Vernal Pools:
A Small Ecosystem with a Big Splash

Many people have become familiar with these small, temporary basins that typically fill with water during the fall and winter, and dry up during summer. Although easily overlooked, vernal pools are home to an incredible diversity of life, including not only the wood frog and spotted salamander, but also dragonflies, caddisflies, diving beetles, midges, clams and snails, fairy shrimp, and many other organisms.   

The majority of vernal pools in our watershed were created during the last ice age, approximately 10,000 years ago. Large chunks of ice deposited as the glaciers retreated resulted in scores of small depressions scattered across the landscape. In addition, the grinding action of the ice created depressions in bedrock and on hilltops. Water flowed into these basins from rain and snow, runoff, flooding from adjacent rivers and streams, and groundwater. During more recent times, vernal pools have been created when a river changes course and leaves a small fragment cut off from the main channel. Human activities contribute to vernal pool formation: small, isolated quarries, detention basins, and farm ponds.
adult wood frog
Adult wood frog

The vegetation that falls into pools from trees, shrubs, and flowering plants is the driving force behind vernal pool ecology. Bacteria, fungi, and protozoans decompose these plants. The same organisms also decompose algae and dead animals from the pools. Shredders break down plant materials by chewing pieces into smaller sizes. Both adult and larval beetles, caddisfly larvae, and some crustaceans are considered shredders. Other animals scrape algae off leaves, stones, and other substrates; these animals are called grazers (e.g., snails, mayfly larvae). Filter feeding species harvest microscopic animals and plant particles from the water. Examples of filter feeders include fairy and clam shrimp, mosquito larvae, and fingernail clams. Predators feed on the invertebrate and vertebrate organisms found in pools. Vernal pool predators include many insect larvae: diving and water scavenger beetles, dragonfly and damselfly larvae, caddisfly larvae, and phantom midge larvae. Wood frog tadpoles and salamander larvae are included in the predator category. Organisms that eat both plants and animals are called omnivores, e.g., chironomid midge larvae and water boatmen.

HO Cook Vernal Pool

Vernal pools are able to support a unique community of animals because they lack fish predators which allows a specific suite of animals to take advantage of these small ecosystems. These “obligate species” require vernal pools to breed successfully. In Massachusetts, the Natural Heritage and Endangered Species Program (NHESP: part of the Division of Fisheries and Wildlife) lists the following as obligate species: wood frog, mole salamanders (spotted, marbled, blue-spotted, and Jefferson), and fairy shrimp (a small, shrimp-like crustacean). Other animals use vernal pools for breeding, feeding, and/or shelter.

Highand Park VErnal Pool with ice

During a visit to a pool on a warm, sunny day in late spring or early summer, pools are alive with activity: adult dragonflies cruise the water surface for gnats and mosquitoes, water striders glide along the surface, and green frogs hide in the leaves along the edge, waiting for prey. Birds, turtles, snakes, and mammals visit pools between spring and fall, to prey upon the pool’s inhabitants, or for water and shelter. To learn more about the animals that may be observed at a vernal pool, we suggest the book, “A Field Guide to the Animals of Vernal Pools” by Leo Kenney and Matt Burne. To order this book and learn more, go to http://www.vernalpool.org/fldgide.htm.