Cows create homes for tadpole shrimp

This is the rare and endangered tadpole shrimp in a vernal pool atop Big Table Mountain. The shrimp grow to be about 1 inch long.
This is the rare and endangered tadpole shrimp in a vernal pool atop Big Table Mountain. The shrimp grow to be about 1 inch long. MARK CROSSE / THE FRESNO BEE

Endangered tadpole shrimp are flourishing this week in massive mud puddles on a flat-top foothill -- and they owe their good time to a bunch of grass-munching cows.

Well-timed rain and a cool spring have set the stage for the shrimp to reproduce in abundance, but nothing much would have happened here without those visiting cows.

Over several years, state officials have brought in the cows to clear out the grass that was stealing water from the huge temporary pools that form in winter and spring - vernal pools - where the shrimp live.

Biologists say bovine intervention has actually nurtured a swath of nature on the Big Table Mountain Ecological Preserve northeast of Fresno where wildflowers, tadpole shrimp and many other creatures needed help.

Though the effort won't get the shrimp off the endangered-species list, it is a step in the right direction, state biologists say. And it took courage. State officials got opposition from environmentalists who knew poorly timed grazing could lead to trampled pastures and streams.

The table top had been privately owned in the past, and cattle had grazed on it for many years, officials said. Officials didn't realize the cows were thinning out the invasive grass that had been introduced by European settlers more than a century before.

After acquiring the property, state officials tried to return it to a more natural state by removing cows in the early 1990s. But the elegant ecosystem was overrun with grasses that are not native to California. The flat-top mountain overlooking Millerton Lake became a sea of green.

Vernal pools - which form in low spots - were fast disappearing. The pools support many kinds of life, such as the tadpole shrimp. If they disappear forever, birds and other creatures would lose an important food source in the ecological web of life. Well-managed grazing was the obvious solution.

"We thought we were being smart by taking the cows off the land in the '90s," said Chuck Peck, executive director of the Sierra Foothill Conservancy, which assists in the effort. "But if you think about it, the best vernal pool areas are usually on ranches where there is grazing."

Since 2000, state officials have allowed ranchers to lease the land and send in cows to devour grasses on the preserve. Grazing takes place only from October to early March so the animals wouldn't obliterate wildflowers and important habitat for birds or other creatures.

Now biologists from the state Department of Fish and Game see an impressive diversity of life.

As she hiked the table top this week, Annee Ferranti, senior environmental scientist with the state, spotted an array of wildflowers - goldfields, fiddleneck, sidalcia, Chinese houses, meadowfoam and many others. Different species of flowers come and go as the weather warms up. The display happens almost in secret. The public can see it on tours with the Foothill Conservancy, but otherwise access is quite difficult.

For many Central Californians, the table tops are mysterious-looking flat foothills visible from Highway 41 and other roads in the area.

Geologists explain that the table tops are actually the course of the San Joaquin River about 10 million years ago. From a plane, they say they can trace the course of the ancient river by following the table tops.

The flat-top foothills were created when lava flowed down the old river channel millions of years ago. The basalt flow coated the granite in the river channel, protecting it from natural weathering.

Over time, the hills around the river eroded and disappeared, leaving the flat, lava-covered river bottom elevated above the countryside. In other words, the surrounding erosion has turned the ancient river bottom into flat-top foothills at about 2,000 feet in elevation.

Depressions in the table top basalt easily hold water, allowing vernal pools to form. These pools can be filled with creatures, such as the tadpole shrimp or the fairy shrimp. They come alive when the water is available.

"Their eggs are preserved in cysts for many years," said Ferranti. "They will wait in the soil until the vernal pool forms."

State Fish and Game owns the 715-acre preserve, and state biologists Michelle Selmon and Krista Tomlinson are busy cataloguing plants and animals. Whether it's an American kestrel in flight or a northern Pacific rattlesnake out for some sun, they take note.

Around a large vernal pool, they swatted at huge flies and stepped gingerly around cow pies. They found many tadpole shrimp - which means good eating for ducks, egrets, herons and other birds.

"This looks like a good year," Selmon said. "I was surprised by the number of tadpole shrimp here. Looks like they're having a party."