Today’s vernal pools are a gift of the most recent glacier, the Wisconsin, which retreated 10,000 years ago. The soil beneath them is impermeable, either because it has been compacted by the weight of glacial ice or because it consists of very fine claylike glacial deposits. But vernal pools must have existed prior to the Wisconsin. Because so many species depend on them for their survival, biologists believe that vernal pools have been an important part of the landscape for a very long time.
In late summer or fall, a vernal pool is a big, empty crater, without any trees or shrubs, yet surrounded by a forest of trees. It could look like a wonderfully convenient place to pile brush.
This might have terrible consequences for the many salamanders, frogs, invertebrates, and other animals that mate and reproduce when the pool is filled with water in the spring and early summer – good reason to learn to distinguish a mere hole in the ground from a true vernal pool.
Aside from the absence of trees, the most obvious clue is that the leaves at the bottom of the depression are usually covered with sediment, possibly just a dusty, dry sediment if you were to visit on a dry day.
Vernal pools are best identified by the absence of vegetation, even when they are full of water. While other wetland environments are characterized by the type of vegetation growing in and around them, vernal pools – even in spring, when the ephemeral pool actually is a pool – have no pondweeds, no cattails, and no sedges. On rare occasions duckweed and other plants that can tolerate drying up will appear.
Just as salmon reproduction is completely dependent on the natal rivers to which the salmon return, the destiny of many amphibians is linked to nearby vernal pools. These small creatures don’t usually travel over long distances, so the destruction of just one pool could knock out the whole local population of some species.
Each spring, frogs and salamanders are in a race against time and the sun to develop from eggs to swimming tadpoles to terrestrial adults before these temporary bodies of water dry up. To help vernal pools maintain water throughout the spring, cutting trees nearby is not recommended because this may increase siltation into the pool and reduce its depth. More sunlight and wind will reach the surface, perhaps increasing evaporation and hastening the drying process. Leaving at least a 50-foot uncut buffer on all sides of the pool will preserve shady conditions.
But it is important that vernal pools do dry out at some point over the course of a few years so that creatures such as fish, which depend on the uninterrupted presence of water, don’t take up residence and devour all the tadpoles. Dragonfly larvae also love to eat tadpoles and can wrap their powerful mandible around soft-bodied tadpoles that are twice as big as they are. It takes two years for some dragonfly larvae to mature, so dryness at summer’s end will wipe out these predators, at least temporarily.
It is possible that ruts in the woods made by skidders or other heavy equipment could also harm salamanders and frogs. If they are lured into laying their eggs in these watery troughs on a rainy April night, and if the inadequate pool dries prematurely, the half-grown tadpoles that are not yet able to venture on to dry land will die.
Steven Faccio, of the Vermont Institute of Natural Science, attached little radio transmitters to adult spotted and Jefferson salamanders as they were leaving vernal pools after they had laid eggs in the spring to see how far they wandered. Most remained within a 500- to 800-foot radius of their breeding pools, and Faccio believes that ideally all of this habitat should get extra protection. He believes that timber harvesting within this home range may be harmful to salamanders and that as much dead and downed wood as possible should be allowed to accumulate on the forest floor.
Organic matter, especially decaying organic matter, is the bread and butter of the host of organisms on which salamanders feed. Plus, during dry times, larger rotting trunks and branches retain moisture, a feature that is essential to amphibians during all the phases of their lives, not just when they are hanging out at the pool.