Vernal Pools: Birthplace of Amphibians

text by Tony Oppersdorff & Joe Gray
photographs by Susan Spinney (wood frog)
& Tony Oppersdorff (spotted salamander)

Vernal pools — from the Latin vernus (spring) and vernare (to flourish) are very important small bodies of water that fill and recede seasonally and which do not contain fish. They may dry up during the summer but must retain water at least 80 days to enable frogs and salamanders to complete their life cycle. True vernal pools require evidence of indicator species that depend on the pool for breeding. Examples of such obligate species in Maine include fairy shrimp, mole salamanders (family Ambystomatidae) and wood frogs.

Facultative vernal pool fauna are animals found in vernal pools but not specialized to breed in them, such as the four-toed salamander, eastern newt, green frog, American toad, spring peeper, gray treefrog, bullfrog, turtles, fingernail clams, amphibious snails and caddisflies. These species may visit a vernal pool but do not depend on it for existence. Vernal pools are an important gauge of water quality and the health of woodlands, amphibians and other wildlife. Many species, large and small, visit vernal pools during their lifetime; one of the most common is the wood frog (Rana sylvatica). They can hibernate (or be in a state of semi-frozen torpor) at shallow depths under leaves, fallen logs, stones and moss mats. Within five minutes of freezing, they accumulate high levels of glucose in the liver and leg muscles. This natural antifreeze is subsequently released into the blood stream and prevents tissues from freezing.

To sense the arrival of spring, they don’t rely on the calendar or internal clock, but on environmental clues. Often when ice is still on the pond they rouse from their hibernation sites, ahead of spring peepers (Hyla cricifer), and begin migration to breeding ponds when melting snow and spring rains saturate the ground. The males often hibernate close to a vernal pool since there is much competition for females. Wood frogs are in evidence by their duck-like call: wrack, wraaackkkk, wrack.

Wood frogs are explosive breeders, accomplishing their appearance, mating, egg-laying and return to terrestrial habitat in as short as a week, or extend the process to 30 days. On average, the eggs hatch within 20 days and tadpoles metamorphose in 60 – 70 days, after which time they begin to emigrate to upland habitats. Young tadpoles feed on algae and microorganisms, later eating plant material. Adults consume a variety of plant and animal matter, including slugs, spiders and worms. They display a high fidelity to their native pools: often more than 95 percent return to the pool of their origin. Wood frogs have returned to a house site or paved parking lot where once a pool existed.

There is little doubt that climate change is occurring, and in Maine it may well be warmer and drier in the future. With this scenario, it’s possible that the breeding population of wood frogs will decrease as smaller pools dry up. Larger pools will become increasingly important as they remain filled with water to allow the young to complete their aquatic life stage. Since the prevalence of vernal pools depends upon the weather, our best strategy is to maintain a variety of high-quality pools of different sizes throughout the forested landscape to accommodate all possible climate conditions.

During life on earth, amphibians have played a pivotal role as the first vertebrates to leave the water, part-time, for the land, some 360 million years ago. They evolved into 15 major groups over the next 125 million years. With the advent of reptiles and later birds and mammals, amphibian groups have waned and today are represented by only three orders: frogs and toads (Anura), salamanders (Caudata) and a group of wormlike creatures (Apoda) confined to tropical biomes. Living out of water on land required a number of adaptations: limbs for support, and locomotion in air which lacks the buoyancy of water, lungs, eggs that do not desiccate in water, and skin that minimizes loss of water—amphibian skin is not impervious to water loss and confining most amphibians to a moist habitat. The majority of species still return to water to breed. They are not adapted to cold weather and their metabolic rate is too low to generate heat, referred to as cold blooded. Fish, reptiles, birds, mammals and occasionally insects eat amphibians. Most amphibians species have either toxic or noxious substances in their skin mucus to discourage predators. They in turn are predators themselves, feeding on an array of invertebrates and occasionally vertebrates.

To offset losses, they produce a large number of eggs. (One common Maine species, the seven-inch spotted salamander (Ambystoma maculatum) lays 100 – 300 eggs per cluster.) Usually the female sheds her eggs and leaves them to hatch and survive on their own. Most male salamanders deposit a small capsule of sperm and then undertake to pursue the female to uptake the capsule into her cloaca.