On May 7, 2015

The Outside Story

Vernal pools: Hatch, grow and get out

By Barbara Mackay

Three things happened this week: bluebirds and tree swallows returned, my road was graded, and the red maple buds popped. It’s time to search for vernal pools.

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, although 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 backs and tails. 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 lives 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. The Vermont Fish and Wildlife Department recommends keeping 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 remains forested, 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: [email protected]

Do you want to submit feedback to the editor?

Send Us An Email!

Related Posts

Pies, parades, and porch chats

July 2, 2025
“America is a tune. It must be sung together.”—Gerald Stanley lee The month of July is the height of summer, bringing a spirit of celebration to all of us. Our town of Killington may be small, but we know how to celebrate the 4th of July. We start early with the annual book sale at…

Inventing a better ski day: the innovations that drew crowds to Killington

July 2, 2025
By Karen D. Lorentz Editors’ Note: This is part of a series on the factors that enabled Killington to become the Beast of the East. Quotations are from author interviews in the 1980s for the book Killington, A Story of Mountains and Men. “We’ve got a million dollars that says you’ll learn to ski at…

‘Almost Heaven’

July 2, 2025
The stage was simple, designed to resemble a wooden board that resembled the siding of any barn, anywhere across America. It could have been the barn behind my house, or the one that my cousins have down in Georgia. It could have been a barn in Colorado or even West Virginia.  Nothing remarkable at all,…

Getting away from it all

July 2, 2025
My family and I went to the beach this past week. The temperatures were hot, and the weather was sunny, making for a classic seaside vacation. The house we rented was in the harbor of the town where we were visiting, so while we didn’t stare out at the ocean, we were able to sit…