Frogs and other amphibians are often used as indicator species. Their skin is very permeable and they absorb and concentrate toxins in their fatty tissues. Also, amphibians are particularly sensitive to pollution because they live at the threshold of two different habitats. When there are lots of healthy frogs in an area, scientists know that the ecosystem is also relatively healthy. However, when populations suddenly dissipate, die off, or when there is an increase in mutations, something has clearly changed for the worse in their environment.