Furthermore, the reliability of the data you give us is higher with calling observations. It is much easier to identify frogs by their calls than by their appearance, since several species look very similar. For example, you would have to be very close to see three warts per dark spot on a Fowler's toad as compared to only one or two on an American toad. Finally, monitoring calls makes it easier for you. As a general rule, frogs are more often heard than seen, especially if you are monitoring at a breeding site such as a wetland.
Some species are particularly sensitive to environmental change.
For this reason they are called indicator species. Frogs and toads make exceptionally good indicator species because they are very susceptible to a variety of environmental problems such as increased ultraviolet radiation, global warming, chemical pollutants and habitat loss due to urban development. Because frogs spend part of their life in water, and part on land, they also provide information about both ecosystems. As well as amphibians, other commonly used indicator species include insects such as mayflies, stoneflies and caddis flies.