Hotspot: California on the Edge
Central Valley Vernal Pools at the Jepson Prairie Preserve

Central Valley Vernal Pools
Ephemeral, Unique and Threatened

In spring they are splashes of vivid blue, yellow and white amid expanses of green grasslands. In summer they are uniformly golden brown and still. Vernal pools are seasonal wetlands that form where natural depressions occur in non-porous soils. Most are found in grasslands, but some form in chaparral, oak woodlands, deserts or coniferous
forests. Winter rain collects in these shallow basins and cannot percolate into the hard soil. When the rains stop, the water eventually evaporates. These ephemeral wetlands remain flooded long enough to prevent typical grassland vegetation from taking hold. The result is a unique assemblage of plants and animals, some of which are found nowhere else.

Vernal pools are like snowflakes – no two are exactly alike.  Each has its own soil composition and water chemistry, and supports a different combination of plants and animals. Depending on rainfall, a pool can be very different from one year to the next.
 These fragile ecosystems are in danger of disappearing. Over 90% of California’s original vernal pool habitat has already been lost, but efforts are being made to save what remains.

As a strategy for survival in a quickly evaporating pool tadpole shrimp produce cysts instead of eggs. Cysts provide a protective shell that prevents the embryo inside from drying out during summer. When the pools fill with winter rains, the embryos complete their development and hatch.

Tadpole shrimp are usually found near the bottom of the pool where they use their shield-like carapace to dig in the mud for food. They will eat almost anything including fairy shrimp and freshly-molted members of their own species.

vernal pool tadpole shrimp
Learn more about vernal pool tadpole shrimp >

Fairy shrimp cysts have elaborate architecture unique to each species. Cysts can tolerate extreme conditions and have survived near-boiling temperatures, months of freezing and 10 years in near-vacuum conditions.

Look closely - these tadpole shrimp have 3 eyes, but only two are used for sight. The third eye only detects variations in light. 

They have 35 to 71 pairs of legs, called phyllopods, that move from front to back and propel the shrimp forward.

Tadpole shrimp are crustaceans related to crabs and lobsters and look very much like their ancestors that lived 220 million years ago. They have short life spans of 20-90 days and can grow to be 3 inches (7.6 cm) long.

In the Central Valley, the summer tadpole shrimp eats rice seedlings and is considered a pest.

Fairy shrimp hatch as soon as there is water and mature in 4-60 days depending upon the species.

Mid-valley Fairy Shrimp
Branchinecta mesovallensis

This species appears to do best in shallow, cool-water pools, but can tolerate warm water. It hatches, matures and produces cysts within 16 days. Adults die when the pools dry up.

Giant Fairy Shrimp
Branchinecta gigas

The largest fairy shrimp in the world preys on the alkali fairy shrimp, Branchinecta mackini. It lives in plant-less, desert lakes in northeastern California and can grow to nearly 6 inches (15 cm).

Conservancy Fairy Shrimp
Branchinecta conservatio
Known from just 8 sites in California, it is only found in playa-type pools. These large pools, like Olcott Lake at Jepson Prairie Preserve, remain filled for a long time. This fairy shrimp needs an average of 50 days to mature.

California linderiella
Linderiella occidentalis

This is the most common vernal pool fairy shrimp in California. Its eye color, either red or black, is determined by the food it eats. Males of this species have elaborate appendages used to attract females and mate.

Vernal Pool Fairy Shrimp
Branchinecta lynchi

Found in vernal pools of the Great Central Valley, Jepson Prairie Preserve, and Solano, Contra Costa and Monterey counties, it needs just 18 days to complete its life cycle. This animal is threatened by extinction due to habitat loss from development and agriculture.

Since California tiger salamanders rely on rodent burrows for shelter, removal of pocket gophers and ground squirrels may also have negative effects on salamander populations.
California tiger salamanders are also threatened by hybridization with non-native tiger salamanders, introduced diseases and predation by introduced species.

California Tiger Salamander

Since being listed in 2004 as an endangered species, a coalition of researchers, land owners, the U.S. Fish and Wildlife Service and non-government organizations have been working together to ensure the survival of the California tiger salamander.

The land that provides critical habitat for the California tiger salamander’s survival is also some of the most sought-after for development in California.

 Research shows that adults, and particularly juveniles, roam far and wide, migrating a mile or more from their breeding pools. A minimum of several hundred acres of upland habitat surrounding a pool are necessary for the species to survive.

Reserves of multiple breeding ponds surrounded by 1000 acres (4 km²) or more of undisturbed habitat are required to ensure their survival. Unfortunately, such large tracts of undisturbed land are extremely rare in California’s Great Central Valley.

The good news is that cattle ranching and California tiger salamanders can share the same land. Ranchers are viewed as key conservation partners for the salamanders.

