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Argument 38
The following is a letter to the editor of an environmental magazine.
"The decline in the numbers of amphibians worldwide clearly indicates the global pollution of
water and air. Two studies of amphibians in Yosemite National Park in California confirm my
conclusion. In 1915 there were seven species of amphibians in the park, and there were
abundant numbers of each species. However, in 1992 there were only four species of
amphibians observed in the park, and the numbers of each species were drastically reduced.
The decline in Yosemite has been blamed on the introduction of trout into the park's waters,
which began in 1920 (trout are known to eat amphibian eggs). But the introduction of trout
cannot be the real reason for the Yosemite decline because it does not explain the worldwide
decline."
The author of this letter concludes that a worldwide decline in the number of amphibians is
an indication, or result, of global air and water pollution. To support this assertion the author
first notes a decline in amphibians in Yosemite Park between 1915 and 1992, and
acknowledges that trout, which eat amphibian eggs, were introduced there in 1925. But, the
author then claims that the introduction of trout cannot be the reason for the decline in
Yosemite because the introduction of trout in Yosemite does not explain the worldwide decline.
I find this argument logically unconvincing in three critical respects.
First, the author fails to provide any evidence to refute the strong inference that the
amphibian decline in Yosemite was indeed caused by trout. Because the author provides no
affirmative evidence that pollution---or some other phenomenon--was instead the reason for
the decline, the author's broad assertion that a worldwide decline in amphibians indicates
global pollution is entirely unconvincing.
Secondly, even if I were to concede that the introduction of trout was not the cause of
Yosemite's amphibian decline, the author provides no evidence that the decline was caused by
pollution--rather than some other phenomenon. Perhaps some other environmental factor was
183
instead the cause. Without ruling out all other possible explanations the author cannot
convince me that pollution is the cause of the worldwide amphibian decline--or even the
decline in Yosemite alone.
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