Related page: significant vernal pools
Photo courtesy of Woodlot Alternatives, Inc.
Vernal pools or "spring pools" are shallow depressions that usually contain water for only part of the year. In the Northeast, vernal pools may fill during the fall and winter as the water table rises. Rain and melting snow also contribute water during the spring. Vernal pools typically dry out by mid to late summer. Although vernal pools may only contain water for a relatively short period of time, they serve as essential breeding habitat for certain species of wildlife, including salamanders and frogs. Since vernal pools dry out on a regular basis, they cannot support permanent populations of fish. The absence of fish provides an important ecological advantage for species that have adapted to vernal pools, because their eggs and young are safe from predation.
Species that must have access to vernal pools in order to survive and reproduce are known as "obligate" vernal pool species. In Maine, obligate vernal pool species include wood frogs, spotted and blue-spotted salamanders (two types of mole salamanders) and fairy shrimp. While wood frogs and mole salamanders live most of their lives in uplands, they must return to vernal pools to mate and lay their eggs. The eggs and young of these amphibians develop in the pools until they are mature enough to migrate to adjacent uplands. Fairy shrimp are small crustaceans which spend their entire life cycle in vernal pools, and have adapted to constantly changing environmental conditions. Fairy shrimp egg cases remain on the pool bottom even after all water has disappeared. The eggs can survive long periods of drying and freezing, but will hatch in late winter or early spring when water returns to the pool.