Prying Into the Lives of Frogs
Details of amphibian biology hold clues to the decade-old mystery of the creatures' disappearances and deformities
- Kathryn Phillips
- Oct 09, 1999
To the casual observer, the western end of San Mateo Creek is a typical Southern California stream, parched much of the year. But as herpetologist Dan Holland stands in the dry creek bed one March afternoon, he sees something special. "This is some of the best arroyo toad habitat in the world," he says. The sandy beaches are ideal for the toads' burrowing. Vegetation is sparse, leaving wide-open spaces for insect catching. The watercourse is suitably gravelly--a surface that a toad's tiny toes can grip.
This section of the creek has one of the world's densest remaining populations of the pale, small, portly toads. Last year, Holland and his colleagues found an average of 22 of the creatures along one half-mile stretch of the creek, gathered on banks and in shallows, the males whistling for mates on warm nights. And that makes this spot a good place to study the endangered animal.
Such research could help scientists fully comprehend the reasons the arroyo toad is missing from 75 percent of its historic range. And the work could offer clues to help stem the worldwide decline of amphibians, the ongoing trend that first made headlines 10 years ago. That's when ecologist Martha Crump, then at the University of Florida, reported that the famous golden toad of Monteverde, Costa Rica, had dropped from sight. Ever since, scientists have warned that human-caused environmental problems are hurting amphibians around the world, although the exact causes of some declines remain unknown.
Much to learn: The mystery is due in part to a dearth of knowledge about the habits, behavior and needs of frogs, toads and salamanders. "Even for most North American species, we can't answer the questions of how long does an individual live, how far does it range," says neuroanatomist Mike Lannoo, coordinator of the U.S. section of the international Declining Amphibian Populations Task Force (DAPTF), which supports scientists' efforts to understand amphibian declines. "We need to be able to answer these questions to be able to successfully manage for amphibians."
Still, there are plenty of obvious reasons for the creatures' problems. Even the arroyo toad's relative paradise on San Mateo Creek is far from a perfect home. Roads run along both sides of the waterway, and traveling toads get squashed by passing cars. Introduced bullfrogs, opossums and house cats prey on incautious toads. If the toads lounge in the farm fields that sit just on the other side of one road--and toads love the abundance of insects in farm fields--they risk being chopped and flattened by farm equipment or sprayed by pesticides. Exotic pest plants threaten to change the creek's open bed.
In short, these toads face many of the dangers scientists agree contribute to amphibians' problems worldwide. Ranking high among these dangers are "really basic things that our society has a difficult time addressing," says herpetologist Gary Casper, also with the U.S. section of DAPTF. "These include habitat loss, habitat fragmentation, habitat degradation and pollution." Each of those environmental ills affects its amphibian victims in specific ways. Some need no further sleuthing: If habitat disappears, it stands to reason that its residents will too. Remove the sandy beaches of San Mateo Creek, for example, and an arroyo toad has no place to hide from predators and the heat of day.
Vulnerable on land and water: Other potential problems are harder to pin down. Frogs, like most amphibians, spend part of their lives in watery homes and part on land. A frog might risk being run over by crossing a busy street and then meet up with pesticide-laced runoff in the pond in which it rehydrates and lays eggs.
Warming or drying trends might stress amphibians, which have porous skin that acts as a sort of third lung and must stay moist to help the creatures breathe. It is possible that certain levels of moisture or dryness make amphibians vulnerable to maladies. Frogs also need live food; the flutter of an insect's wings signals to a frog that the bug is food. Pesticides or other toxic pollutants that don't kill a frog outright may kill its food.
Not only do such variables complicate studies of frogs, scientists still don't know the status of many North American populations of amphibians, much less those in the rest of the world. With nearly 5,000 species in the world (about 200 in the United States), including about 4,400 frogs and toads (about 80 in the United States), there are simply too many to evaluate easily. Nevertheless, it is clear that in the United States alone, at least 20 species are in trouble in all or parts of their natural ranges. That accounts for about 10 percent of U.S. amphibians. The biggest declines are in western states.
