Masthead: Kaweah Range

Sierra Nature Notes, Volume 1, January 2001

Vernal Pools - Ephemeral Oasis of the Foothills
by Alisa Durgarian, M.A. CSU Fresno

VernalPool.GIF (263798 bytes)
The San Joaquin Valley is home to a unique habitat – vernal pools – that exist largely unnoticed by most of us. Agriculture and urban development are causing vernal pools to disappear before many of us have the chance to learn about, much less appreciate, all they have to offer.

How They Form
Vernal pools occur only where a narrow range of favorable conditions exist. First, a Mediterranean climate is necessary. This means most of the rainfall occurs from October to April during the cool winter months. The rainy season must then be followed by a hot, dry season where the pools completely dry out.

Topography is another key ingredient. A shallow depression is required, underlain by some soil substrate such as clay or basalt that is impervious to water percolation. In California, there are three geomorphological situations that provide for this: coastal terraces, broad alluvial valleys such as the San Joaquin and Sacramento valleys and ancient basaltic lava flows. Soils of vernal pools are typically very high in clay but can be derived from a variety of parent materials. These clay soils can be "expandable" clays – meaning that when they are dry, there is a network of large cracks throughout the surface to the subsoil level. As the pools fill with water, the clay soils begin to swell and the cracks are blocked off, creating the impermeable soil layer.

Hydrology is the next key ingredient to making a vernal pool. Specifically, water depth and duration of standing water play an important part in whether these areas can function as vernal pools. Water depths typically range from 10-60 cm (4 in. - 2 feet) deep. Pools need to remain inundated long enough to allow plants, invertebrates, and amphibians dependent upon them to complete their life cycle. Inundation can begin as early as November and go all the way until June in a very wet year. Shallow pools can fill with water, dry up and then refill again several times during a season. Typically, though, a vernal pool is filled with water for only 3-4 months, from about December through March. Vernal pools range from southern Oregon down into Mexico, south of San Diego but the majority of vernal pools occur on California’s coastal terraces and in the Great Central Valley.

Who Lives There?

The Tiny Critters
As vernal pools begin their cycle each winter, they create an environment that is host to a unique assemblage of characters. A patient naturalist with a hand lens will find a variety of protozoa, such as rotifers, that can occur. They survive by feeding on algae, yeast, bacteria and even other protozoa. Flatworms and segmented worms such as leeches often hatch out early in the season and subsist on decaying vegetation, organic matter or algae. Leeches can typically be found towards the bottom of the pool, avoiding light. To survive during the dry periods, they burrow into the mud as the pools dry out.

A step up from these single-celled denizens, are the arthropods, or joint-legged animals. Arthropods have an external skeleton as well as well-developed circulatory, digestive, reproductive, and nervous systems. There are three main types. The first group are the crustaceans. These include seed shrimp, clam shrimp, copepods, isopods, water fleas, tadpole shrimp and fairy shrimp.

One of the more interesting crustaceans is the whimsically named Vernal Pool Fairy Shrimp (Branchinecta lynchi). These small organisms are largely restricted to temporary pools and have evolutionarily adapted to their ephemeral situation. They have eleven pairs of legs and swim on their back, beating their legs in a fluid motion. Fairy shrimp first hatch from eggs at the bottom of the vernal pool when water temperatures reach 10 degrees Celsius. They undergo a series of molts before reaching maturity and in about 2 weeks are approximately 5-20 mm (0.2 in.- 0.8 in.) in length. They can live up to 4 months–just about the average time a vernal pool exists!

During mating, the male Fairy Shrimp holds onto the female with a second pair of antennae that are attached near his head. The female develops an egg sac behind her legs to hold the fertilized eggs. The female then secretes a chemical substance that solidifies the egg, making them durable enough to survive the dry season and hatch next winter.

Other arthropods include various insects and water mites and spiders. Some common aquatic insects include predaceous water beetles, water scavenger beetles, water boatman and backswimmers. Dragonflies and damselflies mate in mid-air and then the female deposits her eggs in floating algae or plant masses where the eggs complete their life cycles anywhere from 3 months to 5 years. Water mites live in floating vegetation and feed on small worms, crustaceans and insects. One spider, called the fisher spider spends its life around the water and can dive and remain submerged for extensive periods of time.

