What is an Amphibian?
Amphibians are a class of terrestrial vertebrates with permeable skin. Reptiles, birds, and mammals have impermeable skin. Amphibian skin is highly permeable to water and gases. Instead of drinking water and breathing, amphibians absorb water and oxygen through their skin. Amphibians have eggs without shells, versus birds and reptiles who have eggs with shells. There are three main groups (orders) of amphibians:
- Frogs and toads (Anura) are the most familiar group of amphibians. They have short bodies, lack tails, and have limbs suited to jumping, hopping, or swimming. Most frogs and toads have an aquatic tadpole life stage and terrestrial adult stage.
- Salamanders (Caudata) have linear bodies and tails, similar to lizards. Some salamanders live their entire life on land while others live their entire life in water. Salamanders in the family Plethodontidae lack lungs, lay eggs on land, and do not have a tadpole stage.
- Caecilians (Gymnophonia) are worm-like amphibians without legs and are proficient burrowers. They occur underground or in water. Caecilians occur in tropical areas but not in North America north of Mexico.
Amphibians Are Energy Efficient
Amphibians are cold blooded (ectotherms) with a heart containing two atriums and one ventricle. Birds and mammals are warm blooded (endothermic) with a heart containing two ventricles and two atriums. Amphibians need much less energy than birds and mammals and are more efficient in converting energy into new tissue. Amphibians convert more than 50% of the energy they obtain into tissue. For mammals, it is just 2%.
Amphibians Are Compact
On average, amphibians are small-sized. Amphibians typically weigh less than 10 grams while birds and mammals typically weigh more than 10 grams. Most frogs and salamanders are compact, just a few inches in length, and easy to miss. In addition, most amphibians are nocturnal, which makes them especially difficult to find.
Amphibians Live in Diverse Habitats
There are approximately 7,200 species of amphibians worldwide in diverse habitats on all the continents except Antarctica. Typical amphibians have an aquatic larval habitat (such as tadpoles in ponds) and a terrestrial adult habitat (such as frogs or salamanders in forests). Amphibians are not found in the ocean. Many amphibians have unique habitats. Some salamanders live exclusively in caves, and some frogs and caecilians burrow deep into the ground. Hellbenders live under flat rocks in fast-flowing streams in the northeastern United States. Many tropical frogs live in rainforests. Amphibian life characteristics and habitats make them vulnerable to human impacts. Frogs and salamanders can lose their habitat when wetlands are disturbed or forests cleared. Pesticides can enter through their permeable skins. Fungal disease has decimated many amphibian populations worldwide.
Amphibian Hot Spots in the United States
A few areas of the United States have a relatively high diversity of amphibians. According to W.E. Dulleman (1999), there are three amphibian hot spots in the United States:
- Pacific-Cascades-Sierra Nevada Mountain Ranges (California, Oregon, and Washington)
- Southern Appalachian Mountains and associated plateaus (North Carolina, Tennessee, and Virginia)
- Southern Coastal Plain (Alabama, Florida, Georgia, Mississippi, North Carolina, and South Carolina)
These areas have a high number of amphibian species and endemic amphibian species not found elsewhere.