On the first rainy nights of winter, adult salamanders emerge and migrate to large vernal pools where they mate and lay eggs. Adults leave soon after egg-laying, and return to their underground retreats. After hatching, young salamanders eat small crustaceans and insects, including mosquito larvae. Later they eat larger aquatic insects, tadpoles of
Pacific treefrogs and even each other.

Three to six months later, juvenile salamanders become adults. They leave the pools and settle into the underground refuges of small mammal burrows before the dry summer season begins.

California Tiger Salamander Nymph

Their unique biology makes these salamanders extremely vulnerable to habitat modification by humans.

Juveniles take 4 to 5 years to reach sexual maturity, and most do not breed more than once in a lifetime. In very dry years, mating may not take place at all.

These black-and-yellow California tiger salamanders are mainly terrestrial, living within a mile or so of a vernal pool. However, they are seldom seen by people, since they live almost exclusively underground in the cool, safe burrows of pocket gophers and ground squirrels.

The California tiger salamander is one of the most distinctive, geographically-restricted species of vernal pool animals.

Over 60 plant species are endemic to vernal pool habitat, being found there and nowhere else.

Most vernal pool plants germinate during the winter as the pools are filling with rain water. They grow rapidly and begin to flower as the water starts to recede. Brightly colored, concentric rings of flowers are produced and gradually shrink toward the center as the pool dries. 

Plants that germinate or flower underwater often have floating leaves and air-filled stems. Long stems help leaves float on the surface where they shade sunlight from competing seedlings.

Many endemic vernal pool plants co-evolved with specific insect pollinators. They depend on each other for pollination and food. 

For plants and animals that live in and around vernal pools, speed is of the essence. They all share similar adaptations for surviving the cold wet winters and hot dry summers of our Mediterranean-type climate.

When the rains come, growth and reproduction must happen quickly before the pools dry up. Many plants and animals germinate or hatch underwater or very close to water.

For most vernal pool organisms, a dormant stage during which activity is reduced or suspended is necessary to withstand the summer’s drought and desiccating heat.

They also have fine-tuned mechanisms for detecting and reacting quickly to small changes in temperature and soil moisture.

Dwarf Downingia
Downingia pusilla

The petite white flowers of this vernal pool endemic are less than ¼ inch (2-3 mm) in size. It flowers in early spring before most other vernal pool plants. Dwarf downingia is threatened by habitat destruction throughout most of its range. 

White Meadowfoam
Limnanthes alba

This plant germinates soon after the rains begin.  It is pollinated by solitary bees that are directed to the nectar by “honey guides” on the petals. The plant produces oily seeds which float on top of the water.

White-headed Navarretia
Navarretia leucocephala ssp. leucocephala

Migrating painted lady butterflies are attracted to the nectar produced by this endemic vernal pool plant. It germinates underwater and grows very slowly until the pools begin to dry up. 

Colusa Grass
Neostapfia colusana

This rare native grass only occurs in large, deep vernal pools with muddy soils.  Much of it’s habitat has been converted to agriculture and urban development. It now exists in only 45 locations in California.

Competition from invasive native and non-native plants poses a problem at several sites.  

About 144 million years ago, during a time called the Cretaceous, most of California lay underwater.

The sea that covered the central part of present-day California teemed with life including many types of invertebrates, as well as fish, marine mammals, and large swimming reptiles. 

The sediments that eroded from the mountains formed the layers that record the position of California’s ancient shoreline.

Ammonites, marine reptiles, clams and even plants have been found as fossils imbedded in the layers of sediment that now fill the Great Central Valley. Ammonites are mollusks that went extinct about 65 million years ago, and are related to octopus and squid.

Inside their coiled shell, a siphon could expel water and propel the animal backwards.
Ammonites probably ate fish and crustaceans. They were likely preyed upon by marine reptiles.

Unlike most members of its kind, this beetle emerges in winter rather than spring and is active during the day instead of the night. Its good eyesight and speed make it an effective predator of fly larvae and other small invertebrates. It has two color forms, one spotted, the other not, and is well-camouflaged.

Delta green ground beetle

First described in 1878 from a single specimen, it was not found again until 1974.  It is known from only 10 square miles (25.9 km2) surrounding Jepson Prairie Preserve and is listed as threatened by the United States Fish and Wildlife Service.

The entire life cycle of these solitary bees revolves around a single vernal pool plant, goldfields, Lasthenia californica.

The bees over-winter in underground tunnels and emerge in early spring when their host plant flowers. Males who have been waiting near flowers mate with females as soon as they emerge from their tunnel. After mating, the female digs a main tunnel with 6
to 8 side tunnels, each ending in a brood chamber.

