Vernal Affairs
by Robin Meadows
In many parts of the world,
temporary pools are a lifeline for many aquatic animals.
In the temperate eastern U.S., these areas—called vernal
pools—come to life in spring, and sometimes in fall.
You can find these hidden habitats if you know what
to look for....
The woods in much of the eastern third of the U.S. contain
countless shallow depressions. To the unpracticed eye,
these lowlying areas are unremarkable until it
rains in the spring. Then they fill up, forming ephemeral
bodies of water aptly called vernal pools. Even when
full of water they appear at first to be no more than
puddles. But vernal pools are actually oases bursting
with life.
"When you walk over to a vernal pool, you have to change your focus and think small," says Betsy Colburn, a freshwater ecologist at the Massachusetts Audubon Society. "You won't necessarily see anything big and flashy right away. You have to look for things that are an eighth to a quarter of an inch long and that often blend in with the background. But if you sit still you will see tiny invertebrates, tadpoles, and beetles darting across."
The pools are home to many invertebrates found in no other habitat, notably fairy shrimp (Eubranchipus sp.), inchlong, bright pinkorange crustaceans that swim by continuously undulating their 11 pairs of legs. Equipped with eyes that swivel on stalks, fairy shrimp are a sight not to be missed.
Ephemeral bodies of water are found in many places around the world. In eastern North America they are particularly common in low-lying areas such as those found in woodlands. Vernal pools are the only breeding sites of the wood frog (Rana sylvatica) in the U.S., and of many of North America's 15 species of mole salamanders in the genus Ambystoma. Wood frogs, which grow up to three inches long and range from rosy pink to dark brown, are noted for being the only frog to live north of the Arctic Circle. Mole salamanders get their common name from their habit of living several feet underground in moist burrows made by shrews and other small mammals. Ranging from three to eight inches long, depending on the species, most mole salamanders are black with spots of a contrasting color such as yellow or blue.
This striking coloration is associated with noxious skin compounds and serves to warn wouldbe predators that the salamanders might not be a tasty mouthful afterall. "I've tasted them by accident and felt a burning sensation," says Randy Cassell, who teaches biology at both Cumberland Valley High School in Mechanicsburg, Pennsylvania, and Messiah College in Grantham, Pennsylvania, and has studied mole salamanders and vernal pools for much of his life.
Unfortunately for the species that depend on them, vernal pools in the U.S. are in trouble. Many of these temporary waters fail to meet the official criteria for wetlands and so are not protected under federal law. In addition, people often destroy vernal pools unwittingly because they are easy to overlook when dry.
Despite their name, not all vernal pools fill in the spring. Due to fall rains and rising groundwater levels, some vernal pools fill in the fall and stay wet into the spring. But most come to life in spring, when spring rains and snowmelt fill them. While many vernal pools in the eastern U.S. are in forests, they are also found in meadows, river floodplains, and coastal dunes. Typically small and shallow, vernal pools can be only a few feet across, and the largest ones are rarely more than 150 feet across and three feet deep. Some of the pools persist well into the hot days of summer, much longer than would be expected based on their size, because they are lined with clay deposited by glaciers.
Living in an environment that cycles between extremes—from inundated to dry and from hot to cold—is a challenge vernal pool species meet in a variety of ways. Plants drop seeds that lie dormant when the pool is dry and then sprout the next time it fills again. Fairy shrimp and other invertebrates endemic to vernal pools take a similar approach, leaving drought and coldresistant eggs that winter over in dry basins and hatch when the pools fill in the spring. In fact, the eggs of many fairy shrimp species must dry out and freeze before they can hatch. The eggs remain viable for years, ensuring that the species survive even during long periods of drought. Fingernail clams (including Pisidium sp.) take another approach to surviving when their pool dries up: these up-to-halfinch bivalves burrow into the mud and seal themselves off until the pool refills.
Mole salamanders use two strategies to increase the likelihood of their survival: they live a long time and generally breed in the pools where they were born. The blackandyellow spotted salamander (Ambystoma maculatum), the most common species of mole salamander, lives as long as 20 years. The salamanders' longevity increases their chances of reproducing successfully because even if a pool does dry up too early one year (that is, before the young are ready for terrestrial life), the adults will live to breed again the following year. The strategy of returning to their birth pool increases the chances that the young will grow up in suitable poolsthat is, ones that generally stay wet long enough for the young to mature. Spotted salamanders have been known to return to the same vernal pools for 15 to 20 years.
