Zoology 10th Edition by Stephen Miller,John Harley

Exercise 13

Amphibians are especially vulnerable to environmental disturbances because of their double life on land and in water and because of their thin, permeable skin. Climate change and susceptibility to a chytrid fungus have resulted in alarming population declines. Wetland preservation, population monitoring, and captive breeding programs are needed to help save amphibian populations.
In what ways are amphibians serving as an "environmental warning system"?