Frogs are more likely to be out calling when it is warm and wet. Scientists have long considered amphibians as early indicators of environmental problems because they live on land and in water. Most amphibians have permeable skin, which allows substances to be absorbed and concentrate in their bodies. Declines have been attributed to a variety of causes, including habitat destruction, chemical contamination, changes in the ozone layer, acid rain, parasites, and viruses. Yet not all amphibians are in decline, and within a single location, some species may be in decline while others are stable.
Amphibian populations tend to fluctuate a lot.
So the best thing to do is collect data over a wide area and over a long period of time. Most scientists agree that although declines are evident across the globe, there is no single factor responsible. We lack the long-term research data to make the type of conclusions we want to. According to Sam Droege, biologist and head of monitoring development for the USGS at the Patuxent Wildlife Research Center in Maryland, the scientific design of the frog-calling survey is quite rigorous. Researchers at the USGS at Patuxent help set up monitoring programs in each state, acting as advisers and consultants.