In a vernal pool grassland, rainwater partly sinks into the ground while the rest flows into streams or depressions, but a hardpan layer of clay or minerals prevents water from moving deeper, so once the soils are saturated, perched water collects in landscape depressions as vernal pools. The water leaves these pools only slowly through the ground or by evaporation. Vernal pools are a special kind of wetland, a transition between water and dry land, and wetlands hold rainwater that might otherwise cause flooding, purify storm runoff by removing sediment and nutrients, and provide habitat for hundreds of plant and animal species, many of them rare and endangered because wetlands have been destroyed. In winter, vernal pools support dozens of aquatic species linked in a complex food web, with decaying matter, bacteria, algae, and protozoa at the bottom and fairy shrimp, tadpole shrimp, and tadpoles higher up, while many animals found there are important food sources for migratory waterfowl and some live nowhere else on Earth. As spring warms and water evaporates, concentric rings and patches of wildflowers appear, with each pool showing a different mix of species shaped by differing needs for water, warmth, and light; goldfields and sky blues are among more than 60 plant species in California that live only in vernal pools. During summer and fall, the pools are dry and vegetation is brown, but microscopic eggs, cysts, spores, and seeds remain to survive the hot dry season, and seeds from flowering plants feed insects, birds, voles, and gophers, while hawks, coyotes, snakes, and toads also use the grasslands and pools. California's vernal pools are unique for their distinctive plants and animals, yet nearly 90% of the state's original wetlands have been destroyed by agriculture and other development, and continued preservation of vernal pools and other wetlands is vital for rare and endangered species and for water quality.