By Peter Montague
The decline and disappearance of frog populations worldwide remains a mystery, despite efforts by hundreds of scientists to determine the causes. The other major problem facing frogs — massive deformities observed since 1995 in California, Iowa, Minnesota, Missouri, Ontario, Quebec, South Dakota, Texas, Vermont and Wisconsin — is now better understood.
During the past six months, press interviews with research scientists and published studies have shed a bit of light on both problems, though true consensus has not yet emerged on either one.
Some scientists still doubt that frogs are disappearing worldwide. They prefer to believe that the simultaneous declines and disappearances of frog populations in North and South America, Europe and Australia reported since 1980 are nothing more than the normal ups and downs of any wild population.
However, Scientific American said in August that the "majority viewpoint" among scientists now is that the widespread declines and disappearances are "highly abnormal".
"I think we're close to consensus now", says David Wake, a well-known frog researcher at the University of California at Berkeley.
Combination of causes
There are roughly 5000 species of amphibians worldwide. Of these, 242 inhabit the US. A recent study by the Nature Conservancy and Natural Heritage Network identified 92 of these 242 (or 38%) as endangered, imperilled or vulnerable (meaning they are likely to become extinct within five, 20 or 100 years if present trends continue).
James La Clair, at the Scripps Research Institute in La Jolla, California, says, "Although amphibians have lived on this planet for over 300 million years ... reports within the last three decades have shown that numerous amphibian species are either suffering from serious population loss or have disappeared altogether".
La Clair says there are very likely "a collection of causes", but they can all be traced back to "the expansion of humankind". Loss of frog habitat — chiefly wetlands — is probably the biggest single cause.
Global warming and accompanying droughts may contribute, because frogs develop from eggs that thrive in water. The artificial stocking of streams with trout and bass plays a role, too. Pesticides and other chemicals certainly exacerbate the problem.
Laboratory experiments have shown beyond doubt that ultraviolet light from the sun can interfere with the development of frogs' eggs. Acid rain may contribute to the problem as well. Humans eating frogs' legs in large quantities are not helping. And there are other causes, such as infectious agents.
A group of Australian researchers reported this year that they have identified one particular fungus that is killing frogs in locations as far apart as Queensland and Panama.
The fungus — which has never before been reported to harm any vertebrate species — causes changes in the skin of frogs, somehow contributing to their deaths. The mechanism is not understood, but frogs breathe through their skin, and the fungus may cause suffocation.
No-one knows why an ancient fungus would suddenly start killing frogs in places far apart. It is conceivable that the fungus was transported to these places only recently.
Immune systems
Another possibility is that the fungus has been present in these locations for a long time, but frogs are now succumbing to it because their immune systems have been impaired by recent changes in the environment.
One candidate would be increased ultraviolet light, which is well known to damage the immune systems of many animals, including frogs. In recent years, chlorinated chemicals released by humans have thinned the protective layer of ozone in the upper atmosphere, thus allowing about 10% more ultraviolet light from the sun to reach the surface of the Earth.
Certain industrial chemicals released into the environment may also be damaging the immune systems of frogs. One particular class of chemicals — called retinoids — has come under strong suspicion because retinoids can cause severe birth defects in many animals, including frogs and humans.
The deformities now being found in large numbers of frogs at many locations in the US and Canada are grotesque. Herpetologists have reported finding frogs with missing legs, extra legs, misshapen legs, paralysed legs that stick out at odd places, legs that are webbed together with extra skin, legs that are fused to the body and legs that split into two half-way down.
They have also found frogs with missing eyes and extra eyes. One one-eyed frog in Minnesota had a second eye growing inside its throat.
Dr David Gardiner, a research biologist at the University of California at Irvine, has been studying retinoids for at least a decade, and in recent years he has probed frog deformities. To him, retinoids are the obvious culprit because of the peculiar kind of limb deformities being observed.
"There is no other known mechanism for this", Gardiner says. "Much of early development is controlled by retinoids", he says. "Our body [and the body of a frog] is completely dependent on them", he told a reporter.
Exposure to retinoids could also make frogs more susceptible to infectious diseases, Gardiner says: "The kinds of chemicals that would target development of limbs would target all organ systems", including the immune system. Frogs with abnormal legs would also very likely have abnormal immune systems.
This could explain why some frogs are now suddenly falling victim to infectious agents that they resisted for millions of years.
Insecticide
James La Clair and his associates recently showed that a popular anti-mosquito insecticide, called S-methoprene, breaks down in the environment to several different kinds of retinoids. Under laboratory conditions, La Clair was able to show that the ultraviolet light in sunlight causes S-methoprene to break down into half a dozen retinoids, and that these retinoids in turn can cause frog deformities of the kind being seen in the US and Canada.
S-methoprene was introduced in the 1970s to control mosquitoes, which breed in water.
It is also widely sold in flea powders. La Clair calculates that the amount of flea power used to treat a 10-pound [4.5 kilograms] pet one time contains enough S-methoprene to contaminate 110,000 litres of water to a level that would cause deformities in frogs.
S-methoprene is also widely used in agriculture to treat cattle gazing areas, tobacco and certain grain crops. It is sometimes added to cattle feed.
S-methoprene mimics a hormone that inhibits developing pupae from moulting; thus it is known as an "insect growth regulator". Because vertebrate species do not have a pupal stage, scientists assumed that S-methoprene could not harm amphibians or mammals. When fed to mammals, S-methoprene is about as toxic as sugar.
Now La Clair's work has shown that this seemingly harmless chemical can be transformed into a potent teratogen by exposure to sunlight for just a few hours. The implications of this research, which was reported in Environmental Science & Technology, a journal of the American Chemical Society, are profound.
Retesting
For one thing, it means that once again the pesticide regulators at the US Environmental Protection Agency have missed a key feature of a chemical whose safety they regulate.
Secondly, it shows once again that relying on risk assessment leads to bad public health decisions. EPA's risk assessments have routinely failed to evaluate the breakdown products of the pesticidal chemicals that the agency has deemed safe enough to allow as residues on dinner plates.
Third, it means that thousands of pesticides now in common use need to be retested to see if their breakdown products are dangerous to humans or other species. However, this is unlikely to occur any time soon, because EPA currently estimates that it is at least 15 years behind schedule in safety-testing the pesticides to which people — and the frogs — are currently being exposed.
Indeed, the situation is worse than the agency makes it out to be.
Congress ordered EPA to re-evaluate and modernise all pesticide safety tests in 1972, and it demanded that the agency complete the job by 1977. Since 1972 the agency has been doing its best to comply, but each year new evidence has come to light showing that pesticides can harm humans and the environment in ways that no-one imagined, so additional tests have been required.
Thus La Clair's work is just the latest surprise in a long chain of unpleasant surprises. EPA officials in 1996 estimated that they will complete their pesticide safety re-evaluations in the year 2011 — 34 years late — if they can keep the work on schedule. Meanwhile, the frogs and we continue to be exposed to thousands of poorly understood government-approved industrial poisons.
La Clair's research into the deformed frogs of North America serves to remind us that pesticides are now too dangerous to be safely regulated, even by the most powerful government the world has ever known.
Or is it that pesticide manufacturing corporations are now too dangerous to be safely regulated, even by the most powerful government the world has ever known? It's a fair question.
[From Rachel's Environment & Health Weekly. Like Green Left Weekly, Rachel's is a non-profit publication which distributes information without charge on the internet and depends on the generosity of readers to survive. If you are able to help keep this valuable resource in existence, send your contribution to Environmental Research Foundation, PO Box 5036, Annapolis, Maryland 21403-7036, USA. In the United States, donations to ERF are tax deductible.]