Since 1993, scientists worldwide have been trying to understand why
frog populations are reported to be steeply declining in relatively
unspoiled environments on several continents. And since 1995 scientists
have been struggling to explain why deformed frogs are being observed
in large numbers in a broad swath across the mid-northern region of the
In both cases, the first problem has been to determine whether the
observed changes represent natural fluctuations, or whether they
represent unusual events that might signal something important about
declining environmental quality. A consensus now seems to be emerging:
** worldwide, many frog populations ARE declining to unusually low
levels (including, in some instances, extinction); and
** frog deformities are definitely occurring in unusually high numbers
in some locales.
Many factors have been identified as contributing to declines in frog
populations. They include:
** introduction of exotic predatory fish; stocked populations of bass,
for example, can clear a stream of all frog eggs and tadpoles in short
** habitat destruction (draining wetlands, for example);
** landscape changes (clearing woods, building roads, etc.) that
isolate particular frog populations;
** increased ultraviolet radiation, caused by industrial chemicals that
have thinned the stratospheric ozone layer;
** clearing wild lands for agriculture;
** acid rain;
** humans eating frogs' legs;
** global warming, causing elevated temperatures and drought;
The identified causes of deformed frogs include these:
** increased numbers of amphibian surveys, thus more and better
** parasite infestations; a parasite called a trematode may be involved
in some frog deformities. Trematodes burrow into the limb buds of
tadpoles and can, in fact, cause at least one of the deformities seen
in Minnesota frogs.
** toxic contamination (pesticides, heavy metals, acidification);
** predation (partially-successful predators may remove parts of frogs,
which may then grow back incorrectly);
** ultraviolet radiation;
As we review recent scientific literature and press reports of
scientific studies and meetings, what seems to stand out is a growing
awareness that industrial toxins --especially agricultural biocides --
are implicated in frog population declines and in frog deformities.
FROGLOG is a publication of the Declining Amphibian Populations Task
Force of the World Conservation Union's Species Survival Commission. In
recent issues, FROGLOG has reported the following:
** The 1996 RED LIST OF THREATENED ANIMALS, published by the
International Union for Conservation of Nature (IUCN) lists 156
amphibian species as extinct, critical, endangered, or vulnerable to
extinction. This represents 25% of all the amphibians on Earth.
The Nature Conservancy, a U.S. organization, in 1996 surveyed the
status of 20,481 species of plants and animals in the U.S. and reported
that 37.9% of amphibians are in danger of becoming extinct.
** Researchers at Widener University in Chester, Pennsylvania, and at
Benedictine College in Atchison, Kansas, have shown that acid rain can
stress frog populations by harming their immune systems. Frogs
raised in water with a pH of 5.5 had significantly more bacteria in
their spleens, and a significantly higher death rate, than frogs raised
in waters with a pH of 7.0. The researchers attribute the increased
numbers of bacteria to reduced efficiency of bacteria removal by white
blood cells --part of the frogs' immune defenses.
** Researchers at the University of Windsor in Ontario, Canada, have
correlated high levels of organochlorine pesticides with reduced frog
populations in several parks and wildlife reserves along the northern
edge of Lake Erie. At Point Pelee National Park in Canada, only 5
frog species remain, and DDT residues in these frogs average 5000 to
47,000 micrograms of DDT per kilogram of body weight. At the Holiday
Beach Conservation Area 40 kilometers [24 miles] east of Point Pelee, a
dozen species of frogs thrive and the DDT in their flesh averages only
6 micrograms per kilogram. This study doesn't prove that DDT has killed
off the frogs in Point Pelee, but it certainly points in that
** Researchers discovered numerous frogs and toads with missing back
legs in ponds and ditches exposed to pesticide runoff in the St.
Lawrence River valley in Quebec, Canada in 1992 and 1993. Of 854
individual amphibians (among 3 species of frogs and one species of toad
examined in 14 agricultural habitats), 106 (12%) had hind limb
malformations. The authors hypothesize that the main cause of the
deformities was exposure to pesticides. They say that such leg
deformities are "virtually unknown" among frogs and toads in the wild.
** A population of leopard frogs (RANA PIPENS) exploded on a western
Michigan farm after the farm converted to organic (pesticide-free)
growing techniques. In 1988, a survey of the farm had revealed that
leopard frogs were nearly absent, but the population rebounded quickly
as soon as pesticide use ceased. Researchers reported that the
number of different frog species on the farm also increased after 1988.
** The Australian government in 1997 took an unprecedented action,
banning 84 herbicide products for use near water because of their
harmful effects on tadpoles and frogs.
All of the 84 banned products contain Monsanto's glyphosate as the
active ingredient. However, the harmful component appears to be not the
glyphosate itself but an "inert" ingredient --a detergent or wetting
agent added to the herbicides so that droplets of liquid spread out and
cover the target leaves.
Detergents interfere with the ability of frogs to breathe through their
skin, and tadpoles to breathe through their gills. Michael J. Tyler of
the Department of Zoology at the University of Adelaide, Australia,
says, "Although the herbicide [glyphosate] is claimed to be
'environmentally friendly,' it is clear that users have been lulled
into a false sense of security."