The female stocks each chamber with a goldfields pollen ball, lays a single egg on it and seals the chamber.  In a few days the egg hatches and the larva begins to
feed on the stored pollen. Reaching adulthood in the fall, the bees will spend the winter underground.

How the bees detect when goldfields are in flower is not fully known. Researchers at University of California, Davis suspect changes in soil temperature and moisture may trigger the synchronous flowering of goldfields and emergence of the bees. In drought years when plants do not flower, bees can remain waiting underground for
up to 4 years.

Vernal pool wetlands provide rest stops for Pacific Flyway migratory birds and wintering habitat for 19% of the waterfowl in the continental United States.
 Pools form links for migrating birds moving between and among wetland habitats.
When flooded, these ponds provide optimal foraging depths for many waterfowl.
Birds like killdeer, avocets, greater yellowlegs, and cinnamon teals help disperse vernal pool plant seeds and eggs or cysts of animals among adjacent pools.

This extensive soil series was mapped in vernal pool areas. The soils range in age from
100 to 450,000 years and formed from glacial sediments eroded from the Sierra Nevada.
Undisturbed layers of soil record climate change in the Great Central Valley. The older soils formed over a period of time that experienced dozens of climate changes. They provide scientists with insights into long-term climate change and geochemical processes.

Over time, the parent rock is changed into soil by weathering and vegetation. A soil profile reveals soil horizons with different color, texture, structure, pH and organic content.
Surface horizon: Organic matter accumulates here and more intense biological activity produces a darker color.

Subsoil horizons: As soil forms, processes create horizons with different properties that give a characteristic structure appearance and composition.

Color: As minerals in the soil weather, iron is released and reacts with oxygen in the presence of water to form red iron oxides. In California, older soils tend to be much redder than young soils.

Claypan: Clay content increases as you move down the horizons. Like a sponge, the clay soil expands when wet and shrinks when dry. When clay is saturated with water, the soil particles swell and create an impermeable layer called claypan.

Hard pan: Found 20 to 40 inches (51-102 cm) below the surface, hard pan is a water-proof layer of silica that dissolved out of minerals.

Botta’s pocket gopher
Thomomys bottae

These small mammals live in burrows with many connecting tunnels. Burrows can be 200 feet (61 m) long. 

In vernal pool grasslands, they dig close to the surface because the hardpan layer keeps them from going deeper.

Pocket gophers may be responsible in part for the formation of mounds surrounding vernal pools.

Sandhill cranes are migratory birds that spend their entire life in or near wetlands. Each winter they return to warmer climates to feed, socialize and roost together. In spring, they migrate to their nesting sites in northeastern California and southern Oregon. 

Sandhill cranes forage by picking and probing with their long bills both at, and below the water's surface, as well as on land. They prefer grain, but eat a wide variety of foods including berries, insects, reptiles, amphibians and small birds and mammals. Steady loss of wetlands adversely affects crane survival.

These nocturnal toads spend most of their life underground, using their flattened, spade-like hind feet to burrow.

They re-surface when they detect the sound of the first rains pounding the hard soil above, and enter vernal pools to breed and eat.

Adults can eat 11% of their body mass in a single feeding. Within a few weeks they have enough energy reserves to survive the annual dormancy period.

Male and female birds of the same species can often look very different. Males are usually more brightly colored while females are more camouflaged. In some birds, the difference is so striking that they were once thought to be entirely different species. This sexual dimorphism is usually related to courtship display and mating success.

Habitat Loss
Native grasslands and vernal pool habitats have been reduced to about 1% of their original extent by human development including agriculture, urbanization, and water diversions.

Invasive Non-native Species
Bullfrogs from nearby permanent streams, lakes and wetlands eat endangered tadpole shrimp and tiger salamander eggs, juveniles and adults. Non-native grasses have pushed out the native bunchgrass prairie.

Jepson Prairie Preserve, one of the last intact vernal pool habitats, is preserved and managed by the Solano Land Trust and the University of California Nature Reserve System.

Restricted access to Mather Air Force Base saved many vernal pools. After the base closed however, vernal pools were lost to development and many still await protection.
Most remaining pools are on privately-owned lands and partnerships with landholders is crucial to protecting them. Flying M Ranch, one of the most productive rangelands in the U.S., Llano Seco Ranch and Crane Ranch are successful examples of vernal pool stewardship.

What You Can Do


Above: Vernal pools at the Jepson Prairie Preserve

Vernal Pool Shrimp

vernal pool tadpole shrimp

Shrimp Diversity Magnified

California Tiger Salamander

California Tiger Salamander

Vernal Pool Plants


Delta green ground beetle

delta green ground beetle

Solitary Bee

Flyway Rest Stop



Habitat Loss

Invasive Non-native Species


What You Can Do