It's Not Easy Being Green: A Tale of Texas Frogs
Frogs play a key role in ecosystems, and we are just beginning to understand how they have adapted.
By Wendee Holtcamp
It’s a sunny, blue-sky Texas day in late October and I’m about to go frogging with Dan Saenz, U.S. Forest Service wildlife biologist in Nacogdoches. A Gary Larson Far Side cartoon with a banjo-playing frog adorns his door. The frog is singing something to the effect of: “My baby’s left my lily pad, I’ve got the greeeens, oh baby, I’ve got the greeeens.”
There are cat people and dog people. Then there are frog people. More than 15 years ago I first went frogging in Australia with a self-described “frog freak” and we scoured the reed marsh looking for frogs. My son was born a frog person. Since he was a tow-headed two-year old, he has been catching frogs in our East Texas yard — little ones, big ones, green ones, brown ones. And it appears that Saenz is a frog person too. “I always knew what I wanted to do when I grew up,” he tells me.
“And what is that?” I ask, as if it weren’t obvious.
“I knew I wanted to be a scientist and study frogs. I was always bringing them home — it drove my mom crazy!”
“I was sure my now 7-year-old son would want to be a wildlife biologist and study frogs,” I say. “But when I asked him, he said he wants to be a cartoonist!”
“Gary Larson was a biologist by training,” Saenz replies.
We arrive at a small pond, surrounded by tall pines and hardwoods, with an understory of yaupon holly and beautyberry bushes. Saenz grabs a string on the edge of the pond and pulls up a black mesh trap. Lifting it out with two hands, he shakes it a bit and peers inside. “Nope, nothing in there,” he says. “That’s pretty normal for this time of year.” In the spring, he tells me, he catches all kinds of things — frogs, tadpoles and tadpole predators, fish, snakes, crawfish, turtles, salamanders.
Saenz, who has worked for the Forest Service since 1991, returned to school a few years back to earn his doctorate under Lee Fitzgerald, associate professor of wildlife and fisheries sciences at Texas A&M University, who specializes in the evolutionary ecology of reptiles and amphibians. Still employed by the Forest Service, Saenz studies frog community dynamics in the national forests of East Texas, and his research informs both scientists and natural resource managers.
Frogs play an important role in food webs as both predator and prey. Adult frogs will consume insects and pretty much anything that fits in their mouths. And likewise, frogs and their tadpoles are food for fish, birds, turtles, snakes, crawfish, dragonfly larvae and giant water bugs.
But frogs have not evolved equivalently. Just as different adult frogs have different niches, their tadpoles exploit different ecological niches from one another. Some have big tadpoles, while others have smaller ones. Some grow fast while others develop slowly. Some are found in ephemeral pools — puddles and such — while others use seasonal or permanent water bodies.
East Texas boasts a rich variety of frog species — from tiny cricket frogs, spring peepers and chorus frogs to the larger tree frogs, leopard frogs and super-big bullfrogs. Saenz and his colleagues want to learn more about how East Texas frogs interact with one another, their predators and the environment. By comparing information from one region to another, scientists can discover which factors are genetically fixed in a species, and which ones are influenced by local factors such as temperature, rainfall and altitude. When animal features or behaviors change with environmental conditions, such changes are known in scientific circles as phenotypic plasticity.
Saenz regularly monitors eight different ponds in two East Texas national forests, and we will visit all of them today. By monitoring ponds with differing ecological conditions year-round, he has started piecing together the “assembly rules” of the frog communities in these East Texas forests. “Really what we’re doing is getting a good idea of what the tadpole community and the tadpole predator community looks like,” he explains. This, in turn, sheds light on the breeding behavior of different frog species.
Even though this first pond’s traps are empty, that result has more to do with the time of year — late October — than the pond itself. “This pond has a good tadpole community,” he explains. “It’s a clear pond, and there are no predatory fish. Later we’ll see some very murky ponds where the traps are covered in algae.”
After checking both traps, he shows me another tool he uses to gather information: a froglogger. It’s a waterproof frog-call recording contraption made by a friend of his, Philip Blackburn of Blackburn Transmitters. Saenz pulls off the froglogger’s plastic cover and presses a button on the mini tape recorder inside. A computer voice states the date and time. “It comes on and records for one minute at six different times each night from 9 p.m. to 2 a.m.,” he explains. The frog-
logger runs on batteries and has a talking watch that announces the time when it begins recording. Saenz removes the tape and inserts another.
As we walk back to the truck, Saenz points to a white circular object on a tall pine tree: a data logger that records temperature and relative humidity every hour. The frog-call data combined with the weather data sheds light on what’s known as “breeding phenology” — in other words, how, when and where different frog species call for mates.
