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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. This ensures the absence of fish, a chief characteristic of all pools.

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 four species of mole salamanders.

In some northern areas, tadpole shrimp are also common.

Flora

Vernal pool flowers, with different species occurring in bands related to soil moisture and temperature gradients formed as the pool dries out.
Sacramento National Wildlife Refuge, Calif.
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 niche 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 Diegomarker area. Another example is the wildflower Lasthenia conjugens, which is found in limited parts of the San Francisco Bay Areamarker. A third example is the herb Limnanthes vinculans endemic to Sonoma County, Californiamarker.

Fauna

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.

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.

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