On rainy summer nights in Florida, the frogs come out to breed. In puddles, ponds, surrounded by sand and pines, the males send up their idiosyncratic bleats, chirps, choruses, or trills. They wait, sometimes until 3 a.m., while the listening females come hopping, crawling, clambering to their sides; they couple, spilling their gelatinous eggs like marbles into the pond, like films onto the water surface, like thickened soap-bubbles among the reeds, or like hardened drops of dew stuck to the undersides of leaves.
There are more types of frogs in northern Florida than anywhere else in the United States. Even south Florida, with its more uniform habitat, is outdone. Which makes the ponds around Tallahassee a good place to find out what acid rain may do to frogs.
Since the late 1980s, biologists around the world have noticed a decline in "herps" (what enthusiasts call amphibians and reptiles; from the Greek herpeton, "creeping thing"). Acid rain has been suspected as a culprit in the animals' decline, but few data exist on herps' tolerance to acid waters.
Ponds in northern Florida are naturally quite acidic, according to Penn State graduate student Susan Warner who, with her adviser, Bill Dunson, received a grant from the Florida Freshwater Fish and Game Commission to study the state's frogs. "Pines tend to be acidic," Warner explains, "and sandy soil doesn't have many cations to help buffer the soil acidity. You'll have a higher pH in sites that are on the karst plain or the Marianna lowlands, because of limestone outcrops. But the thing that causes the greatest variation is seawater influx. Seaspray brings a lot of cations and salts with it."
Of the 115 ponds Warner tested, the most acidic pH was 3.46; the median was 4.9. (Lemon juice, by comparison, is pH 2.3; "clean" rain, about 5.5; pH 7 is neutral.)
To see the effects of low pH on the herps' survival, Warner collected eggs from 10 local species and raised them (50 embryos to a bowl, 1,000 eggs per species) under different pH regimens. The species she chose were the southern toad (the common Bufo found in the woods), the spadefoot toad ("they burrow down in the sand for years and come up and breed, typically, only when you get 5 inches of rain"), the narrowmouthed toad ("they sound like sick sheep when they call; they're sort of slimy, not lumpy like normal toads"), the southern leopard frog ("the frog you use in biology labs is the northern leopard frog"), chorus frogs ("called ornata, or ornate, because they come in brown, green, or gray"), spring peepers, and four kinds of tree frogs.
"The pine woods tree frog was the most tolerant of any species," Warner says. These could tolerate water of pH 3.35. "The rest ranged up to 4.26 for another little tree frog that often breeds in the same places, the squirrel tree frog. He sits up in the tree and goes, ak, ak, like a squirrel."
To determine if the acidity of the water could change the mix of species in a pond, Warner raised eggs of the most tolerant species, the pine woods tree frog, and eggs of one of the less tolerant, the barking tree frog, together in 36 large cattle-watering tanks under varying degrees of acidity. "If I had put squirrel tree frogs in with the pine woods tree frogs, Warner explains, "it would have been very, very hard to tell them apart."
When the tadpoles metamorphosed, the emergent frogs would clamber out of the water onto a ledge under the rim of the tank; lifting off the screen lid, Warner would try to catch them, weighing those whose tails had been totally absorbed. "If they're bigger at metamorphosis," she explains, "they're usually bigger as adults. They lay more eggs and survive better." Some days, recalls Warner, who remembers as a child in Connecticut catching a bucketful of leopard frogs, "I would have 100 tree frogs coming out of the tanks in one day."
After replicating the experiment three times, Warner concluded, "When you change the pH, you change the outcome of competition. The barking tree frog won out at a high pH—it ate all the food. But at a low pH, although the pine woods tree frog couldn't outcompete the other species, it could hold its own."
Sturdy proof that acid rain could shift the balance of species in Florida's ponds. Still, Warner is not as concerned about acid rain as she is about other threats to Florida's frogs. "Right outside Tallahassee," she says, "there's a huge national forest where several of us have our field sites. The first summer I was down there, I went out with the director of the grants program to a pond where one of the species of special concern in the state, gopher frogs, was living." (A "species of special concern" is not yet endangered, but getting close.) "While we were standing there, a jeep went through the pond. The director turned to me and said, 'Well, Susan, you can forget about that pond.' It seems it's a big macho thing down there, to drive through the ponds in big wheel trucks and see how much water you can spray out.
"This habitat destruction is probably much more of a detriment to the frogs than the pH of the rain."
Susan Warner will receive her Ph.D. in December 1994 from the department of biology, Eberly College of Science, University Park, PA 16802; 814-863-0278. Her adviser was William A. Dunson, Ph.D., professor of biology; 865-2461. This study was funded by the Florida Game and Freshwater Fish Commission.