We drive to another location and take a short walk to a pond filled with murky brown water. “This pond has a lot of green sunfish,” Saenz says. “They’re voracious predators.” I’m wondering if perhaps they’re like piranhas but no, they just eat a lot of frogs and tadpoles. “They have a very wide gape,” Saenz explains, “kind of like a bass mouth. They’re also tolerant of the skin toxins that some tadpoles have.” I’m curious which tadpoles are tasty and which are yucky to fish. “Bullfrogs are distasteful, leopard frogs are distasteful,” he lists them off. Bronze frogs are also unpalatable to some degree, he says. We pull up the algae-covered traps, which are empty. “In three years,” he says, “I have only caught one tadpole here — a bronze frog.”
Finished checking the ponds and frog-loggers in the Davy Crockett National Forest, we head to the Stephen F. Austin Experimental Forest, which is a study site within the Angelina National Forest. As we arrive at the first pond there, I hear little squeaks followed by a splash. Frogs are jumping in from the water’s edge. I pull up the mesh trap using the attached string. I shake the trap a bit and look inside. “Oh, look! There’s a tadpole.” I hand it to Saenz. He pulls it out and shows me the coppery gray, 2-inch-long tadpole.
It’s a leopard frog tadpole. Saenz calls leopard frogs “superfrogs.” These med-ium-to-large frogs have an amazingly raucous, cackling call. But the reason he calls them superfrogs is because they threw a curveball into his research predictions on community assemblages.
“The really interesting thing is that southern leopard frogs seem to always be in the system,” Saenz says. “You can always find their tadpoles. And they call every time of the year.” Saenz has caught them, or heard them calling, in all the eight ponds with their varying ecological conditions. And they call for mates at the warmest temperatures and at the coldest temperatures of all species.
“Do you think their skin toxins are why leopard frogs are so omnipresent?” I ask. Saenz explains that unpalatability is likely just one part of a complex natural history. Mostly, their abilities to escape predators and to use both ephemeral and permanent ponds has made them so adaptable.
He tells me about some laboratory research done with different tadpoles and a predatory dragonfly larva. “The current paradigm in anuran community ecology,” Saenz explains, “is that species that inhabit ephemeral ponds have tadpoles that move around more and metamorphose quickly. Those that use more permanent sites have longer larval periods and have lower activity levels and metamorphose slower.” The reason is that puddles don’t have predators, but they evaporate quickly. More permanent water bodies have predators but don’t dry out fast, so the tadpoles can take more time to grow.
In laboratory experiments, Saenz studied how well leopard frogs, spadefoots, chorus frogs and spring peeper tadpoles could elude a green darner dragonfly larva, which eats tadpoles. “The spadefoots were terrible,” he says. “They swim around in circles drawing attention to themselves.” This was expected: spadefoots lay eggs in very ephemeral water bodies, and since puddles don’t have predators, spadefoot tadpoles don’t have behavioral adaptations to elude predators.
Spring peepers and chorus frogs had low activity levels in the experiments, and moderate-to-low abilities to elude dragonfly larvae. These two species breed in seasonal wetlands and temporary pools, but not in the extremely ephemeral sites that spadefoots use. Leopard frogs are adapted to live in permanent water sources, so it’s not surprising that they were best able to escape the predatory dragonfly larvae. These superfrogs surprised Saenz again with a quirk in their predator-escape behavior: Instead of staying still to avoid the predator, they make quick bursts away from it, and then stop.
Several of Saenz’s fellow graduate students at Texas A&M study different aspects of frog biology, including Gage Dayton, who is looking at community structure in Big Bend National Park and Deborah Cowman, who studies sublethal effects of chemicals on amphibian behavior.
Amphibians are called “nature’s canaries” because they tell us whether the environment might be in trouble. “Their biphasic life cycle makes them vulnerable to insults in water and on land,” explains Cowman, who works for the U.S. Geological Survey.
Amphibian declines first started making headlines in 1995 when deformed frogs found by Minnesota schoolchildren made national news. Across the nation and around the globe, scientists began looking for deformed frogs and talking about possible global amphibian declines. It wasn’t until 2000 that biologist Jeff Houlahan and colleagues published a paper in the journal Nature quantifying the global amphibian decline. The much-talked-about decline was real. Now scientists have to do the hard work of figuring out why.
Several scientific ideas about the causes of amphibian declines exist, and they’re not necessarily mutually exclusive. Chemical pesticides, fertilizers and acid rain are thought to have contributed. Habitat loss has definitely affected amphibian populations. A killer “chytrid” fungus affects populations in both North and South America. Increased ultraviolet radiation (UV-B) has also been implicated.
“What really interests me is how contaminants might affect an animal’s ability to withstand other diseases due to the lowering of their immune system,” Cowman explains. “I knew someone who had AIDS, and what they passed away from was a fungal infection. I kept thinking that the more assaults an animal has on its system, the less they will have to fight something new that comes along.” Cowman thinks that ultimately scientists will find out that the amphibian declines are not caused by any one thing, but some combination.
