Vernal Pools

These fleeting spring wetlands are factories of biodiversity. Unusual winters threaten vernal pools, as do hot, dry summers.

March 20, 2017 | | Environment

I approach the vernal pool in a greening world. Emerald-hued moss caps escarpment stone. Bloodroot and spring beauty spangle the woods and parasols of mayapple unfurl. Near the pool, a northern waterthrush sings exuberantly and a broad-winged hawk, newly arrived from the south, utters a plaintive cry. Peering into the pool, I see a profusion of life in miniature. Caddisfly larvae, protected by their fanciful cases, animate the detritus on the bottom. Diving beetles speed by on urgent business, and fairy shrimp swim leisurely, powered by the wavelike motion of their feathery legs.

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Caddisfly larvae, protected by their fanciful cases, animate the detritus on the bottom. Photo by Don Scallen.

Released from winter’s grip, spring quickly reveals its abundant life. Vernal pools are focal points of this abundance. They are stunning repositories of wild and lovely things. And here in the hills we are graced with a profusion of these temporary wetlands. The varied topography of the Niagara Escarpment and the Oak Ridges Moraine provides pockets and creases that capture snowmelt and allow vernal pools to form. They are generally small, ranging from the size of a child’s splash pool to about an acre, and are rarely more than two or three feet deep. They are usually isolated from other wetlands, except perhaps through groundwater connections.

Vernal pools evaporate in summer’s heat, but during their temporary presence they are factories of biodiversity, nurseries for some of the most beautiful and important woodland creatures.

They remain little known, however, and underappreciated. Scott Sampson of Credit Valley Conservation laments, “Many landowners and planners are still not aware of vernal pools or their significance … They may even have negative associations with the pools, seeing them as largely valueless – havens for mosquitoes and dangerous snakes.” CVC sometimes receives complaints about the noisy frogs hosted by these pools, says Sampson. And displaying profound indifference, a few callers have even asked if there is something they can spray to get rid of the frogs.

People with such extreme views may never be persuaded to change their minds, but I hope others can be convinced of the important contributions these wetlands make to our local ecology. I hope too that some of you may be inspired to explore vernal pools. Anyone at all curious about the natural world will be amazed.

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  • Salamanders are the nocturnal stars of vernal pools in early spring. Sampson remembers his first experience with these amphibians many years ago. On an early April evening, he had accompanied me to one of my favourite pools in north Halton. “I still tell the story of that magical night,” says Sampson. “I remember it being cold. There was rain and wet snow, and I thought maybe we were too early in the year. Then in my flashlight beam I noticed a Jefferson salamander. And then the ground was just literally erupting with salamanders. We had to step carefully to avoid the pond-bound amphibians.”

    The endangered Jefferson salamanders that astonished Sampson inhabit the Niagara Escarpment at least as far north as Mono Cliffs Provincial Park. Along with spotted salamanders and the smaller blue-spotted salamanders, they are considered vernal pool obligate species – animals that depend on vernal pools for all or most of their reproduction. The wood frog is another one.

    This excerpt from my nature journal recounts a memorable experience with these frogs:

    Early April, sunny and 10°C. Red maple flowers bloom against azure skies. Spring peepers raise a throbbing cacophony, and muted in the background, the “quacking” of wood frogs is just discernible. Closer now to the vernal pool. I wade in, toward the frenzied action of hormone-driven frogs tumbling and tussling on the surface of the water. Frantic ripples reveal the staging area. I approach as stealthily as my lumbering chest-wader-clad body can, but the wood frogs fall silent and take cover. I stand in their arena and wait. Soon bug-eyed heads begin to pop to the surface. The urgency of mating trumps fear. Males – desperate suitors – grapple with other males and accost pairings of frogs already in nuptial embrace.

    vernal_ss551658574_pool vernalpool_WoodFrogAmplexusMating_8738 vernalpool_SpringPeeper vernalpool_DivingBeetleLarvae vernalpool_EasternRedSpottedNewtEft_5301
    Eastern red-spotted newt, juvenile or eft phase. Photo by Robert Mccaw.

    Entrusting precious eggs and larvae to ponds destined to dry up seems an absurd strategy for amphibians. If the water evaporates before the larvae transform, they die. Why tempt fate in this manner? The answer is the absence of fish. Amphibian larvae in vernal pools need not worry about being eaten by these voracious predators.

