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Why are frogs croaking?

Amphibians—frogs, toads, and salamanders—have been around for a long time. They watched the dinosaurs come and go. But today amphibian populations around the world are in dramatic decline, with more than a third of the world’s amphibian species threatened with extinction. Why?
Amphibians—frogs, toads, and salamanders

Biologists work to answer this question by making observations and doing experiments. A number of factors may be involved, and one possible cause may be the effects of agricultural pesticides and herbicides. Several studies have shown that many of these chemicals tested at realistic concentrations do not kill amphibians. But Tyrone Hayes, a biologist at the University of California at Berkeley, probed deeper.
Amphibians—frogs, toads, and salamanders
Hayes focused on atrazine, the most widely used herbicide in the world and a common contaminant in fresh water. More than 70 million pounds of atrazine are applied to farmland in the United States every year, and it is used in at least 20 countries. Atrazine is usually applied in the spring, when many amphibians are breeding and thousands of tadpoles swim in the ditches, ponds, and streams that receive runoff from farms.
Amphibians—frogs, toads, and salamanders
In his laboratory, Hayes and his associates raised frog tadpoles in water containing no atrazine and in water with concentrations ranging from 0.01 parts per billion (ppb) up to 25 ppb. The U.S. Environmental Protection Agency considers environmental levels of atrazine of 10 to 20 ppb of no concern; the level it considers safe in drinking water is 3 ppb. Rainwater in Iowa has been measured to contain 40 ppb. In Switzerland, where the use of atrazine is illegal, the chemical has been measured at approximately 1 ppb in rainwater.
In the Hayes laboratory, concentrations as low as 0.1 ppb had a dramatic effect on tadpole development: it feminized the males. In some of the adult males that developed from these larvae, the vocal structures used in mating calls were smaller than normal, female sex organs developed, and eggs were found growing in the testes. In other studies, normal adult male frogs exposed to 25 ppb had a tenfold reduction in testosterone levels and did not produce sperm. You can imagine the disastrous effects these developmental and hormonal changes could have on the capacity of frogs to breed and reproduce.But Hayes’s experiments were performed i n the laboratory, with a species of frog bred for laboratory use. Would his results be the same in nature? To find out, he and his students traveled from Utah to Iowa, sampling water and collecting frogs. They analyzed the water
Amphibians—frogs, toads, and salamanders
Frogs Are Having Serious Problems An alarming number of species of frogs, such as this tiny leaf frog (Agalychnis calcarifer) from Ecuador, are in danger of becoming extinct. The numerous possible reasons for the decline in global amphibian populations have been a subject of widespread scientific investigation.

Amphibians—frogs, toads, and salamanders
for atrazine and examined the frogs. In the only site

where atrazine was undetectable in the water, the frogs

were normal; in all the other sites, male frogs had
abnormalities of the sex organs.
Like other biologists, Hayes made observations. He
then made predictions based on those observations,
and designed and carried out experiments to test his
predictions. Some of the conclusions from his experiments, described at the end of this chapter, could have profound implications not only for amphibians but also
for other animals, including humans.



Ref: life the science of biology 9th edition
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Location: New York, NY, USA

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