** Researchers in Sri Lanka report that frogs are nearly absent from
tea plantations where herbicides are heavily sprayed, but their
populations rebound shortly after spraying stops.
"Conversion to organic [pesticide-free] tea production in this region
has contributed greatly to the re-establishment of populations of local
frogs," they say.
** According to the MINNEAPOLIS STAR TRIBUNE, Swiss researchers
reported earlier this year that a fungicide used heavily in Minnesota
can stunt the growth of tadpoles and retard the sexual development of
frogs. The Swiss researchers have not produced the kinds of
deformities seen in Minnesota frogs, but they say the fungicide
triphenyltin could harm frog populations by delaying their growth,
which would allow more time for predators to eat them.
** Another class of industrial compounds called retinoids has been
implicated in frog deformities. Retinoids are a class of molecules
including vitamin A and similar compounds, including retinoic acid,
which is a potent hormone. Exposure to excessive amounts of retinoids
can cause birth defects in all vertebrates, from fish to humans. (The
retinoid-based acne treatment Accutane has caused birth defects in
humans.) At least one pesticide, methoprene, acts like a retinoid.
Methoprene is an insect growth regulator that prevents young insects
** Researchers some years ago identified a pesticide spill into Lake
Apopka as the cause of mature alligators with penises so small that
they could not reproduce. (See REHW #372, #377.) Scientists assumed the
trouble was confined to that one lake. But recently alligator problems
have come to light all across southern Florida. In the Everglades,
which are contaminated with numerous pesti-cides, full-grown alligators
weigh hundreds of pounds less than alligators elsewhere in Florida. And
in Lake Okeechobee, Florida's largest lake, juvenile alligators have
levels of reproductive hormones in their blood that are far below
normal --another possible sign of disruption by pesticides.
The case is not airtight. There is still much to be learned.
Nevertheless, evidence continues to accumulate indicating that
amphibians and reptiles are being harmed by industrial chemicals
released into the environment. Are humans exempt from similar harm? It
seems very unlikely.
--Peter Montague (National Writers Union, UAW Local 1981/AFL-CIO)
 Gee Chow, "Pesticides and the Mystery of Deformed Frogs," JOURNAL
OF PESTICIDE REFORM Vol. 17, No. 3 (Fall 1997), pg. 14.
 Tim Halliday, "1996 IUCN Red List," Froglog No. 21 (March 1997),
pg. 2. FROGLOG is edited by John W. Wilkinson, Department of Biology,
The Open University, Walton Hall, Milton Keynes, MK7 6AA, United
Kingdom. FROGLOG is available on the world wide web at http://acs-
 William Dicke, "Numerous U.S. Plant and Freshwater Species Found in
Peril," NEW YORK TIMES January 2, 1996, pg. B12. The Nature Conservancy
can be reached in Arlington, Virginia: (703) 841-5300.
 Marc Brodkin and Martin Simon, "The Effects of Aquatic
Acidification on RANA PIPIENS," FROGLOG No. 20 (January 1997), pg. 3.
 Ronald W. Russell and Stephen J. Hecnar, "The Ghosts of Pesticides
Past?" FROGLOG No. 19 (November 1996), pg. 1; a more formal report is
R.W. Russell and others, "Organochlorine pesticide residues in southern
Ontario spring peepers," ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY Vol. 14
(1995), pgs. 815-817. And see Ronald W. Russell and others,
"Polychlorinated Biphenyls and Chlorinated Pesticides in Southern
Ontario, Canada, Green Frogs," ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY
Vol. 16, No. 11 (1997), pgs. 2258-2263.
 Martin Ouellet and others, "Hindlimb Deformities (Ectromelia,
Ectrodactyly) in Free-Living Anurans From Agricultural Habitats,"
JOURNAL OF WILDLIFE DISEASES Vol. 33, No. 1 (1997), pgs. 95-104.
 Patrick D. McKown and Samuel M. DeFazio, "Froglog Shorts," FROGLOG
No. 21 (March 1997), pg. 4.
 Michael J. Tyler, "Herbicides Kill Frogs," FROGLOG No. 21 (March
1997), pg. 2.
 Ranil Senanayake and others, "Frog Tea?" FROGLOG No. 23 (August
1997), pg. 2.
 Greg Gordon, "Fungicide used by Minnesota farmers is found to harm
frogs," MINNEAPOLIS STAR TRIBUNE February 18, 1998, pg. unknown.
 William Souder, "A Possible Leap Forward on Amphibian
Abnormalities," WASHINGTON POST March 16, 1998, pg. A3.
 Cable News Network (CNN), "Pesticides suspected in Florida gator
decline," CNN INTERACTIVE March 15, 1998. Available on the world wide
web at http://www.cnn.com/EARTH/9803/15/gator.woes/index.html .
Descriptor terms: pesticides; frogs; alligators; mn; wi; fl; species
loss; wildlife; endocrine disruptors; wetlands; ozone depletion;
ultraviolet radiation; agriculture; global warming; parasites;
trematodes; heavy metals; mercury; lead; cadmium; nature conservancy;
red list of endangered animals; acid rain; ddt; glyphosate; australia;
herbicides; fungicides; inert ingredients; triphenyltin; retinoids;
retinoic acid; vitamin a; accutane; methoprene; lake apopka;