When is a Pool Not a Pool?

When it's a vernal pool, a habitat that comes and goes with the seasons

• Mark Cheater
• Apr 01, 2000

It's nearing midnight on a chilly Tuesday evening in May, and Mike Hayslett is lost in the woods.

He crashes through huckleberry bushes and fallen hickory branches in the Blue Ridge Mountains of western Virginia, his headlamp cutting a bright swath in the darkness. The 34-year-old biologist has spent the past three hours in the woods looking for tiger salamanders some of the colorful creatures that breed in the small, temporary wetlands known as vernal pools. This next pool, he says, will certainly have tigers. He just has to find it.

Stopping suddenly, he twists his head to the right and cups his hand around his ear. "Hear that?" he says. In the distance, there's a faint clicking noise like a Geiger counter with a low reading. "Cricket frogs. Our pool is over there," he says, then turns on his heels and heads off again.

Hayslett is one of a growing cadre of people who willingly don hip boots and head out into the dead of night to study vernal pools and their denizens. Vernal pools are the most humble of wetlands -- sometimes no more than glorified puddles. They form from rainwater or snow melt in spring (thus the moniker 'vernal,' derived from the Latin word for spring) and usually disappear by June or July. These ephemeral oases may not look like much, but they shelter many imperiled species (including amphibians, a group of animals in decline worldwide) and play a vital role in the ecology of many forests and prairies.

Under Siege

The pools go by a variety of colorful terms: prairie potholes, whale wallows, hog wallows, sinks, kettles. But one term most people haven't attached to them is 'important.' Because their biological significance is little known, and because they often lie unprotected on private land, these pools are under siege in many parts of the country.

Although there are no nationwide figures on the number of vernal pools, experts agree that these unique habitats are disappearing. "They're small, they're easily overlooked and they're easily destroyed before they're even discovered," says Hayslett, founder of the Nearby Nature Center in Lynchburg, Virginia, and one of the state's leading vernal pool advocates.

What brings Mike Hayslett and others out into the woods on cold spring evenings is the chance to observe the strange assortment of creatures that gathers in vernal pools. Among these animals are the pond-breeding salamanders, which slither out of their subterranean dens each year to mate and lay eggs in small woodland ponds often in the course of just a few nights.

Hayslett points out that the tiger and similar salamanders are rarely seen above ground. "Every breeding season there's this phenomenal emergence and migration to their maternal ponds," says Hayslett. "It's often over and done with in 2 or 3 nights, then they go back. So, even though these animals may be fairly common, most people never see them. You have to be in the right place at the right time."

Of Fish and Salamanders

What lures the reclusive creatures to vernal ponds, as opposed to other bodies of water, is what these ponds lack: fish. Because the ponds dry up every year, fish can't survive in them and so salamanders can breed there safely. "That's the key, because fish are effective predators of aquatic organisms such as small salamander larvae," says Hayslett.

Unlike fish, salamanders and other amphibians are only water dwellers for part of their lives. Hayslett demonstrates this earlier in the evening by swishing a small net through the dark, leaf-filled waters of one of the vernal pools he comes across. He pulls the net out and, in the glare of his headlamp, a dozen brown, shiny, thumb-sized spotted salamander larvae squirm. These larvae, he explains, spend the first 2 to 5 months of their lives swimming in the ponds (feeding throughout the pool at night, thus the nocturnal trips by vernal pool enthusiasts). Then, as the pools begin to dry out in the summer, the larvae absorb their feathery gills and tail fins and emerge from the water, spending the remainder of their lives on land.

Several species of salamanders--spotted, marbled, Jefferson and tiger, among others--are almost completely dependent on vernal pools for breeding. But they aren't the only creatures that use these unique habitats. "About one-third of all amphibians in the eastern United States are strongly dependent on vernal ponds as breeding sites, and many other species use them less frequently,"  says Jim Petranka, a biologist at the University of North Carolina at Asheville.

Among the other vernal pond breeders are the thimble-sized cricket frog, the 2-inch-long wood frog and the spring peeper. Like the salamanders, they congregate in the water in late winter and spring, where they mate and lay bunches of sticky, gelatinous eggs. The frogs' croaking or creaking is often a giveaway to the location of remote vernal pools.

Remarkable Residents

The most remarkable of vernal pool residents may be a creature called the fairy shrimp. These inch-long crustaceans, whose diaphanous bodies are tinted in varying shades of blue, orange or yellow, are as ephemeral as their habitat. The shrimp mate and the females lay eggs that fall to the bottom of the pool. The eggs go into suspended animation when the pool dries out, sitting in the sediment for months or years, if necessary, until the pool fills up again. Once immersed in water, the creatures come to life, breed and die--all in as few as 15 days.

