A Springtime Phenomenon

Vernal pools serve as a key player in the New England ecosystem, aiding wildlife throughout the region. 

Spring has sprung, and you know what that means? It’s time for the snow to melt, the tulips to bloom and the vernal pools to come to life.

Vernal pools are a big springtime phenomenon. They’re created when a depression in the forest floor fills up with water from melting snow and rain. They typically dry up later in the year, around late summer or early fall. Because of this wet/dry cycle, they are a specialized habitat for many amphibians and a number of invertebrates, that breed specifically in these vernal pools. Vernal pools are special because they cannot hold species such as fish, which are predators to those animals that rely on vernal pools to breed; fish would normally pose a threat to amphibian eggs, larvae and tadpoles.

Another special characteristic of vernal pools is their very high biodiversity. There have been around 550 different species documented that utilize New England vernal pools, not only for breeding, but also for foraging and feeding. According to Brett Amy Thelen of the Harris Center for Conservation Education, another way vernal pools help animals is that they are “kind of like wetland stepping stones in the woods for turtles, small birds and small mammals that need protected wet areas in order to move through the landscape.” These vernal pools provide them with a temporary shelter along their travels.

Thelen notes there are two things that come into play when it comes to identifying a vernal pool.

“Technically, in order to be considered a vernal pool, there is the physical definition: It is a body of water, isolated from other bodies of water,” she said. “They exist on their own and aren’t connected to existing wetlands, streams or lakes and rivers, and they eventually dry up. But there is also a biological definition: There is breeding by what is called obligate or indicator species. These are species that rely on vernal pools for breeding, such as wood frogs, spotted salamanders, Jefferson salamanders and fairy shrimp.”

With both the physical and biological definitions, it would be considered a vernal pool. “If you have a body of water in the woods and none of those species are breeding there,” Thelen added, “then technically it is not a vernal pool.”

Generally, vernal pools begin in the spring and then dry up in the late summer or early fall. However, Thelen said “every vernal pool is different depending how big it is, how deep it is, where it is located, how much rain we’ve gotten that year, or how much snow.” Some pools don’t dry up every year; some dry up every third or fourth year. Vernal pools are typically quite shallow — rarely more than 4 or 5 feet, often no more than two feet.

“One great way to find vernal pools is to listen for the sounds of wood frogs,” Thelen said. Wood frogs are little brown-orange frogs, about 3 inches long. They are a common early spring frog species. The males do what is called “chorusing,” which is when they make a noise or sing, which sounds a lot like quacking ducks. This chorusing is part of their courtship, so they’ll do it for two or three weeks after the first warm, rainy night. If you follow the sound, chances are you will come to an active vernal pool. There are many in the region.

“I can say that there is a complex of eight to 10 vernal pools in the Robinhood Park woods,” Thelen said, “and if you spend enough time walking through the woods in April, and you keep your ears open for wood frogs, you have a pretty good chance of finding a vernal pool.”

Vernal pools are an important habitat, and they can be easy to miss and very vulnerable to development and impact. The Harris Center has initiated a great effort in helping the public learn about and identify vernal pools to help protect them when possible. To learn more about vernal pools, identifying them, or identifying eggs and species within them, visit harriscenter.org.

The Harris Center will be holding a Zoom talk on the basic ecology of vernal pools on April 3 at 11 a.m., as well as a moderately-strenuous, 1.2-mile (round-trip) hike to a vernal pool on April 21 from 1 to 3 p.m. Online registration is required; visit harriscenter.org to register.