What are vernal pools? Are they important?



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    Few habitat types better illustrate the massive impact seasons have had on the evolution of life in temperate regions than vernal pools. Vernal pools are transient bodies of water which often form in spring (hence, ‘vernal’) from snowmelt and rainfall, and again in fall from rainfall. They are often little more than depressions carved by boulders with the help of retreating glaciers. They dry out in the summertime, and spend winter frozen solid. The fact that they do dry up periodically is the key to their importance for a variety of organisms.

    Because these pools are usually little more than dry ditches on a regular basis, they cannot support fish or other large aquatic predators. Any aquatic organisms that can exploit these bodies of water during the few months they are present will be free from the risk of being sucked into the large (or small, as the species may be) mouth of a bass.

    One group of animals is particularly well-suited to take advantage of these ephemeral habitats: amphibians. Only part of their life is spent as tadpoles, which are obligatorily aquatic. The trick is to be able to metamorphose to a more terrestrial form before the pond dries up. In my region (the Northeast of the U.S.), one species of frog and a group of salamanders have become specialized to do just this.

    Wood frogs (Rana sylvatica) is the classic vernal pool frog. These are largely terrestrial frogs that spend most of their time in woodlands (hence the name), not necessarily tied to permanent bodies of water as many other frogs are. When it’s time to breed in spring, however, they will sojourn to vernal pools and lay their eggs. The sooner they breed the better, because their offspring will be racing the eventually dessication of the pools they live in.

    grenouille rousse / red frog

    Mole salamanders (genus Ambystoma) follow a similar pattern. These chunky salamanders spend most of their adult life underground, in burrows or deep beneath leaf litter and logs. Like wood frogs, they depend on vernal pools for their breeding habitat. Three of our species (spotted, blue-spotted, and Jefferson’s salamanders) breed in spring, and the fourth (marbled salamander) breeds in fall.


    Some of the inhabitants, however, are not quite so noticeable or familiar. Daphnia (genus Daphnia) and fairy shrimp (order Anostraca) — two groups of tiny freshwater crustaceans — are also vernal pool specialists. So much so that the latter are often used as the official determinant when deciding whether or not to classify a given body of water as a vernal pool.


    These tiny organisms do not escape the pool the way their amphibian neighbors do. Instead, they lay eggs which are able to endure the drought and the winter, to hatch again when water is present. Given their speedy life cycles of these minuscule organisms and the remarkable endurance demonstrated by their eggs, they are excellently equipped for this lifestyle.

    Though vernal pools occur in many regions throughout the world, I’ve focused this answer on the ones most familiar to me — those found in my native New England. Nothing to me is more representative of the seasonal nature of life here; the way organisms have adapted to deal with constantly shifting temperatures, water levels, and weather patterns. Vernal pool specialists around here have found a way to utilize a habitat that is usually wet for about half the year, dry for a quarter, and frozen solid for the remainder. If that’s not a beautiful example of adaptation, I don’t know what is.


    Wikipedia: Vernal Pool
    EPA: Vernal Pools
    Ohio Vernal Pool Partnership
    The Vernal Pool Association

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