The Carlisle Mosquito Online

Friday, March 24, 2006


Vernal Pools

Woodfrog. (Photo by Kay Fairweather)
The vernal equinox, which marks the beginning of spring, occurred on Monday, March 20, and is one of the two days in the year when the length of the day is equal to the length of the night. I joined Tom Brownrigg at the Benfield Land and watched the sun come up over the treetops approximately 30 degrees to the east of where it rose on the winter solstice. I thought this would be a good week for an overview on vernal pools.

Definitions: There are many definitions of vernal pools, some in popular use and others with specific meanings to scientists and regulators. A general definition would be a pond that dries up every summer (or every few summers) and comes back every year. The Field Guide to the Animals of Vernal Pools defines them as "any fish-free temporary wetland that supports indicator species." This is a good workable definition once you know the indicator species, and if you don't mind relative terms like "temporary." Just think of the temporary Rourke bridge set up across the Merrimack River in Lowell in 1983 and still in use today. The definition of temporary, for a vernal pool, would be a period of time short enough to prevent breeding populations of fish from becoming established, and long enough to support the life cycles of the indicator species.

Indicator species: There are two kinds of indicator species — those that are completely dependent on vernal pools, and those that use both vernal pools and other wetlands. The former, called obligate species, are the wood frog, four types of mole salamanders, two types of fairy shrimp, and the spadefoot toad (a threatened species, known in Plum Island and probably not occurring in Carlisle). The other group, called facultative, has a lot more species and includes spring peepers and other frogs and toads, many turtles, amphibious snails, leeches, fingernail clams, and the larvae of many insects. The obligate species are seldom seen. This is due to a combination of their reclusive life style and our inattention. It is not indicative of their numbers or their significance to the health of the ecosystem. Sometime soon we will be hearing from hundreds of wood frogs as they turn on their cell phones — all with the same quacking call signal, all on high volume — and frantically, urgently calling around for a date. The noise more than compensates for the quiet nature of the mole salamanders and the tiny fairy shrimp. The presence of mole salamanders is usually confirmed by finding their eggs, which have various distinguishing characteristics. Fairy shrimp are small crustaceans, up to 1.5 inches long, and are well camouflaged by the leaf litter in the pond. I will write a column featuring the fairy shrimp when and if I catch one — or anyone can take this as a challenge to beat me to it.

Spotted salamanders. (Photo by Kay Fairweather)
Salamander migration: On the first few rainy nights in early spring when the temperature is 40 degrees or more, the spotted salamanders and blue-spotted salamanders migrate en masse to vernal pools to breed. The Jefferson salamander migrates earlier, sometimes when the ground is still partially frozen and ice is still on the ponds. The fourth of the mole salamanders, the marbled salamander, uses a different strategy. It seeks out a dry vernal pool in August or September, lays its eggs in a nest, and tends them until they hatch when the pool fills in the fall. If the pool does not fill before winter, the female abandons her nest to find a place to hibernate.

Certification: A vernal pool in Massachusetts may be certified, a process which officially establishes its existence and affords it some legal protection from habitat disruption. Anyone can send evidence of a vernal pool to the state for certification. Everything you need to know about the three methods of certification is at the Natural Heritage web site listed below in references. All methods are based on the presence of indicator species. You need to learn the indicator species, collect the evidence, and document it using the appropriate forms. Tom Brownrigg has certified some Carlisle vernal pools and is willing to help others who are interested.

April 23, save the date: The Trails Committee is hosting a vernal pool walk on April 23 at a site yet to be selected. Chris Kavalauskas, an expert in this topic, will be the leader.

References: Leo P. Kenney and Matthew R. Burne, Field Guide to the Animals of Vernal Pools — I highly recommend this book for children or adults. It has lots of color photos and excellent descriptions and is available at the Town Hall for $10. Also, the Natural Heritage and Endangered Species Program web site (go to and click on "Natural Heritage."

Submissions to Biodiversity Corner are encouraged from everyone. you can write the column or tell me what you saw and I will write it. the only requirements are that the species exists in the wild and was seen in Carlisle. Send a message to Kay Fairweather at

2006 The Carlisle Mosquito