One possible reason for the decline of some frog population is acid rain.
Acid rain is formed when invisible gases are released into the air and mixed with rain clouds.
Most of these gases come from businesses that burn large amounts of fossil fuels such as coal and oil. Another source of harmful gases comes from driving cars that burn gasoline. These gases combine with the moisture in the clouds and fall back to the ground not as clean rain but as acid rain. The acid in the water is harmful to plants, water systems, such as lakes and rivers, and animals such as salamanders and frogs. In eastern Europe, acid rain has actually destroyed forests.