Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe development of natal amphibian and insect species unable to withstand competition or predation by fish. Certain tropical fish lineages (such as killifishes) have however adapted to this habitat specifically. Vernal pools are a type of wetland. They can be surrounded by many communities/species including deciduous forest, grassland, lodgepole pine forest, blue oak woodland, sagebrush steppe, succulent coastal scrub and prairie. These pools are characteristic of Mediterranean climates, but occur in many other ecosystems. Generation and annual development During most years, a vernal pool basin will experience inundation from rain/precipitation, followed by desiccation from evapotranspiration. These conditions are commonly associated with Mediterranean climate such as the Central Valley of California. Vernal pool basins are often characteristics of Mediterranean climates. Most pools are dry for at least part of the year, and fill with the winter rains, spring snowmelts and rising water tables. Some pools may remain at least partially filled with water over the course of a year or more, but all vernal pools dry up periodically. Typically, though, a vernal pool has three phases each year: it is inundated in the winter (inundated phase) with the vernal pool holding onto the water from 10–65 days, it dries slowly during the spring (flowering phase), and it dries completely during the summer (dry phase). Vernal pools favor native species because many non-native species cannot tolerate the extreme seasonal changes in environmental conditions. Some vernal pools have an underlying impermeable clay layer (also known as a hardpan) that reduces water percolation. The impermeable layer is hydrophobic and it prevents water from draining into lower soil layers, allowing vernal pools to become inundated for a very long period of time. This feature of vernal pools means that the water is allowed to slowly evaporate instead of draining. This is a key factor in the development of vernal pool plant communities as it keeps the soil at the waters edge just wet enough for vernal plant communities to flourish while those closer to the center of the pool are more inundated, leading to zonation of plant communities as the water level recedes. This clay layer also allows pools to exist long enough to prevent