Last July, Dr. Donald F. Anthrop wrote a letter,
“Pesticides killing Frogs? Poppycock” (HCN, 7/7/03: Pesticides
killing Frogs? Poppycock), criticizing an earlier report by Cosmo
Garvin (HCN, 5/26/03) about possible effects of pesticides on frog
populations. At the end of Dr. Anthrop’s letter, he stated:
“This is a sorry excuse for scientific research.”
I think
it is important that the question of pesticide effects on wildlife
be taken seriously, and I see lots of good evidence (such as
research published in peer-review scientific journals) that
Garvin’s story was based on standard and acceptable
scientific research.
Dr. Anthrop’s first point was
that “despite the fact that pesticide residues found in mountain
frogs are far below lethal levels, the argument seems to be that
since declines in frog populations have occurred in mountain
regions downwind from agricultural regions, pesticides are
responsible.”
I find this statement to be misleading,
because “lethal” typically means that individuals exposed to the
chemical die in a few hours or days. Sub-lethal exposure levels are
much more frequently observed than lethal levels, because pesticide
applications are typically greatly diluted by air and water. The
important ecological point is that even at very low sub-lethal
exposure levels, solid scientific research has shown that
pesticides adversely affect development, physiology, hormone
activity, behavior, and reproduction of amphibians (including
frogs). Sub-lethal pesticide effects can adversely affect wildlife
populations, even if there is no increased mortality. For example,
if male frogs can’t croak, they won’t attract mates,
and the population will decline. Published results of careful
scientific research by several ecotoxicologists show that, at very
low (parts per billion) levels, many kinds of pesticides can
interfere with normal development, functioning and reproduction of
a variety of organisms, including frogs. A reasonable hypothesis
(and area for research) is that, over generations of time, wildlife
populations are declining due to sub-lethal adverse effects of
pesticides. This kind of ordinary and acceptable science is being
done, reviewed and published every day.
Dr.
Anthrop’s second point was that “if pesticides constitute the
culprit, why didn’t the frog populations decline long ago?”
The major decline of many amphibian populations occurred in the
1960s or 1970s (you can check this out by “Googling” your favorite
amphibians). Pesticides have been used in exponentially increasing
amounts since the 1950s. To take a specific example, Atrazine,
introduced in the 1960s, is the most commonly used herbicide in
North America. Research by several scientists has shown that
Atrazine adversely affects sexual development and behavior in
frogs, at levels far below those that kill frogs outright. For
pesticides in general, and in the specific case of Atrazine, there
is a reasonable historical match between increasing pesticide use
and decreasing amphibian populations. The historical pattern sets
the stage for research to test hypotheses about whether specific
sub-lethal mechanisms are in fact causing the decline. To me, this
sounds like a reasonable scientific approach.
We may not
have the whole story on which specific pesticides to implicate in
the amphibian decline, but it seems reasonable to me, as a
practicing ecologist and toxicologist, to be concerned about
pesticide effects on wildlife.
Stanley
Dodson
Madison, Wisconsin
The author, who became a naturalist in Grand Junction, Colorado, is a professor of zoology at the University of Wisconsin.
This article appeared in the print edition of the magazine with the headline Pesticides are killing frogs.
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