U.S. species in trouble: The California red-legged frog, the model for Mark Twain's famous jumping frog of Calaveras County, is among the known amphibians in trouble. A victim of wetland destruction and predators including humans, exotic fish and bullfrogs, the red-legged frog is now federally listed as endangered. Also in California, the Yosemite toad, the mountain yellow-legged frog and the foothill yellow-legged frog all have declined. Only one known wild population of the Wyoming toad remains. The Cascades frog is in decline in Oregon, and the western spotted frog is nearly gone in Washington. In the Colorado Rockies in the mid-1970s, boreal frogs went through a dramatic and so far inexplicable decline. They also are missing or in decline in New Mexico, Wyoming and the mountains of Utah and Idaho.
Northern leopard frogs, which may tie with the eastern bullfrog for the average American's image of a classic frog--long legs, green skin with spots, soulful eyes--have been in decline in the Upper Midwest for decades. Today they also are gone from much of their old hopping grounds in Canada. The tiny Blanchard's cricket frog, once found throughout the Upper Midwest, has disappeared from Canada and most of Minnesota and Wisconsin. Even the bullfrog, considered a pest in places where people have introduced it, appears to be in decline in some of its native habitat in eastern states.
Elsewhere in the world are other distressing reports. In Australia, at least 27 frog species are in decline, and several have not been seen in years. In tropical Central America, a one-time paradise for moisture-loving forest amphibians, dramatic declines have taken place in several locations. Since 1987, at least 20 of the 50 amphibians found in the forests surrounding Monteverde, Costa Rica, have declined.
Evidence has mounted that climate change is a culprit in Monteverde. Scientists including J. Alan Pounds of the Monteverde Cloud Forest Preserve and Tropical Science Center think that climate change has raised the altitude of the clouds that typically hang over the mountains in the winter dry season. Temperatures have increased, and moisture levels have decreased, causing changes in bird, reptile and amphibian populations. During El Niño years in particular, when rainfall levels have dropped in Monteverde, stream flows and ground moisture have decreased. All of that is potentially bad news for amphibians, with their need for a certain moisture level. Most amphibians lay eggs that have a gelatinous coating but no shell. The eggs must stay moist to develop, and a long dry period could desiccate not only an adult but its eggs as well.
Easy targets: Dry spells could also make the animals more vulnerable to sickness. During a dry period in the 1980s, Pounds noticed that harlequin frogs along one stream grouped together more closely than usual as they vied for space. The frogs became easy targets for a parasitic fly that patrols streams. Pounds says it is possible that the effects of two decade's worth of climate change may have made amphibians in the Monteverde area more susceptible to parasites and disease. The conditions could have stressed the amphibians' immune systems, caused behavioral changes that put them in harm's way or increased the virility of diseases or parasites.
Lately several scientists have been looking into the role of disease in declines. Dead or dying frogs infected with chytrid fungus have been found in Central America, Australia and the United States. Scientists have linked other diseases to other frog deaths, including the decline of the Wyoming toad.
Also, researchers have suspected for some time that pesticides may be contributing to declines. That suspicion gained weight in 1995, when schoolchildren in Minnesota found dozens of deformed frogs in country ponds. Today, the link between the deformities and declines remains elusive. Malformed frogs have been found in nearly every state, but only in a few hot spots have such frogs been found in numbers that significantly exceed what researchers estimate is a natural rate.
Two of those places are in the Midwest and Quebec's St. Lawrence River Valley, where scientists have linked chemical compounds, including pesticides, to the deformities, mostly in the form of missing legs. Exactly which compounds are at fault is uncertain. In some other locations--in California, Oregon, New York and Arizona--scientists confirmed this spring that parasites that target tadpoles are to blame for deformities, especially multiple limbs. Whether something makes the tadpoles susceptible to the parasites is unknown, and so far the parasites have not been found at midwestern sites of frog deformities.
Even if pesticides do not cause deformities, they can threaten a frog's survival. Studies show some pesticides slow tadpole reflexes or increase involuntary twitching, making it harder for frog offspring to evade predators. The chemicals also can compromise immune systems and decrease reproduction. "It's all these subtle secondary effects that may accumulate to a point where there is a system breakdown," says DAPTF's Casper.
