As Earth warmed and dried over millions of years, amphibians had to specialize and adapt strategies for a wide variety of environments.
This has driven the development of the thousands of species we have today and enabled amphibians to survive in very hot deserts, rainforests, temperate ponds and streams, and even frozen Arctic tundra. Amphibians live all over the world, on every continent, except Antarctica. They are abundant and diverse in tropical regions, where the damp environment most suits them.
Amphibians are divided into three main groups: frogs, salamanders, and caecilians (pronounced seh-SILL-ians).
There are between 4,000 and 6,500 species of frogs and toads, about 400 species of salamanders, and fewer than 200 species of caecilians. Caecilians are legless amphibians that live underground or in water in the tropics. The word amphibian comes from the Greek word amphibios, which means two lives. This description refers to their life cycle, which begins in water and moves to land. Because they are dependent on both aquatic and terrestrial environments, amphibians are particularly sensitive to changes in those environments. Their skin also makes them sensitive. They have no scales, feathers, or fur to protect them, so amphibians are vulnerable to pollution, attacks by predators, and even sunlight. For this reason, they are generally nocturnal. Their skin is also highly permeable (they can take in oxygen, or breathe, through it), making amphibians more vulnerable to pollutants, especially pesticides, fertilizers, or acid rain.