Vernal pools, also called vernal ponds or ephemeral pools, are temporary pools of water. They are usually devoid of fish, and thus allow the safe development of natal amphibian and insect species. Certain tropical fish lineages (such as killifishes) have adapted to this habitat specifically, however.
Most pools are dry for at least part of the year and fill with the winter rains or snow melt. 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.
They are called vernal pools because they are often, but not necessarily, at their peak depth in the spring ("vernal" meaning of, relating to, or occurring in the spring).
Despite being dry at times, once filled they teem with life. The most obvious inhabitants are various species of 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. There may be vegetative indicators of vernal pools but further study must be conducted to verify the relationships between vernal pools and their characteristic flora and fauna.
In some northern areas, tadpole shrimp are also common.
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The underlying soils of the vernal pool are a fundamental part of the vernal pool habitat. These soils are often referred to as hydric. In most cases there is a hard pan layer which causes the retention of water in the pools. The entire process of the normal soil turning into hydric soil is often referred to as "base retention". Professor Kenneth Pearson first discovered this process in 1856 when he found a body of water that had suddenly appeared after a rainy day in what is today Leominster, Massachusetts. He noticed the water had remained for about 3 weeks, and wondered why this was. When he studied the samples of soil he had retrieved from the site, he named the process "base retention" for the soils conversion into a retentive state.[1]
In vernal pools, flowering occurs simultaneously because of the seasonality of favorable conditions. Vernal pools are home to many endemic species because of the unique environmental niches created by acidity and salinity gradients.
Different species are suited to different moisture levels, and as water evaporates from the edges of a pool, rings of flowers blossom around it. The color patterns change as the wet season wears on. The rings may form swirls and layers, with the green of new grass surrounding the whole pattern.
Flora commonly found at vernal pools include Downingia and lupine species, yellow pansies, several sweet-scented clovers, a variety of goldfields, button parsleys, yellow and bright lavender monkeyflowers, star lilies, yarrow, and endangered grasses such as Solano grass.
Because of their characteristics, vernal pools are often threatened by development in the same way that wetlands are.
As a result, most pools have been converted into residential zones, roads, and industrial parks. That is why most extant pools occur on protected or private land such as national parks, and ranches.
A large number of rare, endangered species, and endemic species occur in vernal pool areas. For example, the San Diego mesa mint, a highly endangered plant, is found exclusively in vernal pools in the San Diego area. Another example is the wildflower Lasthenia conjugens, which is found in limited parts of the San Francisco Bay Area. A third example is the herb Limnanthes vinculans endemic to Sonoma County, California.
Many of the amphibians that breed only in vernal pools spend most of their lives in the uplands within hundreds of feet of the vernal pool. Eggs are laid in the vernal pool, then the juveniles leave the pool two or three months later, not to return until the following spring to breed. Therefore, the upland areas surrounding a vernal pool are critical for the survival of these species. In New York state the endangered tiger salamander (Ambystoma tigrinum) is dependent on vernal pools to breed as described above. A few other obligate vernal pool species are the marbled salamander (Ambystoma opacum), Jefferson's salamander (Ambystoma jeffersonianum), the blue-spotted salamander (Ambystoma laterale) and the spotted salamander (Ambystoma maculatum).
Some other species, notably the fairy shrimp, lay eggs capable of entering a state of cryptobiosis. They hatch when rains replenish the water of the pool.