Spring arrives in Wisconsin with a chorus of trillings from bogs and marshes as rising temperatures wake hibernating frogs and toads. In recent years, however, the volume of that chorus has decreased as their numbers have declined.
To try to explain that decline - and possibly halt it - biologists from across the Midwest met at the Milwaukee Public Museum recently.According to Gary Caspar, a herpetologist at the museum who organized the conference, amphibians are receiving increasing attention because scientists view them as early warnings of environmental problems.
Born in shallow water and later moving out onto land, frogs, toads and salamanders have complex lives that depend on a variety of habitats. Also, Caspar said, "the fact that they don't have shelled eggs means that their embryos are intimately in contact with the environment."
Campaigns earlier this century to drain swamps cut back amphibian populations. That trend has continued in recent years as development has cut into wetland habitats. In fact, Caspar said, amphibians are probably at greater risk than many other animals because the wetlands they favor are the ones most prone to be bulldozed.
Frogs and toads like marginal wetlands that tend to dry up during the summer but get refilled with autumn and spring rains. The reason, Caspar said, is simple: "There's no fish in them. Fish are voracious predators of amphi-bians."
Posing additional risk to amphibians is the conversion of wetlands into aquaculture ponds for raising fish, according to one researcher, Michael Lannoo of Ball State University.
Not only do the fish prey on the animals, he said, but chemicals used to clear the ponds of fish predators kill off young am-phi-bians.
Also, Lannoo said, the trend toward isolating wetlands in small pockets surrounded by housing development or farm fields works against amphibians. Periodic droughts can dry up some wetlands entirely for a year or more at a time. If amphibians are unable to migrate to other wetlands, they cannot reproduce.
Researchers are examining the impact on amphibians of toxic chemicals and other contaminants that often become concentrated in wetlands.
Part of the goal of the Milwaukee conference was to champion the denizens of one ecological niche that don't always get the most positive attention. Slimy they may be, but frogs, toads and salamanders play a key ecological role.
They serve as the prey of animals ranging from raccoons to game fish, ducks and owls, as well as predators of wetland insects like mosquitoes.
What's more, some amphibians have evolved potent toxins in their skin to ward off predators. Scientists are investigating those substances for use as anesthetics or other medical purposes.
In an important sense, Caspar said, "amphibians are money in the bank. There are all sorts of things that they do that we don't know much about."
Information on declining amphibians is available on the World Wide Web at: (http://www.mei.com/other/mpm/collect/rvert.html).