Mole salamanders spend most of their lives in upland forests, anywhere from several hundred yards to half a mile from the pool where they were born. This means that when the urge to breed strikes, the salamanders must travel from their forest burrows to the pool. On rainy nights during the breeding season, the time of which varies depending on the species, hordes—hundreds or even thousands-—of mole salamanders migrate en masse to their vernal pools, letting nothing stand in their way. "I've seen Jefferson salamanders migrate across snow," says Cumberland Valley High School's Cassell.
A vernal pool's yearly cycle of salamander breeding sometimes begins in the fall. On warm, rainy nights, the blackandwhite marbled salamander (Ambystoma opacum) migrates to the dry basin of its vernal pool. The time of the migration depends on how far north the pool is: Marbled salamanders migrate as early as September in southern New Hampshire, the northernmost part of their range, and as late as December in northern Florida, the farthest south the species is found. A female lays 50 to 200 eggs in the basin and then usually curls around her eggs to protect them from ants and other predators until the pool fills enough to cover them. Once wet, the eggs begin to develop and the larval salamanders overwinter in the pool.
Other mole salamander species wait until their vernal pools contain water before migrating, typically on rainy nights in early spring when the temperature creeps above freezing. These species include the spotted salamander, the darkchocolate-colored Jefferson salamander (Ambystoma jeffersonianum), and the bluespotted salamander (Ambystoma laterale). Spotted salamander eggs are the easiest to find because they are laid in largest masses, which contain 100 to 200 eggs and are the size of an orange. If you are lucky enough to find a mass of salamander eggs, don't be surprised if it is green—algae grow in the protective jelly layer that surrounds the eggs.
Like these species of mole salamanders, wood frogs migrate to their vernal pools in early spring, often arriving before the ice has melted completely. A local population can number in the hundreds to thousands and males can arrive at their pool within a few hours of each other. Unlike salamanders, which are quiet barring an occasional chirp, courting wood frogs make a racket with their quacklike calls. "A full chorus...can be almost deafening if you are in the midst of it," says Leo Kenney, a biology teacher who founded the Reading Memorial High School Vernal Pool Association in Reading, Massachusetts.
Wood frog eggs can be quite conspicuous because the frogs often lay their eggs together and there may be as many as 300 fistsized egg masses in one spot on submerged parts of plants. "The presence of wood frog eggs is perhaps the easiest method for identifying a vernal pool," says Kenney.
Amphibians lay so many eggs because very few of the young reach adulthood. At best only about 10 percent of the tadpoles and salamander larvae survive long enough to leave their vernal pool because they become meals for a great variety of animals including snapping turtles, great blue herons, and aquatic insects such as diving beetles (family Dytiscidae) and giant water bugs (Lethocerus americanus). Diving beetle larvae, also called "water tigers," hang upside-down from the water surface and grab their prey with strong, sickleshaped jaws. The up-to-threeinchlong giant water bug, also called a "toebiter," subdues its prey by stabbing with its beak and injecting anesthetic saliva into the hapless creatures. And as if all of these predators weren't enough, salamanders must contend with cannibalism: salamander larvae are voracious carnivores and larger ones readily eat smaller ones.
Although undeniably devastating to young amphibians, predators are natural. The real threat to the species that live and breed in vernal pools is destruction of the pools themselves. People use dry vernal pool basins as dump sites and treat full pools with BTI (a strain of Bacillus thuringiensis), which kills larvae of mosquitoes, midges, and other related flies. "It wipes out a whole section of the invertebrate community, a whole part of the food web," says the Massachusetts Audubon Society's Colburn.
In addition, many vernal pools have been and continue to be dredged, filled, and developed. "They're temporary wetlands, so they look like great building sites most of the year," says Laury Zicari, the U.S. Fish & Wildlife Service's (USFWS) Northeast Regional Coordinator for permits. Even if vernal pools are not disturbed directly, they can still be adversely affected by development. "The closer vernal pools are to disturbed areas such as lawns and roads, the more sediment and the fewer invertebrates the pools have," says Colburn.