    The lack of fish not only improves the survival chances of amphibian larvae, but also enables invertebrate life to flourish, including the lovely centimetre-long fairy shrimp. These freshwater crustaceans are another vernal pool obligate species, found rarely, if ever, in other wetlands. Tens of thousands can inhabit a typical vernal pool. Their lives are ephemeral, but as with spring’s glorious woodland wildflowers, that is part of their appeal. Theirs is a precious, fleeting beauty.

    Vernal pools are alive with other invertebrates: backswimmers and water boatmen, caddisflies and fishflies, snails and clams. And though devoid of fish, vernal pools are hardly predator-free. There are dragonfly larvae with alien-like extendable mandibles, sickle-jawed diving beetle larvae three centimetres long or more, as well as what may be Canada’s most fearsome invertebrate predators – giant water bugs that wield piercing beaks to drain the bodily fluids of prey as large as adult frogs.

    The isolation of vernal pools in the landscape means these creatures need ways to move between them. Most do this in the usual fashion, on foot or on wing. But invertebrates lacking such appendages have tapped into nature’s infinite creativity to find travel alternatives. Leeches use their sucking mouthparts to cling to turtles and amphibians. Pill clams, common vernal pool molluscs, clamp onto the toes of newts, so that as the pools dry, the newts will drag them to other water bodies.

    And fairy shrimp? With the evaporation of the pools, fairy shrimp females die, but not before leaving dustlike eggs behind. It is thought these eggs ride summer breezes to other woodland depressions that fill with water come spring.

    I realize a vernal pool’s abundance of “bugs” won’t convince everyone of their value. Rarely do invertebrates inspire respect and esteem. But they absolutely should. The profusion of insect larvae, including mosquitos, cradled in vernal pools transform into winged adults – an aerial bounty that nourishes birds. Yes, mosquitos do inflict seasonal torment on us, but if we cherish our songbirds, it’s a small price to pay.

    Sampson notes that bats feed on the insects when they rise into the air over vernal pools. Several species of bats are currently being destroyed by a devastating disease called white-nose syndrome. Vernal pools help support the survivors.

    The pools also offer water to thirsty woodland animals, as well as food to birds and mammals that forage in their shallows. So the value of vernal pools to the environment far exceeds their small footprint on the landscape. They are biodiversity engines, driving the ecological health of the surrounding forest.

    But the biological argument isn’t the only argument in favour of vernal pools. Consider some of the benefits they offer landowners. Sampson touts their value from a hydrological perspective – the ways they help to naturally manage the woodland water supply. “They mitigate the effects of rapid snowmelt and heavy rain by capturing some of that runoff,” he says. “They recharge groundwater through the gradual seepage of water into the soil and help maintain moisture in the adjacent forest.”

    Sampson also notes that a visit to a vernal pool, by kids and adults alike, makes for a real get-your-hands-dirty nature experience, a healthy alternative to the virtual reality of our ubiquitous screens. Dipping with a small net for pond creatures is lots of fun and a great way to discover the biotic wealth of these habitats.

    In southern Ontario, it’s calculated 70 to 85 per cent of wetlands have been lost over the past century. It’s hard to assess the loss of vernal pools because they are small and often hidden among trees, but it is likely that in the near-urban countryside most are gone. Forests that harbour vernal pools have been cleared, and much of the landscape has been drained and levelled for agriculture and urban development. It is sobering to think an hour or two of earth moving is all it takes to fill in a vernal pool – and that this brief act of ecological vandalism can diminish local biodiversity forever.

    It is important to dispel the ignorance that allows this to happen. As Sampson says, “We need to let landowners know what they’ve got.” Then, with knowledge of the ecological and hydrological value of these pools, they can be protected and enhanced.

    According to Paul Biscaia, senior co-ordinator of wetland restoration at Credit Valley Conservation, landowners can protect vernal pools by “maintaining the tree canopy surrounding them and by establishing natural buffers if needed.” This would involve planting native shrubs and trees to shade the pools and integrating them into the forest ecology. Sampson adds that ditches dug in the past to drain low-lying areas can be plugged, bringing vernal pools back to life.

