Silent Night? Yeah, Right: Surveying Frogs in Hillsborough County Nature Preserves

Frogs are hard to find and catch, so we're surveying them by listening to their nocturnal calls!

If you think of the night as a quiet, peaceful time, you probably haven’t done much night hiking.

Hillsborough County’s Nature Preserves are absolutely chock full of exciting sounds after dark (you can find out yourself by signing up for one of our guided night hikes, which are advertised in the Trailblazer newsletter each month and on our Community Calendar. Guided hikes will resume after the Safer-at-Home Order is lifted, when the county administrator allows events to resume). Some of the sounds, like bat calls, are harder to hear, but crickets, cicadas, owls, and frogs are all easily detectable by the human ear, and can even be deafeningly loud in some places.

It’s a good thing for the Environmental Lands Research Analysts Team (EL-RAT) that frog calls are so audible. EL-RAT is currently surveying frogs in nature preserves, and these small creatures are extremely difficult to find and catch. Not only are they quick and agile, but many of Florida’s frogs are also extremely well-camouflaged and difficult to see even if you’re looking right at them (see below for two excellent examples). Rather than try to find these elusive amphibians, Bernie Kaiser, Gillian Seymour and Rose O’Donovan, the members of the EL-RAT team, are using battery-powered acoustic monitoring meters to listen for frog calls. These meters record continuously from 30 minutes before sunset to 30 minutes after sunrise, taking in the whole spectrum of night noises. 

Well-camouflaged oak toad, Anaxyrus quercicus 

American bullfrog, Lithobates catesbaeianus, hiding in duckweed

Luckily, the team doesn’t have to listen to hundreds of hours of audio and pick out each individual frog call. They’re using Kaleidoscope, an amazing program that we introduced you to a few months ago in our bat surveying article, to analyze the audio data. The team can change Kaleidoscope’s settings to filter out certain sound frequencies and sound lengths, removing background sounds like droning cicadas and crickets. Kaleidoscope then clusters similar sounds together and creates a spectrogram, or visual representation of the sound.

Kaleidoscope is actually pretty great at identifying different bat calls (up to a point), but it’s not able to auto-identify frog calls because there’s so much noise clutter that gets in the way. That’s why the EL-RAT team has to ID each cluster using their frog call knowledge, with a little help from the Frog Listening Network and the Florida Museum’s Florida Frog Calls website. 

Pig frog, Lithobates grylio, call spectrum

Southern cricket frog, Acrus gryllus, call spectrum

Kaleidoscope has allowed the team to do monitoring and research that would be nearly impossible without technological assistance. The program makes it possible to go through massive amounts of audio data, allowing users to process over 200 hours of recording in 5 or so hours of work – still a good amount of time, but vastly more efficient! Some areas have very few frog calls (depending on the time of year); if a human was listening to and processing all that audio, they’d probably miss these short and sporadic calls, but Kaleidoscope has no trouble picking them out. 

EL-RAT are using the data they collect to add to the species list for each preserve, and so far they’ve identified ten frog species that have never before been documented on any of the several preserves they've monitored. One of the advantages of audio surveying is the ability to hear frogs who live in different habitats all at once; water-dwellers like the pig frog, tree-dwellers like the green tree frog, and burrow-dwellers like the eastern narrow-mouthed toad are all represented in the audio data, and researchers don't have to go hunt them down in their various habitats to survey them. Frogs may be the focus of the survey, but the meters are also picking up some incidental owl calls, coyote songs, and even early-morning bald eagle noises! 

Pig frog, Lythobates grylio (Ken and Nyetta / CC BY 2.0)

Eastern narrow-mouthed toad, Gastrophryne carolinensis (Wilafa / CC BY-SA)

Frogs may be small, but they’re incredibly important as indicator species; their presence can tell us something important about habitat health and quality. Amphibians have permeable skin and are very sensitive to many pollutants and to changes in their habitat. Many frog species also rely on ephemeral wetlands and seasonal ponds to survive; these shallow depressions fill with water in the spring and are integral to the health of many plant and animal species. When ephemeral wetlands aren’t burned enough, duff and leaves collect and fill these depressions in, and an entire miniature ecosystem vanishes. The presence of frog species that rely on ephemeral ponds indicates that the land is being burned enough to keep those ponds and wetlands from filling in. One of these species, the gopher frog, has become extremely rare in Florida and is now listed as a species of special concern. Gopher frogs need healthy, well-maintained ephemeral ponds to breed and lay their eggs in, and habitat degradation due to fire suppression has wreaked havoc on these breeding ponds. Gopher frogs are good indicators of well-maintained upland habitats; they haven't shown up in on a sound meter survey yet, but EL-RAT is hoping to hear one soon. They’re also listening for a less desirable frog species, the invasive Cuban tree frog, which preys on some of Florida’s native frogs. 

The EL-RAT team will continue to survey frogs at different nature preserves in the future, collecting species data from year to year and looking at changes in species composition over time. Analyzing changing frog species data will help us understand how prescribed fire and our other land management activities are impacting these sensitive habitats and help us keep our nature preserves (and our frog populations) healthy and diverse!

Gopher frog, Lithobates capito (Kevin Enge/ CC BY)