Also called “ephemeral wetlands”, 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 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.
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.
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.
You may spot these creatures when visiting a vernal pool in the Deerfield River Watershed
Wood frogs Rana sylvatica (pictured above) have a distribution over the northeast United States, stretching into Canada and across to Alaska with several distinct populations. They are “obligate” vernal pool breeders, meaning this habitat is the only place in which they will breed. Because of this, they will often migrate longer distances than most other frogs to find appropriate breeding locations. They are generally 2-3in in length with the females growing to be slightly larger than males. The adults are brown or rust-colored with a dark eye mask. The underparts are pale with a green or yellow hue. No other species of frog in North America are similar in appearance to the wood frog.
Spotted Salamanders Ambystoma maculatum are quite common in the eastern United States and Canada. They live most of their adult lives in burrows on the floors of deciduous forests. However, like the wood frogs they require vernal pools for breeding to protect their eggs from being eaten by the fish who inhabit permanent bodies of water. An adult spotted salamander is usually 6-10in long. They are usually black, or another dark color, and can be identified by the two rows of yellowish spots that run the length of their body.
Dragonflies Anisoptera are insects with a long, slender body, slightly uneven wings, and compound eyes which can be found on every continent sans Antarctica. An easy way to differentiate between dragonflies and damselflies is the way in which they hold their wings while at rest; dragonflies leave theirs flat, pointing away from the body while at rest and damselflies often fold theirs along the length of the body. Both the airborne adults and aquatic young are carnivorous.
Caddisflies Trichoptera are an order that includes approximately 14,500 species globally. They are also called sedge-flies or rail-flies. The larvae are aquatic and most often observed after having surrounded themselves with a protective casing fortified with sand, sticks, and other debris. Caddisflies are often used as a bio-indicator of good water quality with the different species representing different habitat types.
Diving Beetles Dytiscidae is a family of water beetles that includes about 4,000 species globally. They are usually about one inch long with the largest growing to about 1.75in. Most are dark brown, blackish, or olive in color but some families do have yellow or golden highlights. Because of their voracious predation, the larvae are commonly referred to as “water tigers”
Fairy Shrimp Anostraca is an order of freshwater crustacean that includes 300 species. Their segmented bodies are .25-1in long (except for a species that can be found in western North America that grows up to 6.75in). They swim with their 11 pairs of legs pointing upward and either feed by filtering the water or scraping algae from surfaces. Because they cannot move quickly to flee from fish predators, they tend to live in habitats with fewer predators, such as vernal pools.
Water Boatmen Corixidae (not to be confused with the Notonecta glauca of the UK) is a family of aquatic insects that includes about 500 species. Their flat bodies are about half an inch long with a pair of wings that lay flat along their back, four long hindlegs and two shorter front ones. Their four back legs are covered with hairs and shaped like oars which help them move through the water. Unlike their relatives, backswimmers which swim upside-down near the surface of the water, water boatman swim near the bottom of the body of water in which they live. They are one of the only families within their order that are herbivores, feeding on plants and algae.
Damselfly Zygoptera are a suborder of insects found on every continent except Antarctica. Both the airborne adults and the aquatic nymphs are carnivorous. Similar in appearance to dragonflies, damselflies have long thin bodies, two sets of wings, and compound eyes. Simple ways to tell them apart are their wings, which damselflies rest with folded along their body; their eyes, which are further apart; and their body size, which is often smaller than that of a dragonfly.
Green Frogs Lithobates clamitans are a species of frog that includes the two subspecies bronze frog and northern green frog native to eastern North America. Adults range from 2-4in long, excluding legs with females often larger than males. The ridges that run along either side of their back make them easy to differentiate from bullfrogs. It is easiest to observe green frogs when they are resting on the shore before they jump into the water upon approach.
Spring Peepers Pseudaceis crucifer are frogs that are more often known by their distinctive voice rather than their appearance. There are two subspecies of this small chorus frog that can be found throughout eastern North America. The adults are 1-1.5in long, brown, green, gray or tan in color and spend much of their time in the forest. The expansion and contraction of the vocal sac located near the throat allows this little frog to make it’s distinctive peeping call.
Field Guide To the Animals of Vernal Pools by Leo Kenney & Matthew R. Burne