    Caring for our vernal pools takes on even more urgency in the midst of climate change. Sampson sees ominous signs in recent winter weather. “The snowpack hasn’t formed adequately,” he says. “Because of the alternating freeze-thaw cycle, water is moving through the system all year as opposed to piling up in the winter and being released all at once.”

    Unusual winters threaten vernal pools, as do hot, dry summers. The summer of 2016 was a disaster for vernal pool obligate species. Most of the amphibian larvae in Headwaters’ vernal pools died before they had a chance to transform into air-breathing juveniles, a process known as recruitment. Jim Bogart, professor emeritus at the University of Guelph, notes, “Last year there was no recruitment of Jefferson salamanders from a large population in the Dundas Valley. The pond dried in June before the larvae had transformed, and the water was so low in May that predators, mostly crows, could easily catch and eat the larvae.”

    There have always been years in which heat and drought have caused an early shutdown of the biological processes of vernal pools. This is the trade-off obligate species accept to avoid being eaten by fish. But climate change may lead to more frequent hot, dry summers. If this happens, vernal pool life will suffer and the negative effects will ripple outward into the surrounding environment.

    I stand with a friend, shivering after dark alongside a vernal pool on the Niagara Escarpment. We watch in awe as scores of spotted salamanders roil and churn in the cold water. Most are standard issue – jet-black bodies highlighted by two rows of brilliant yellow spots. Spellbinding. But nature celebrates diversity. Swirling in the tumult of salamander bodies are some without any spots at all and a few that are partially leucistic – lacking normal pigmentation. As a woodcock twitters overhead, the breeding salamanders display in our flashlight beams.

    vernalpool_SpottedR_JeffersonsL_9013 vernalpool_JeffersonSalamanderComplexLarva vernalpool_JeffersonSalamander_5123 vernalpool_SpottedSalamanderM
    Salamander egg masses; Jefferson at left and spotted at right. Photo by Robert Mccaw.

    This wondrous spectacle has occurred in these hills every spring for thousands of years. May it always be so.

    More Info


    In Piotr Naskrecki’s wonderful book The Smaller Majority, the entomologist, photographer, author and Harvard professor advocates passionately for the conservation of invertebrates and other small creatures so important to the health of our planet.

    Naskrecki writes, “Public appreciation of the beauty and importance of these animals is our strongest ally in this conservation work.” He encourages us to “kneel down, look closer and discover the beautiful world around our feet.”

    If you want to kneel down for a closer look at vernal pools, one of the easiest ways to find one is to follow the shrill calls of spring peepers. Though the peepers do not limit their breeding to vernal pools, they do use them extensively. When you find the water, listen for the distinctive “quacking” 
of wood frogs. If you discover wood frogs, you’ve almost certainly found a vernal pool. Finding fairy shrimp will also confirm your discovery.

    Don’t limit your visits to daytime. Many vernal pool creatures, including breeding salamanders, are far more active after dark.

    Though no equipment other than warm clothes is necessary, your experience will be enhanced by taking rubber boots, dip net, camera, magnifying glass, small plastic containers for viewing animals and close-focus binoculars.

    Please remember your excursion should be guided by respect. Don’t trespass. And if you capture creatures for a closer look, treat them gently and return them quickly to their aquatic realm. And one important caveat: If you are lucky enough to find Jefferson salamanders, don’t touch them at all. As an endangered species, Jefferson salamanders are protected by law, so harassing them in any way is considered illegal.


    Ontario Nature, a province-wide conservation organization, runs a vernal pool mapping project to document the location of vernal pools. The objective is to identify priority areas for research and conservation efforts. If you find a vernal pool, you can report it on the Vernal Pool Search form at

    You can also put on your citizen science hat and collect information for Ontario Nature about your vernal pool’s biological and physical features.

    If you are a landowner in the Credit River watershed who wants to learn more about conserving vernal pools and other natural features on your property, consider attending one of Credit Valley Conservation’s workshops on the topic. For one-on-one help with your land management and stewardship goals, you can also talk to a CVC stewardship co-ordinator directly. See “Your Land and Water” at for information on workshops and how to contact a stewardship co-ordinator.

    For one-on-one help with achieving your land management and stewardship goals, you can also contact a CVC stewardship co-ordinator directly. Information is available at

    About the Author More by Don Scallen

    Don Scallen enjoys sharing his love of nature through his writing and presentations. Check out his blog "Notes from the Wild".

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