Logging practices can also destroy vernal pools. "[The pools] tend to be in nice flat areas in the woods and loggers say, 'Oh good, here's a place to put wood,'" says Colburn. Even if loggers don't destroy a pool basin directly, their heavy equipment leaves deep ruts that confuse migrating salamanders. When the ruts fill with water, salamanders lay their eggs in them by mistake. This is disastrous for the young salamanders because the ruts dry out much sooner than the pools do. Moreover, heavy equipment undoubtedly crushes adult salamanders in their burrows.
Vernal pools also suffer when loggers cut the trees around the basin, exposing the pool to direct sunlight. As a result, the water in the pool gets warmer and evaporates faster, increasing the chances that young amphibians will be stranded in mud before they are ready to leave the pool. In addition, vernal pools that lose their shade often become overgrown with plants and seem to attract bullfrogs and other amphibians characteristic of permanent bodies of water rather than mole salamanders and wood frogs, says Cumberland Valley High School's Cassell.
To help loggers protect vernal pools, foresters and wildlife biologists have developed voluntary guidelines. They recommend keeping the pool basin entirely undisturbed; establishing 50foot buffer zones around the pools, which means no clearcutting and no heavy equipment; and establishing 50- to 200foot "low ground disturbance" zones around the pools, which means in part operating heavy equipment only when the ground is frozen so it won't leave ruts.
Voluntary protection of vernal pools is essential because their legal protection is inadequate. "Vernal pools have two things against them," says the USFWS's Zicari. "They are small and sometimes dry up before the start of the official growing season." To be defined as a wetland, an area has to be wet for a certain amount of time during the official growing season. This season is based on when agricultural crops grow and so is irrelevant to the life cycles of vernal pool plants and animals, which usually start growing very early in the spring.
"The confusion over where vernal pools fit into the federal definition of wetlands impedes their protection," says Colburn. "The real issue with vernal pools and wetlands is that we tend to look at this as if there is no relationship between a pool and the rest of the ecosystem. But amphibians also use uplands." In other words, protecting a pool basin is not enough: Part of the surrounding land must also be protected.
Vernal pools—and other wetlands—fall under the jurisdiction of the Army Corps of Engineers, a branch of the federal government historically dedicated to maintaining navigable waters in the U.S. Over the years, the Corps' jurisdiction has crept upstream and it now includes wetlands and headwaters. According to Zicari, the Army Corps of Engineers takes advantage of a huge loophole in wetlands protection: isolated waters and headwaters are less protected than other bodies of water, such as rivers and lakes. "Under Nationwide Permit #26, the Army Corps of Engineers can expedite permitting [to build] on up to 10 acres. And developers basically have federal preapproval for sites that are less than one acre," says Zicari.
Legal status aside, ignorance can play a large part in vernal pool destruction. Many landowners are simply unaware that the pools exist at all, let alone on their land. "Public education is one of the most important parts of vernal pool protection," says Colburn. "People may not recognize what a unique habitat they've got. They're not just mosquito holes."
Vernal pools must be identified before they can be protected. To find vernal pools near you, look for contained depressions on topographical maps; take evening walks through the woods in early spring, listening for chorusing wood frogs and looking for migrating salamanders; or contact your state wildlife division for tips on how and where to find pools.
Full vernal pools are relatively easy to find, and once you know what you're looking for, you can even find dry basins. "Vernal pools often have black, mucky soil that smells of sulfur because its anaerobic," says Zicari. "If you dig underneath the surface of some pools they're battleship gray because the iron has washed away." Another clue that a lowlying area is a dry vernal pool basin is that the leaf litter is blackened and compressed rather than light brown and loose. Also, you may find freshwater clam shells and discarded cases of caddisfly larvae, which can be about an inch long and are made of fragments of leaves or stems, small stones, or even empty snail shells, depending on the species and the materials available.
Another clue is that trees growing in a vernal pool basin can look unusual. First, the lower parts of trunks often have darkly stained bark, indicating the pool's high-water mark. Second, the roots are often twisted and deformed and the trunks are sometimes buttressed, which presumably makes the trees more stable when the ground is saturated with water.
So the next time you're hiking through the woods, keep your eyes and ears open. You might be rewarded with the sights and sounds of one of nature's wonders, a vernal pool.
Robin Meadows is a contributing editor to ZooGoer.
(ZooGoer 24(1) 1995. Copyright 1995 Robin Meadows.
All rights reserved.)