While the fairy shrimp are around, they serve as an important food source for hungry waterfowl and other birds migrating north during the spring. This is especially true in the Midwest, where temporary and permanent wetlands known as prairie potholes annually host hundreds of thousands of migratory birds that feed largely on the shrimp.

But perhaps the most important aspect of vernal pools, says David Skelly, an ecologist at Yale University, is that "they're home to a host of organisms that really can't live anywhere else."

Trouble in California

Nowhere is that more true than in California's Central Valley, where vernal pools shelter more than 60 species of plants and animals found nowhere else in the world. These include several types of fairy shrimp that are found in only a few pools. Among the unique plants are at least seven species of unusual mints in the genus Pogogyne. These flowering plants are part of a springtime explosion of color around California's vernal pools that naturalist John Muir referred to as "a continuous bed of honey-bloom."

Despite their colorful displays, California's vernal pools are fast disappearing. Less than 7 percent of the state's original pools remain, and many of those are on private land. The rest have been drained and plowed for cropland, or leveled and turned into housing or office developments.

Similar threats face ephemeral wetlands across the country. "Vernal ponds have been lost at an alarming rate and are becoming increasingly isolated from one another," says Petranka. The problem, Hayslett says, is that many people view the pools as soggy nuisances, "so there's a real disparity in the public's understanding of their biological significance."

Draining Experience

To demonstrate this, earlier in the day Hayslett drives through the rolling tobacco-farming country south of Lynchburg, turns down a freshly surfaced gravel road and pulls off onto a field. He walks across the road and down an embankment into a vernal pool that looks like a small blackwater swamp about 50 feet across, dotted with heath shrubs and shiny-leaved tupelo trees and red maples.

Hayslett puts on his rubber hip boots and wades out into the 2-foot-deep water. "This is covered by trees, but look around the perimeter," he says, pointing to stumps just 30 feet away. "The whole surrounding area has been logged." Since adult amphibians live in the forests adjacent to vernal pools in many parts of the country, the loss of these buffering woods can be just as devastating as the loss of the vernal pools themselves.

But that's not the only threat to this pool. Hayslett sloshes over to a new, 4-inch- diameter metal drainpipe installed next to the pond and under the road, which has siphoned off part of the pond s crucial water. One of Hayslett's friends and a fellow vernal pool enthusiast, David L. Dawson, was driving on this road earlier in the spring--not long after the salamanders had laid their eggs--and noticed state workers resurfacing the road and installing the pipe. "They had no clue as to the importance of this site," says Hayslett. "When David spotted it, the swamp had lost a lot of water because of the drain." Hayslett and Dawson came back soon after and scooped more than 200 of the gelatinous egg masses into coolers, then moved them to a more-protected pool about a mile away.

"This pool is pretty much trashed," says Hayslett. This is an all-too-common fate of vernal pools as more roads, houses and parking lots are built. The traffic on roads provides an additional hazard to amphibians as they move to and from the pools in the spring, he adds. "The road mortality when animals are migrating in and out can have a holocaustic effect on a population--25 percent could get wiped out on the way in, another 25 percent on the way out."

Schools for Pools

To help stem the loss of Virginia's vernal pools and their denizens, Hayslett has launched a grass-roots program to raise awareness about the pools. This program, Schools for Pools, aims to get students to adopt and protect local vernal pools. The program is modeled after an effort by Leo Kenney, a biology teacher at Reading Memorial High School in Massachusetts, who trains students in his state to identify vernal pools. Kenney's program, now in its eighth year, has led to the certification and protection of hundreds of vernal pools in Massachusetts and spawned similar programs in at least three other states.

"Most vernal pools are on private lands," Hayslett says. "The only way we've got to protect them at this time is through grass-roots education. Who's a landowner going to listen to more than the youth in their own community? I'm convinced that it's the single most effective way to protect them in our state."

In the darkness of George Washington National Forest later that night, Hayslett is still looking for the pool in the woods. "It's a real trick working out here at night," he says. "Many a researcher has gotten turned around--myself included." He follows the clicking of the cricket frogs, which gradually grows louder. Then the forest opens to reveal an 80-foot-diameter pond, its surface shimmering with the reflection of a waxing moon.

He splashes into the pool and glances back and forth looking for salamanders. No tigers, but his headlamp picks up a 2-inch-long spotted salamander larva in the tea-colored water. "Look at him--he's just floating in the water. That's what they do at night--they float and anything that happens along, they latch onto it."

Hayslett looks up and glances around the pond, its perimeter of trees softly silhouetted against the moonlit sky. "When we come back in September, this whole area will be dry, like a huge glen. Vernal pools are here today, gone tomorrow."

Senior Editor Mark Cheater went vernal pool hunting with biologist Michael S. Hayslett last spring.

To find out more about how you can help protect vernal pools and their residents, check out the following Web sites:

The Vernal Pool Association
Frog Force
NWF's Backyard Wildlife Habitat™ program