Some people say they sound like an old man snoring. I think they sound like someone dragging a stick across a large ribbed washboard, or maybe even a gator bellowing. But whatever you think they sound like, Carolina gopher frogs simply aren’t sounding off their characteristic low-frequency vibrating call in as many places as they used to. And where they do remain, there are just less of them. (Compare a gopher frog call here, with a gator bellow here.)
Carolina gopher frog (Rana capito) numbers mirror the downward trend line of so many amphibians worldwide, unfortunately. And their last habitat hold-outs in N.C. are on public lands and military installations. I wrote about Carolina gopher frogs, and the monitoring work that NC State Univ. Nick Haddad is doing to help them, in the Charlotte Observer a few weeks ago. You can read the story, “Rare frogs find a military home,” here. As the story states, only about 100 to 150 of the rare frogs are thought to live on Fort Bragg and they are listed as “threatened” by the state. On Fort Bragg, they have a deep affinity for the artillery impact zones on the base because fires sparked by munitions groom longleaf pine ecosystems which the frogs depend upon.
Scientists are still trying to untangle the specifics of this animal’s life history. They don’t know if they breed every year, for example, or if they skip a few years and breed on intervals. The frogs spend much of their lives holed up in underground burrows. When they emerge to breed, they hop to a nearby pond (often, they will travel up to a mile or so). They show strong site fidelity, returning to the same ponds year after year. At Fort Bragg, Haddad and several graduate students are working with the military’s Endangered Species Branch to monitor all the amphibians on the base with the goal of collecting baseline data. And so it was that I got to trail NC State Univ. PhD canidate Will Fields around one morning while he documented the amphibians coming and going from several ponds known to be used for gopher frog breeding. (Read the Observer story for particulars of his research.) Haddad says that of the seven years he’s been studying the frogs, he’s only seen them come out to breed in strong numbers once. The frogs need several nights of rain and warm temperatures in early March to spark their breeding response. They also need ponds that will stay of a sufficient depth until late May or early June. Their offspring mature so slowly, and are so large when they emerge from the ponds, that they require pools which stay deep and free of fish for several months. And here is where they run into trouble, because the ponds dry too early according to Haddad and Fields.
And actually, the “ponds” of Fort Bragg where the Carolina gopher frogs meet up for breeding are not really true ponds at all. They are “borrow pits” where the military has scraped away dirt for projects, and water pools seasonally. The seasonal aspect keeps the water pits free of fish which might eat young tadpoles, so that is good, but they don’t seem to stay wet long enough to allow gopher frogs to develop, which is a problem.
In addition to the litany of environmental things keeping frogs down (pesticides, insecticides, climate change, mosquito-spraying, habitat fragmentation and such), a herpetologist at the NC Museum of Natural Sciences in Raleigh says he believes the frogs historically bred in Carolina bays. Alvin Braswell told me that these unique geological features have been mostly lost. Carolina bays were elliptical, clay-lined depressions that held water for part of the year. Ninety-five percent have been ditched and drained, Braswell said, and their soils exploited for agriculture. But Braswell said that water in Carolina bays was typically in a narrow pH range (between 4.2 and 5.2), and he speculates that the gopher frog eggs require this pH range in order to develop properly. The pH of the borrow pits on Fort Bragg may be too low, which could kill the eggs, he said. Plunging groundwater tables could also be to blame. Lowered ground water, due to extractions by humans, could cause places that used to be temporal ponds to drain too fast and dry out too early for the Carolina gopher frogs’ metamorphs to develop, Braswell said.
“The cards are stacked against them,” Braswell told me. “Large tracts of intact land are this frogs hope. Its future is tied to public lands.”
When I last checked in with Fields last week, he told me that the ponds we’d visited in early March when I trailed him were on the verge of drying. One had already dried, and he was expecting two others to dry any day now. Part of Fields’ work is developing models for water-table depth that will help to predict areas the Carolina gopher frog might prefer for breeding. He’s used topography maps, elevational models and LIDAR imagery to figure out where water would pool on Fort Bragg. He drops simulated water all over the surface of a GIS layer in a model to predict where unknown pools may be on Fort Bragg. Then he and Haddad go out and search for them on foot. So far, they’ve not found any additional breeding pools.
The plight of the Carolina gopher frog saddens me. It’s discouraging to watch a species usable habitat dwindle in the onward march of human-modifications to the landscape. It’s discouraging to watch a species specialized to its environment be unable to adapt to changes. And it’s discouraging to know that human presence and land uses are to blame, but to feel like there is nought an individual can do about it. The Carolina gopher frog’s story also demonstrates that there is much that goes on in the natural world unseen by human eyes, and our human experience on earth will be enriched if we just stop to look, watch and understand other creatures.
Have you ever seen a Carolina gopher frog? Want to share your story? The comments section is always open.