Vernal pools are small areas of wetland that form in the spring and dry up during the summer. Water collects in saucer-shaped depressions that have an impermeable layer of soil, leaves, or debris. Snowmelt and spring rains fill these puddles. Without an inlet to replenish the supply, summer’s sun and heat eventually evaporate the water, though a dense forest canopy helps delay the inevitable drying up. Some vernal pools may refill after a heavy rain, but the main characteristic is their temporary nature.
The shrill, bell-like songs of spring peepers might lead you to a vernal pool. As you approach, the tiny frogs will probably stop singing, but if you stay quiet, they’ll start up again. While you wait, look in the water. If you are at a vernal pool (as opposed to a pond or marsh), you will notice that the water is shallow, typically less than four feet deep, and has no fish or submerged vegetation. In early spring you may see masses of salamander and frog eggs. A few weeks later you can observe the young amphibians developing. It’s a race against time: hatch, grow, and get out before the pool dries up.
In order for their young to survive, some animals require bodies of water that do not have fish. For the most part, they breed in vernal pools. In our area, these species include the fairy shrimp, wood frog, and several salamanders: blue-spotted, spotted (with yellow spots), Jefferson, and eastern four-toed. Spring peepers do not use fish-free pools exclusively, but since vernal pools form in their wooded habitat, they commonly breed right along with the other frogs and salamanders. Other pool inhabitants include fingernail clams, newts, snails, caddisflies, water fleas, damselfly larvae, green frogs, and American toads.
Spotted salamanders are particularly interesting. They are dark brown or gray, six to eight inches long, and have two rows of yellow spots down their back and tail. By count they are our most common salamander, yet they are often difficult to find. They don’t hang around after breeding, and spend much of the year underground in burrows within a quarter-mile of the pool they were born in. Males arrive first and intermingle closely in a large group, called a congress. A male and female pair mate underwater, where the fertilized egg mass attaches to a firm structure such as a twig.
Caddisflies have a strategy to avoid the hatch-grow-exit time crunch. They live their entire life in the pool, so getting out is not necessary. They lay their eggs, called cysts, in the leaves at the bottom of the water. Before dropping the eggs, the females cover them with a substance that solidifies them. The eggs can survive summer drought and winter cold, even accidental ingestion, and hatch the next spring when the pool fills up again. (Unfortunately for an ingested caddisfly egg, if it is released onto permanently dry ground, it will not hatch.)
The lack of fish in a vernal pool does not mean there are no predators. Wood ducks, mallards, and great blue herons sometimes feast on the amphibians where vernal pools are near large bodies of water. Birds, reptiles, and small mammals take advantage of prey who cannot escape. There is also danger within the pool population itself: salamander larvae have a carnivorous diet, feeding mostly on the invertebrates in the pool, but sometimes eating tadpoles and even other salamander larvae.
The importance of vernal pools is hard to exaggerate. Their health depends on forest owners and visitors preserving them. Recommendations include leaving a 100-foot buffer zone, in which there is no clearing of land or permanent development, and a 600-foot zone in which greater than 75 percent of the area is retained in forest condition, with at least sixty percent canopy cover. Compacted soils, rerouted water channels, pesticides, disturbed forest floor, and manmade barriers that prevent amphibian travel are concerns. We can all play a part in keeping these ephemeral ecosystems thriving on a permanent basis.
Barbara Mackay is a teacher and naturalist who lives in northern Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org