The Carlisle Mosquito Online

Friday, April 6, 2001


Vernal pools: Close encounters of the amphibian kind

Within the next few weeks, swamps and ponds throughout Carlisle will spring to life with the familiar songs of spring peepers. These amphibians herald spring with their songs. Only the male frogs sing, to attract females which home in on their calls. In fact, the peep of the spring peeper can be heard as far as a mile away! During the first warm, rainy nights in spring, they are joined by wood frogs and mole salamanders, which travel from nearby woodlands to a special kind of pond known as a vernal pool.

Vernal pools are small, isolated ponds that fill with water during the winter and dry up by late summer. Like gems within the woodlands, these small pools of water are scattered throughout the landscape. Because of their small size and tendency to dry up, fish are unable to live ithere.

Breeding nurseries

Vernal pools are essential wildlife habitat because many amphibians such as the wood frog, spotted salamander, and the state-protected blue-spotted salamander, cannot breed successfully in any other kind of wetland. Frogs and salamanders use vernal pools as a safe place to lay their eggs, and as a nursery for their young. Within the vernal pool, amphibian eggs and young are safe from being eaten by fish.

To reach the vernal pools, wood frogs hop and mole salamanders crawl along the ground and even over melting snow and ice, crossing roads and lawns along the way. Every spring, each wood frog and mole salamander instinctively makes its way to the vernal pool where it was born. If the pool is destroyed, they will still make the pilgrimage to the same site, but will not be able to breed. When a vernal pool is lost, so too are the nearby populations of wood frogs and mole salamanders.

The journey to the vernal pool can be a treacherous one. Many wood frogs and mole salamanders will not reach their destination. Obstacles that must be overcome include walls, predators and roads. Where well-traveled roads must be crossed to reach the vernal pool, mortality rates due to automobile encounters can be quite high. Upon reaching the vernal pool, male wood frogs will sit upon the melting ice of ponds, calling for mates with a quacking call. Wood frogs are unique among frogs, in that they can freeze solid, yet survive the ordeal. Their blood acts as a natural anti-freeze.

You may be familiar with the common redback salamander that lives beneath leaf litter and stumps, but mole salamanders spend the majority of their lives living beneath the ground in woodlands and are rarely seen. They briefly leave their subterranean homes to breed. The most common mole salamander in Carlisle is the spotted salamander, approximately 6 to 8 inches in length, with a stocky build. It is black, with striking yellow spots decorating its body.

By late May, adult wood frogs and salamanders complete their breeding activities, and leave the pools, entrusting the safety of their eggs and young to the protective environment of the vernal pool. The adult wood frogs and salamanders return to the nearby forests to live and hibernate. The eggs soon hatch in the warm waters of the vernal pool. The young salamanders have a voracious appetite and will eat anything that moves and fits into their mouths. This includes many mosquito larvae. Both tadpoles and larval salamanders develop quickly. Most will leave the pool by early summer and join their parents in the nearby woodlands.

Food, water and shelter

The vernal pool is not only a nursery for amphibians, it is also home to a staggering array of plant and animal life. Its waters and vegetated banks offer food, water and shelter for warblers, ducks, wading birds, turtles, snakes, tree frogs, toads, green frogs and deer, as well as a variety of other wildlife species. Its waters teem with a variety of plants and insects, including dragonflies. Inch by inch, the amount of life within a vernal pool rivals that of a rainforest. The abundance of both food and water within vernal pools attracts wildlife from nearby woodlands. Vernal pools are also home to an unusual crustacean, the fairy shrimp. Fairy shrimp can live only in vernal pools. They are the freshwater cousins to brine shrimp. Fairy shrimp are an important food source for growing young salamanders and can often be found in large groups languidly back swimming through the water.

The vernal pool offers eggs, frogs, salamanders and fairy shrimp safety from fish, but not from human land development. Because of their small size, vernal pools often do not meet the criteria necessary to enjoy full protection under the Massachusetts Wetlands Protection Act and the adjacent upland area is rarely protected. A vernal pool without nearby undisturbed upland habitat for adult wood frogs and mole salamanders will quickly lose its breeding population of these amphibians.

Protecting vernal pools

Protecting vernal pools and nearby adjacent upland is not only important for salamanders and frogs, but is also good sense for people. Vernal pools help to protect the groundwater supply, acting as storage basins for snow melt and rain water, which otherwise can flood roads and basements.

The Carlisle Conservation Commission is currently working on revisions to the Carlisle Wetlands Bylaw. These revisions will be presented for vote in the May Town Meeting. The proposed revisions help to ensure that these woodland gems remain a part of Carlisle's landscape for years to come.

Soon the wood frogs and salamanders will start making their way to vernal pools throughout the town. During those first warm, rainy nights, find a flashlight, a rain slicker and your inner child, listen for the peeps and quacks of spring peepers and wood frogs, watch where you place your feet and tires, and go exploring. If you are lucky, you just might have a close encounter with a spotted salamander. You can learn more about vernal pools by visiting

Rachel Landry is a former member of the Carlisle Conservation Commission and a wetland scientist at Earth Tech, Inc. Last year she presented the Vernal Pool workshop to the Association of Massachusetts Wetland Scientists.

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