Nature Close to Home: Vernal Pools – Mud Puddles or Unique Wetlands
Scattered through both the countryside and developed areas, vernal pools are little known types of wetlands that are largely undocumented. Vernal, or spring, pools are depressions in the ground that fill with water during the fall and winter months, then are dry by midsummer.
These little known wetlands can be found in isolated areas throughout Prince William County. They provide an irreplaceable habitat for some very specialized animals such as wood frogs, spotted salamanders and marbled salamanders.
Often called hatcheries for the forest, vernal pools are dynamic mini-ecosystems teeming with life. Come across a vernal pool and you will have discovered a wonderful opportunity to observe some secretive forest residents, who leave their forest homes to breed in these temporary pools. You will also have discovered a unique and valuable habitat important to protect.
The seasonal nature of vernal pools is a double-edged sword for the amphibians that begin their life cycles here. Because they do not hold water year round, vernal pools are unsuitable habitat for fish, major predators of both amphibians and their eggs. However, vernal pools also present special challenges to the animals that depend on this habitat for their survival: animals whose young must be able to survive in upland areas before the water disappears. Life in a vernal pool is a race against time.
One of the first animals to silently trudge their way to vernal pools each year is the marbled salamander (Ambystoma opacum), which breeds in October and November. Rarely seen because of its exceptionally secretive nature, this salamander has white or grey bands across a black body and completes its breeding well before pools thaw in the spring.
Marbled salamanders make nests in scraped-out cavities or under logs at the edge of the vernal pool, where the females lay single eggs. Most females stay with the eggs until autumn rains either raise the water level to reach the nest or wash the eggs into the pool. The eggs hatch in about two weeks and these gilled larvae transform into adults by late June. At first the newly transformed marbled salamanders can be found under debris at the edges of the vernal pool, but disperse rapidly.
Another amphibian that relies on vernal pools is the spotted salamander (Ambystoma maculatum), whose bright yellow spots against a steel-grey background make identification a simple task. Spotted salamanders spend most of the year in moist woods buried in leaves but, come a damp and dreary night in late February or early March, they begin to make their way to vernal pools.
When they arrive, usually to the same pool year after year, the males outnumber the females. Spotted salamanders often court in groups scattered around the shallow areas of the vernal pool. Within two or three days, the females begin to lay eggs on fallen tree branches and aquatic plants. A single egg mass, with an average of 150 eggs, can take up to an hour for the female to deposit. In little more than a month, the eggs hatch into gilled larvae that transform into adults between June and August.
The wood frog (Rana sylvatica), easy to identify by its dark racoon-like eye mask, is another amphibian that spends its life in upland areas and ventures to vernal pools only to breed. Their clue each year to return to the pool is a heavy rain that stretches into the evening, when temperatures reach at least 40 degrees.
In Prince William, wood frogs may begin to breed in February but we hear most choruses in early March. Listen for their duck like quacking, an unmistakable clue. Wood frogs lay globular egg masses, after which the adults return to the woods where they hide under debris or logs. After a week or so in the water, their egg masses spread and rise to the water surface.
Wood frog tadpoles hatch in two weeks to a month, with the eggs laid earlier in the season developing more slowly. These very dark tadpoles get some added protection from their dark color, which matches the color of fallen leaves in the water. In spring, the edges of these vernal pools are swarming with young wood frogs.
Why one vernal pool is heavily populated, yet another apparently barren of life, is a mystery yet to be discovered. What we do know is that many species depend on vernal pools for their breeding grounds and their survival.
One popular myth is that, if the wetland is saved, the species that use the wetland are also saved. Salamanders can live half a mile from where they breed. Since animals use vernal pools for only part of their life cycle, with many spending most of their lives in woods and forests, protecting the vernal pool itself is only half the answer. Saving a vernal pool will ensure that these animals have a place to breed, but development of the surrounding area leaves these same animals with no place to live.
There are no nationwide statistics on vernal pools, but experts agree that this is a rapidly vanishing habitat. Vernal pools are small, isolated and easily overlooked. Many are destroyed before they can be discovered.
Interest in documenting and protecting these isolated wetlands is growing nationwide. State legislation in Massachusetts gave special protection to vernal pools in 1987, provided they were “certified” by the Natural Heritage Program. This gave new impetus to volunteer monitoring and Massachusetts responded by publishing a manual that outlined the procedures and criteria in a straightforward manner.
In Virginia Mike Hayslett, Director of the Nearby Nature Center in Lynchburg, has started a Schools for Pools program. This opportunity provides education and resources, helping document and protect these important habitats.
Although vernal pools are found throughout Prince William County, no one really knows what's out there. Many vernal pools are on private lands. Protection of these special wetlands depends on a widespread understanding that vernal pools are more than soggy nuisance areas and, in fact, have great biological significance.
These special areas need special attention that will come only when community residents understand and value this unique habitat. Take a walk today and find a vernal pool, then visit it regularly and be amazed at the wonders of nature found close to home.