Vernal Pools: Vanishing Habitat
Vol. 4 Issue 1, Winter 1999
By Leslie McCasker
vernal: of, relating to, appearing or occurring in, the spring (Webster’s New World Dictionary, 2nd ed.)
Vernal pools are shallow natural depressions ranging from puddles to ponds, that are filled by the rising water table of fall and winter, by the melt-off of winter snow, and by spring rains. These waters combine with the unique qualities of the soil to create a habitat ideal for plants and animals that occur nowhere else in the world. Most pools hold water only for a few months and are generally dry by late summer. Some are semi-permanent; although they do not dry out completely, as summer progresses the pools are deficient in oxygen and nutrients.
During the last century, vernal pools have disappeared beneath the asphalt and concrete of urban sprawl. The few areas where vernal pools remain are facing continued pressure from non-native species, pollution, and since powerful machines can now breakup the hardpan and drain the soil, it seems that everybody—farmers, developers, highway builders—wants a piece of the vernal pool landscape.
One of the most fascinating aspects of vernal pools is the sudden appearance of life in the just thawed waters of a spring pool. Since the pools dry periodically, organisms that use them have adapted to take advantage of the temporary abundance of water without actually remaining in the pool itself. Some live in adjacent forests and lay their eggs in the pool. Others come to the pool to use it as a feeding resource for themselves and their offspring.
The food chain within the vernal pool begins with leaves that settle into the pools in the fall. Bacteria and fungi begin the decay process and themselves become food for larger micro-organisms. Insect larva feed on the leaves, shredding them as they forage. Insects feed on the leaves and other plant materials, reproduce and become abundant. The organisms that develop in a vernal pool are in a race with time; they must end their dependence on the pool before the waters disappear. The organisms that develop and forage in vernal pools are themselves scavenged by insects, birds, mammals, and reptiles.
Some organisms have evolved so that they must use a vernal pool during part of their life cycle. These are the obligate vernal pool species. If an obligate species is using a body of water, then that water is a vernal pool. The most easily recognizable obligate species in the mid-Atlantic states are the fairy shrimp, the wood frog, and four types of mole salamander.
Fairy shrimp are small crustaceans (about 1 inch in length) which spend their entire lives (a few weeks) in vernal pools. The female shrimp drop their egg cases during the aquatic phase, then die en masse as the pool evaporates. Fairy shrimp produce two types of eggs. One hatches immediately to produce a current generation. The other, laid at the end of the growth season, remain on the bottom of the pool, surviving a cycle of drying and freezing, and then hatch when water fills the pool in the next year. By the time the next generation appears in the fall, most of the predatory insects have disappeared, leaving the tiny hatchlings to grow in relative peace.
Wood frogs and mole salamanders are amphibian species that spend most of their lives on the upland forest floor. During the first night of warm spring rains, after the ground has thawed, the most noticeable vernal pool activity begins. On this night, the wood frogs and mole salamanders migrate to ancestral vernal pools.
These amphibians may journey considerable distances, cross snow drifts, and endure any ice that remains, to return to their breeding pools. After mating and laying eggs, they return to their woodland habitat on another rainy night, to spend the rest of the year. Their young develop in the pool, and eventually emerge to begin their lives on land, following the adults into the forests as the pools dry.
Normally by this time of year, the vernal pools would have started to fill with water. But as I write this article, most of the vernal pools in our area are still dry as a result of the summer drought. The unseasonable warmth has prevented the soil from freezing, and at the same time, precipitation is 30% below normal. Hopefully by spring we will once again see these miraculous habitats return.
If you find a vernal pool, treasure it, study it, and protect it. Observe the full and varied spectrum of its life. Show its wonders to your children and friends. Most of all, understand just how fortunate you are — your pool represents the critical and fragile habitat of numerous species, yet most people are unaware of their existence.
In a time when urbanization and continued development reduce biological diversity and destroy pockets of natural habitat, vernal pools represent a dynamic ecosystem, where generation after generation return to breed and ensure the survival of their species. The continued destruction of vernal pools may signal the decimation of entire populations of valuable wildlife.
Now that you’ve read this article, try our Vernal Pool Word Search Puzzle!