Frogs: Class Amphibia

Amphibians crawled from the water over 370 million years ago and were the first vertebrates to colonise the land. Australia has around 200 species of native amphibians, all of which are frogs, belonging to the order Anura.

Most are still dependent on water to complete their life cycle as they have a larval stage that lives in water. The word 'amphibian' actually means 'two lives' - one in water and one on land. Australia has around 200 species of native amphibians, all of which are frogs. About 37 of these are found in Sydney.

One of the factors that allowed the transition from water to land was the development of specialised skin. The skin of a frog is more than just a covering. It is used for a number of important biological functions including:

• absorbing oxygen and releasing carbon dioxide
• regulating their salt content and absorbing water
• changing colour to camouflage themselves
• secreting mucus to avoid drying out (as well as making them slippery, which is a form of defence)
• oozing poisons to deter predators (only some frogs do this).

Frogs play a key role in many food webs, both as predators and as prey. Frogs are carnivores and mostly eat insects. For many larger animals including snakes and birds, frogs are an essential part of their diet.

By observing frog populations, we can get a good indication of the condition of the environment as frogs are sensitive to environmental change. At present, frog populations are declining all around the world.