Them That Eats the Tiny Critters
The next group or animals that have specifically adapted their life cycle to the ephemeral nature of vernal pools are amphibians. The Pacific Tree Frog (Hyla regilla) is one such species. The Pacific Tree Frog is a chorus tree frog, due to its unique call. The males are the ones that sing the "choruses" to attract females during mating season. After mating successfully, eggs are laid in gelatinous masses in the vernal pools. The eggs are left to hatch on their own. The young tadpoles feed on algae and dead vegetation. As the pools begin to dry, the tree frogs migrate away from the pools and aestivate for the long dry summer in burrows, old logs, crevices between rocks and other protected places.

Western Spadefoot Toads (Scaphiopus hammondi) have a similar life cycle except that when it is time to aestivate for the summer, these toads have the ability to dig burrows into the ground up to 90 cm (35 inches) deep, especially when the soils are relatively soft and sandy. California Tiger Salamanders (Ambystoma tigrinum) are another unique amphibian found in vernal pools. They are active only after the first rains, when they emerge from their underground hiding places to locate breeding sites. The females deposit eggs in the vernal pools and the young complete their aquatic life cycle there.

Other various reptiles, birds and mammals use vernal pools throughout the winter months but do not need them to survive. Gopher snakes, rattlesnakes, and other species common to the area all probably forage around vernal pools.

Many birds use vernal pools: Cinnamon Teals, Mallards, Green-winged Teals, Ruddy Ducks, American Coots and Killdeer can all forage and nest in or within the vicinity of vernal pools. Horned Larks, Lark Sparrows, Mourning Doves, Western Meadowlarks, and Water Pipits can all forage and drink from vernal pools. Bank swallows have been seen taking mud from vernal pools to build their nests. Many mammal species such as California Ground Squirrel and Audubon Cottontail forage on bulbs, seeds and plants in and around vernal pools. Coyotes and Bobcats prey on small mammals attracted to these areas.

A Complex Flower Garden
Vernal pools also support a unique assemblage of plants, several of which are endemic or only occur in vernal pool habitats. Almost 200 species are known to only occur or be commonly associated with vernal pools. For such small ponds, they are incredibly diverse ecosystems. Many of the vernal plant species begin their life cycle during the aquatic phase of vernal pools. Then, as the pools dry out, these plants are able to complete their life cycle.

One of the unique and beautiful phenomena of vernal pools is how various species of plant begin flowering in concentric rings around the pool as the water levels drop. Early on, a ring of color surrounds the pool as meadow foam and yellow carpet begin their bloom. Then, as the pool dries further, a mosaic of color erupts from carpets of goldfields (Lasthenia sp.), blue and purple flowering downingias (Downingia sp.), mint smelling pogogyne (Pogogyne sp.), woolly marbles (Psilocarphus sp.), slender popcorn flower (Plagiobothrys stipitatus), tricolored monkey flower (Mimulus tricolor) with its bright pink corolla, prickly coyote thistle (Eryngium sp.) and annual hairgrass (Deschampsia danthonioides). If the pool is especially deep unique grasses like San Joaquin orcutt grass (Orcuttia inaequalis) or Greene’s tuctoria (Tuctoria greenei) might bloom in late May.

Vernal pool plants have also been found to depend on a unique group of solitary bees, which serve as pollinators. Many of these bees are host-specific to individual plant species such as the showy meadow foam, yellow carpet, and various species of goldfield and downingia. These bees begin their life cycle in the spring when they mate and begin excavation of their nests. These nests are usually located on higher, drier areas adjacent to the vernal pool. The burrow is shaped as a long shaft approximately 10-30 cm (4 in.-1 ft.) with soil at the entrance that serves as a door and a large room at the other end. When the female is out foraging, the entrance is open but upon her return, the door is closed. When the female is out foraging, she is collecting pollen that she ultimately stores in the large room at the end of the shaft. When she has enough pollen, she lays a single egg on top of the pollen mass. The room is then sealed and the female then begins excavation of a new room and starts the process all over.

Where to Visit Vernal Pools
Vernal pools are complex, ecological units that still hold a myriad of questions for us to attempt to answer. More and more people are coming to know and appreciate this habitat and greater efforts for protection are being made. Several preserves are now in place such as the McKenzie Preserve managed by the Sierra Foothill Conservancy and a brand new preserve just established by the Four Creeks Land Trust in Visalia. Education is the greatest key of all, fueling knowledge and appreciation for the uniqueness and diversity of nature. Many threats to vernal pools exist such as development, agriculture and mining but as our awareness increases so do the chances for preserving this amazing ecosystem.


For further reading on vernal pools and related subjects:


Alisa Durgarian did her Master's thesis on vernal pool ecology and now works for Hartesveldt Ecological Consulting Services near Oakhurst, CA.

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