Wetlands are areas where water either covers the soil or soil is saturated permanently or seasonally. Wetlands have soils that have developed anaerobic (lacking oxygen) conditions in the upper part of the soil layer, known as hydric soils. Plant species that have adapted to survive in these conditions are known as hydrophytes. Since water is essential for almost everything on earth, wetlands are one of the most biologically productive ecosystems on the planet and are home to plants, mammals, birds, reptiles, amphibians, and fish that are uncommon in other ecosystems.