HOTSPOT: Vernal Pools

Kathleen M. Wong

Amid the Central Valley's vast sea of grasses lie some of the oddest ecological islands in California. Known as vernal pools, they are home to dozens of tiny plant and animal species that live nowhere else in the world. From microscopic fairy shrimp to dainty white popcorn flowers, each has adapted to a world that floods in winter and dries to a crisp by summer.

Vernal pools owe their existence to a confluence of geology and climate. When winter and spring rains fall on hardpan and claypan soils, the collected waters may linger for weeks or months. The sudden arrival of moisture rouses the pools’ dormant residents to life. During the heady few months when the ponds are wet, a frenzy of hatching, courting, and mating goes on. After all, there isn’t much time. Clouds can disappear and pools evaporate with no whisper of warning.

Delta Green
Ground Beetle

The delta green ground beetle (Elaphrus viridis) is the ultimate vernal pool insider. It spends its whole life near these ephemeral wetlands, scurrying across patches of bare clay soil in search of tasty fly larvae or other small invertebrates. The beetle’s life cycle, like many of its neighbors’, is dictated by the state of the pools. Adults are active in winter and early spring, when the pools are wet. Eggs laid between January and April hatch into larvae that burrow into the moist clay for food and shelter. They spend the scorching summer hunkered down in dry soil. In fall, they pupate, emerging into the cool winter air as adults clad in one of two different spot patterns. Their emerald green hue and oval shape evokes the small leaves that sprout around the pool in winter, and camouflages the beetles from hunting birds and amphibians. This garb certainly helped hide the beetle from scientists; though first described in 1878, it was not recognized again until the 1970s. Now very rare—on a good day, a sharp-eyed entomologist might encounter one or two—the delta green ground beetle is found only in a ten square mile area around Jepson Prairie Reserve in California’s Central Valley. Scientists suspect the beetle once ranged much farther afield, taking advantage of winter floods that left much of the Central Valley underwater.

Françoise Chanut

But just add water, and the spadefoot toads dig their way up from under several feet of soil the first chance they get. Once freed from their earthen cocoons, they set out to catch their only meal of the year. After nightfall, tiger salamanders waddle out of burrows inherited from ground squirrels and look for mates beneath the stars. Fairy and tadpole shrimp cysts—really embryos in suspended animation—hatch into instant adults with the first sustained showers. Their wiggling legs and waving antennae beckon hungry waterfowl migrating along the Pacific Flyway.

As the waters recede, the animals vanish too, hunkering down into the soil for another year. Now the wildflowers emerge, surrounding each pool in rainbow-hued bathtub rings. Yellow tidy-tips, toothy petals edged in white, grow in concentric circles along with violet-bearded downingia, lilac-tinted meadowfoam, and carpets of miniature goldfields. Each week of drying brings one or two new blooms as another species sets seed and fades.

A few yards from the vernal pools swell mima mounds resembling ocean ripples covered in grass. The origins of the mounds remain something of a mystery, but the prevailing belief is that they are the result of digging by generations of gophers and ground squirrels.

Each pool—and there may be dozens in a single field—shelters a unique community of animals and plants. Across California, naturalists have identified more than 100 species that live only in and around vernal pools.

“Their disappearance or decline would mean a significant loss of state biodiversity,” says Jaymee Marty, an ecologist with The Nature Conservancy. Already, California has lost approximately 80 percent of its pools since the early 1970s. Fast-growing Sacramento County has lost more than 30 percent of its pools over the last decade alone.

Biologists are now studying how best to preserve the few precious pools that remain. Most have been rangeland for the last 100 years. To determine whether cattle grazing is good or bad for vernal pool health, Marty studied 72 pools on 12,362 acres of the Howard Ranch in eastern Sacramento County. She allowed cows and calves to graze on some pools, but excluded the livestock from others. She also surveyed the species diversity at each pool annually during the three-year experiment.

“We suspected we wouldn’t find a perfect fit, that for some species grazing would be positive and for others, negative,” Marty says. “But in the end, we found the historic level of grazing actually had the highest diversity for both native plants and aquatic invertebrates.” Without grazing, native plant cover dropped by 20 to 50 percent at both pool edges and upland areas, while exotic grasses increased their territory. Wildflowers and other forbs declined, while grasses began to dominate.

In addition, protected pools dried an average of two months faster than grazed pools. The extra time underwater can spell the difference between life and death for species like the California tiger salamander. “They need 90 days for larvae to turn into adults and walk out of the pools,” says Marty. If the pools dry up too fast, salamanders “are going to be stranded and die.” She suspects much of the water at ungrazed pools was sucked out of the soil by the extra grass.

Tiger Salamander

This young California tiger salamander (Ambystoma californiense), a species restricted to the vernal pools of California Central Valley, is far from ready to venture out of the water. But in a few months, it will have shed its gills, grown legs and lungs, and donned an adult’s slick black sheath splashed with pale yellow. In its new outfit, the young salamander will migrate a mile or more to dry land in search of a pocket gopher tunnel or ground squirrel burrow. There it will hunker down for most of the rest of its life, sheltered from summer sun and winter chill. After four to five years, it will emerge on a rainy winter night to trudge back to a vernal pool and find a mate. Adults return to dry ground soon after mating, leaving the eggs to hatch on their own. Born in the dead of winter, the young spend three to six months in the pool, feeding first on small crustaceans and insect larvae, and later on tadpoles of frogs or other salamanders. The tiger salamander’s unique life cycle leaves it vulnerable to habitat destruction and climatic variations. Some may only mate once in their lifetime, and during prolonged droughts, when vernal pools stay dry, they might not mate at all.

Françoise Chanut

Nonnative grasses pose another threat to this fragile habitat. Most invasive species can’t withstand the dramatic moisture swings and alkaline soil found within the pools. Unfortunately, those defenses don’t faze all nonnatives. Pepperweed, for example, marches right in on spreading subterranean runners. Growing up to three feet tall, it starves petite natives of nutrients, moisture, and sunlight.

Biologist Niall McCarten of Environmental Science Associates managed a CalFed study to determine how to eliminate pepperweed from a 320-acre vernal pool site at the former site of McClellan Air Force Base near Davis. He faced a terrible dilemma. Pulling up the plants and their runners by hand would disturb the soil, and could upset the ecology of the pool. And while herbicide spraying would kill the weed, it might also contaminate pool water and kill native wildflowers, including endangered species. He opted for a more painstaking approach instead. After the pools dried, his scientists trimmed the tops from many pepperweed plants with hand shears. They hand-painted some with the herbicide Roundup, and left the other clipped weeds alone.

A month later, the weeds that had been clipped and painted were dead and the endangered grasses in those plots had increased in size and number. Meanwhile, weeds that had only been clipped were regenerating. Control plots, where pepperweed had been left unmolested, had fared even worse, losing an average of four individual endangered grass plants per experimental plot.

This type of hand-weeding, says McCarten, “is labor intensive, but the results are worth it.” After all, the pools are also the only known habitat for Solano grass, a federally endangered species. “When you’ve got such rare species, we’d rather not take big risks.”

Kathleen M. Wong is Managing Editor of California Wild. This article was funded by the CALFED Science Program.