upland species from developing, while existing for just enough time to prevent aquatic plant species from taking over. Some authorities restrict the definition of vernal pools to exclude seasonal wetlands that have defined inlet and outlet channels. The justification is that such seasonal wetlands tend to be qualitatively different from isolated vernal pools; this is because they are fed by larger drainage basins so that firstly, inflow contributes higher concentrations of dissolved minerals. Secondly, flow patterns increase the periodic scouring and silting effect of flows through or simply into the wetland. Thirdly, longer distance inflow and outflow make for less strictly endemic populations and plants. Low dissolved mineral concentrations of smaller vernal pool basins may be characterized as oligotrophic, and poorly buffered with rapid pH shifts due to carbon dioxide uptake during photosynthesis. Vernal pools are so called because they are often, though not necessarily, at their maximum depth in the spring ("vernal" meaning of, relating to, or occurring in the spring). There are many local names for such pools, depending upon the part of the world in which they occur. Vernal pools may form in forest, but they are more typically associated with grassland and rocky plains or basins. While many vernal pools are only a few meters in width, playas and prairie potholes are usually much larger, but still are otherwise similar in many respects, with high water in wet periods, followed by dry conditions. Some exclude desert playas from the definition of vernal pools because their larger closed drainage basins in areas with high evaporation rates produce higher concentrations of dissolved minerals, with salinity and alkalinity favoring different species. Playas may be inundated less frequently than vernal pools, and inundation typically coincides with colder weather unfavorable for plant growth. Ecology Despite being dry at times, vernal pools teem with life when filled, serving as critical breeding grounds for many amphibian and invertebrate species. The most obvious inhabitants are various species of breeding frogs and toads. Some salamanders also utilize vernal pools for reproduction, but the adults may visit the pool only briefly. Other notable inhabitants are Daphnia and fairy shrimp, the latter often used as an indicator species to decisively define a vernal pool. Other indicator species, at least in New England, are the wood frog, the spadefoot toad, and some species of mole salamanders. While vernal pools can be devoid of fish, in some habitats such