What Cowman contributes to the scientific knowledge base on amphibian decline is how “new generation,” or post-DDT, pesticides affect amphibians. DDT was used extensively in the United States until Rachel Carson’s research-based book Silent Spring was published in 1962, making explicit the link between DDT and mass declines in bird populations. Though the chemical often did not kill non-invertebrates outright, it was persistent in the environment, and became more concentrated as it worked its way up the food web in organisms’ body tissues. When birds ate fish or frogs loaded with DDT, their eggshells thinned and would not hatch. DDT was banned in the United States in 1973.
“[New generation] chemicals were thought to be really great because they aren’t persistent,” Cowman tells me, “but they are highly toxic, and there can be a whole slew of sublethal effects.” Unfortunately, their lack of environmental persistence also makes it hard for scientists to determine whether wildlife deaths or deformed frogs are linked to exposure to these chemicals.
However, scientists can study the chemicals’ sublethal effects on amphibians or other wildlife in the laboratory. She found that when amphibians were exposed to the pesticide carbofuran, it caused temporary paralysis, loss of reflexes, hyperactivity and in some cases permanent nerve damage, whereas the herbicide trifluralin did not.
“Looking back, we may have been comparing apples to oranges,” she says. “It would be interesting to look at long-term chronic effects. Sometimes populations can appear healthy for a long time if the animals are long-lived. But if they’re not reproducing, and not replacing themselves, then their populations will crash.”
Despite the potential for declines from pesticides, habitat loss and other factors, frogs and other amphibians seem to be faring well in most of Texas. East Texas species are doing particularly well, while some frogs and toad populations are declining in South Texas due to pesticides and in West Texas due to salinization of the Pecos River and habitat alteration. But for many species, scientists have little information on historical population sizes. Without a baseline, it becomes difficult to know whether species have declined.
A new TPWD program, Amphibian Watch, enlists volunteers to monitor various Texas locales for frogs on a regular basis. The program already has had some success. During one volunteer training session at Brazos Bend, the program leaders identified Rio Grande chirping frogs for the first time in the region. Since then, volunteers have documented an expansion of this South Texas native into many parts of coastal Texas. The species has radiated out from urban centers because of its habit of bedding down in potted plants.
“The program is too new to see any trends so far,” says Lee Ann Linam, Amphibian Watch coordinator, “but we will keep an eye out for what is there now, and look for any changes in future.”
Gage Dayton, another Texas A&M doctoral student, studies frog communities in Big Bend National Park. Like Saenz, Dayton studies how biotic and abiotic factors influence species distributions. “It’s sort of the building block foundation of conservation in my mind,” says Dayton, “because without knowing why they are where they are, it’s hard to figure out reasons for their decline.”
Two unique frogs found in the Big Bend’s Chisos Mountains are the canyon tree frog and the spotted chirping frog. The canyon tree frog is mottled gray but can change color like a chameleon. Canyon tree frogs breed in mountain creeks, and the tadpoles develop inside the eggs, so little froglets hatch directly out of the egg. Because of this adaptation, they don’t need aquatic environments to breed, although they require some moisture, because of their thin skin. Populations of both of these mountain species appear healthy. “About the only thing might be acid rain fallout, but there doesn’t appear to be any of that,” Dayton says.
One relatively new problem is the invasion of bullfrogs. “They are an exotic species causing a lot of problems in the Southwest, but in Texas they are not as big of a problem,” Dayton says. Bullfrogs are native all the way to mid-Texas, but people introduced them to parts of western Texas. The problems bullfrogs cause are twofold. First, their tadpoles are good at out-competing native tadpoles. And then those that survive to adulthood often get eaten by adult bullfrogs, which are voracious.
Back in the East Texas woods, Saenz and I arrive at the last of the eight ponds, where most frogs are happy and thriving. I pull up one trap while Saenz grabs the other. I look inside and see one tadpole and several invertebrates. “What are these?” I ask.
There’s a dragonfly larvae about two inches long with sharp mouthparts. Then there’s a fierce beetle-looking creature. “That’s a giant water bug. They eat tadpoles and frogs,” Saenz tells me.
Oh my gosh!” I say, “Is that the one that sucks the guts out of living frogs?” I read about it in Annie Dillard’s Pulitzer Prize winning book, Pilgrim at Tinker Creek. They sneak up on them, inject them with a paralyzing poison that liquefies their guts, and then they suck them out with straw-like mouthparts. Pretty creepy. Pretty cool. Just as Dillard describes in the book, nature is always this way. What you observe will surprise and delight you when you take the time to look, listen, and learn.