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 he 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 among frog populations in California,
Iowa, Minnesota, Missouri, Ontario, Quebec, South Dakota, Texas, Vermont,
and Wisconsin1 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. No one is even sure whether the two problems are connected,
though new evidence indicates they are.
Some scientists still doubt that
frogs are actually 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."[2] "I think we're close to consensus now," says
David Wake, a well-known frog researcher at the University of California
at Berkeley.[3]
There are roughly 5,000 species of
amphibians worldwide. Of these, 242 inhabit the United States. A recent
study by the Nature Conservancy and the Natural Heritage Network identified
92 of these 242 (or 38 percent) as endangered, imperiled, or vulnerable[2]
(meaning they are likely to become extinct within 5, 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, nearly 120 times [as long
as] modern man, reports within the last three decades have shown that numerous
amphibian species are either suffering from serious population loss or have
disappeared altogether."[1] La Clair says there are very likely "a
collection of causes," but one way or another 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 (more on
this below). Laboratory experiments have shown beyond doubt that ultraviolet
light from the sun can interfere with the development of frogs' eggs.[4]
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 summer that they have identified one particular fungus that
is killing frogs in locations as far apart as Queensland, Australia and
Panama in Central America.[5] 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 oxygen through their skin and the fungus may cause suffocation.
No one knows why an ancient fungus
would suddenly start killing frogs in places as far apart as Australia and
Panama. It is conceivable that the fungus was transported to these places
only recently on the boots or equipment of researchers studying the disappearance
of frogs. 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 percent more
ultraviolet light from the sun to reach the surface of the Earth.[6]
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 medicine Accutane, prescribed for treating acne, is
a retinoid known to cause major birth defects in humans.
The deformities now being found in
large numbers of frogs at many locations in the United States and Canada
are grotesque. Herpetologists (scientists who study amphibians and reptiles)
have reported finding frogs with missing legs, extra legs, misshapen legs,
paralyzed legs that stick out from the body at odd places, legs that are
webbed together with extra skin, legs that are fused to the body, and legs
that split into two halfway 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.[7]
To him, retinoids are the obvious culprit in the mystery of the misshapen
frogs because of the peculiar kind of limb deformities being observed. "There
is no other known mechanism for this [besides retinoids]," 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.[8]
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.
James La Clair and his associates
at the Scripps Research Institute in La Jolla, California, recently showed
that a popular antimosquito insecticide, called S-methoprene, breaks down
in the environment to several different kinds of retinoids.1 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
now in the United States and Canada.
S-methoprene was introduced in the
1970s to control mosquitoes, which breed in water. It is sold under trade
names like Altosid, APEX, Diacon, Dianex, Kabat, Manta, Minex, Pharoid,
Precor, Yuvemon, and ZR 515.
It is also widely sold in flea powders.
La Clair calculates that the amount of flea power used to treat a ten-pound
pet one time contains enough S-methoprene to contaminate 110,000 liters
of water to a level that would cause deformities in frogs.[1]
S-methoprene is also widely used
in agriculture to treat cattle gazing areas, tobacco, and certain grain
crops. It is also sometimes added to cattle feed.
S-methoprene mimics a hormone that
inhibits developing pupae from molting; thus it is known as an "insect
growth regulator." Because vertebrate species do not have a pupal stage
of growth, scientists assumed 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. For one thing, it means
that once again the pesticide regulators at U.S. Environmental Protection
Agency (EPA) 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 byproducts of the pesticidal chemicals that the agency
has deemed safe enough to allow as residues on our dinner plates. Third,
it means that thousands of pesticides now in common use need to be retested
to see if their breakdown byproducts are dangerous to humans or other species.
However, this additional testing 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 we and the frogs are currently being
exposed.[9]
Indeed, the situation is worse than
the agency makes it out to be. Congress ordered EPA to reevaluate and modernize
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 revelations have come to light, new evidence 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 reevaluations
(which they were ordered by Congress to complete in 1977) in the year 2011
34 years late IF they can keep the work on schedule.9 Meanwhile, the frogs
and we continue to be exposed to thousands of poorly-understood government-approved
industrial poisons.
In sum, Dr. 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.  |
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- James J. La Clair and others, "Photoproducts and
Metabolites of a Common Insect Growth Regulator Produce Developmental Deformities
in XENOPUS," Environmental Science & Technology Vol. 32,
No. 10 (1998), pgs. 1453-1461.
- Rodger Doyle, "Amphibians at Risk," Scientific
American, August, 1998, pg. 27.
- William Souder, "Evidence Grows, Suspects Elusive
in Frogs' Disappearance," Washington Post July 6, 1998, pg.
A3.
- Andrew R. Blaustein and others, "UV repair and resistance
to solar UV-B in amphibian eggs: A link to population declines?" Proceedings
Of The National Academy Of Sciences Vol. 91 (March 1994), pgs. 1791-1795.
- Lee Berger and others, "Chytridiomycosis causes
amphibian mortality associated with population declines in the rain forests
of Australia and Central America," Proceedings Of The National
Academy Of Sciences Vol. 95 (July 1998), pgs. 9031-9036. See also Jocelyn
Kaiser, "Fungus May Drive Frog Genocide," Science Vol.
281, No. 5373 (July 3, 1998), pg. 23; and see Carol Kaesuk Moon, "Newly
Found Fungus Tied to Vanishing Frog Species," New York Times
June 28, 1998, pg. unknown. This is not the first fungus linked to frog
deaths; see Andrew R. Blaustein and others, "Pathogenic Fungus Contributes
to Amphibian Losses in the Pacific Northwest," Biological Conservation
Vol. 67 (1994), pgs. 251-254.
- J.B Kerr and C.T. McElroy, "Evidence for Large Upward
Trends of Ultraviolet-B radiation Linked to Ozone Depletion," Science
Vol. 262 (November 12, 1993), pgs. 1032-1034. See also Mario Blum-thaler
and Walter Ambach, "Indication of Increasing Solar Ultraviolet-B Radiation
Flux in Alpine Regions," SCIENCE Vol. 248 (April 13, 1990), pgs. 206-208.
- See http://darwin.bio.uci.edu/mrjc/Whoweare/Dave.html.
- Maggie Fox, "Common chemical may be to blame for
dead frogs," Reuters wire service August 5, 1998.
- John Wargo, Our Children's Toxic Legacy; How Science
And Law Fail To Protect Us From Pesticides (New Haven, Conn.: Yale
University Press, 1996), chapter 5
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