Meanwhile, discoveries from the frontier of frog field biology offer hope for amphibians. Take the case of California's Sierra Nevada range, where frogs are absent in hundreds of lakes and ponds where private citizens and public agencies have introduced fish. Researchers have recently established that the fish eat the tadpoles of mountain yellow-legged frogs, which have dropped from sight in much of their Sierran habitat. Yosemite toads and foothill yellow-legged frogs also seem to be affected by fish, among other factors.
Enter Vance Vredenburg, a graduate student at the University of California, Berkeley, who decided to test what would happen if he removed fish from a lake in a remote part of Kings Canyon National Park, where scientists had found strong evidence that introduced trout were the cause of the decline of mountain yellow-legged frogs. In 1997, Vredenburg set up camp near a lake that held fish and no frogs. Another, nearby lake held frogs and no fish. For a month, Vredenburg used a gill net to remove more than 500 fish, mostly introduced rainbow trout. "It's incredibly labor intensive removing fish by gill nets," he says. The work paid off: When he inspected the newly fishless lake the following summer, he found mountain yellow-leggeds had recolonized.
Ending the practice of introducing fish into lakes and streams would help protect some amphibians, according to California Academy of Sciences research associate Mark Jennings, who did some of the early research on the effects of fish on frogs. Yet government agencies still plant fish for sports fishing in frog habitat in many states. And as corn prices have dropped, Wisconsin's governor has encouraged struggling farmers to turn seasonal wetlands into fish farms. "I don't see why the frogs need to suffer for the low price per bushel of corn, but that's the way it's playing out," says DAPTF's Lannoo.
That's all too often the pattern for amphibians affected by the habits of people. In the Upper Midwest, to give another example, new findings may help explain the decline of the Blanchard's cricket frog. Suspected causes have included changes in water levels and drainage. Recently, Jason Irwin, a graduate student at Miami University in Ohio, reported that many Blanchard's cricket frogs survive harsh winters by burrowing less than an inch underground near the water's edge. The temperatures in that special zone do not drop below freezing. Exactly why the temperatures stay warmer is uncertain, but the answer may have to do with interaction between organic matter, subsurface water and soil composition. Researchers speculate that changes in subsurface water levels may be reducing those special warm zones, leaving the cricket frog exposed to freezing temperatures.
Peripatetic toads: It isn't clear how such knowledge can help the Blanchard's cricket frogs. But back on San Mateo Creek, Holland and graduate student Paul Griffin have made a discovery about the arroyo toad's habits that could help protect it. Using bean-size radio transmitters and other, less high-tech methods, they have found that arroyo toads will travel more than half a mile away from the floodplain. Holland has found toads on steep hills in thick chaparral, in grasslands and in sage scrub.
Wildlife managers could help protect the toads, concludes Holland, by not building within half a mile of their creek habitat. Then he lists planned construction he knows will degrade upland habitat near San Mateo Creek: a helicopter landing area, a levy and an eight-lane toll road--just the sort of projects that have pushed frogs out of their homes around the world.
Before Holland and Griffin take a visitor on a tour of these sites, the researchers consider a question they hear too often: Why do toads matter? There are many scientific and ethical reasons, they say: Amphibians are an important part of the ecosystem; arroyo toad tadpoles eat algae, and arroyo toad adults eat a range of insects. In turn, the toads feed other indigenous animals in the system. They keep life going. If this toad disappears, its absence will reduce biodiversity, and as scientists have proved time and again, diverse ecosystems are the healthiest.
But it is Holland's initial response to the question, the one that required less than a second of thought, that he likes best. "What good are they?" Holland snaps. "My response is: What good are you?"
Kathryn Phillips is the author of Tracking the Vanishing Frogs: An Ecological Mystery (Penguin, 1995).
NWF Takes Action: May the Frog Force Be with You
Americans everywhere are invited to become part of the "Frog Force," a partnership between the National Wildlife Federation, the U.S. Department of Interior and Martin and Chris Kratt of the television series Kratts' Creatures.
The effort's main goals are to help solve the mystery of dwindling amphibian populations and create frog-friendly habitat such as backyard ponds. "Our goal in this partnership is to educate citizen naturalists about the plight of amphibians and equip them to help find the answers," says NWF President Mark Van Putten.