting those calls and then listening. It’s fun. I think a lot of people will hear frogs in their backyard if they pay attention.”

[fade in – Bullfrog chorus]

Narrator:  This podcast is a product of the U.S. Geological Survey, Department of the Interior. Podcast artwork is by Jeffrey Kemp, and our theme music is by USGS scientist Marty Fitzpatrick.

[fade out – Bullfrog chorus]

[fade in – music]

A special thanks goes out to Hardin Waddle from the USGS Wetland and Aquatic Research Center and the rest of the Outstanding in the Field team: Suzanna Soileau, Hannah Hamilton, Sue Kemp, and Catherine Puckett. I’m Marisa Lubeck. Thanks for listening.