Seasonal water
The length of time a pool holds water—its hydroperiod—shapes which animals can reproduce there. Some pools dry every year; others remain wet longer and dry in some years.
A temporary pool, a whole world of life.
For a few months, a shallow woodland hollow becomes a nursery. Frogs call, salamanders return to breed, and tiny creatures move among submerged leaves. Later, the water may disappear—but the habitat remains.
Explore seasonal pools of Wisconsin and the Midwest, with a look at how they differ elsewhere.
Vernal pools are small, shallow wetlands that hold water seasonally rather than permanently. Rain, snowmelt, and sometimes rising groundwater fill a depression. A recurring dry phase usually prevents fish from maintaining a permanent population.
The length of time a pool holds water—its hydroperiod—shapes which animals can reproduce there. Some pools dry every year; others remain wet longer and dry in some years.
A pool’s repeating wet–dry cycle and biological community matter more than its appearance on one visit. Not every flooded ditch or duckweed-covered pond is a vernal pool.
Water collects in a low spot while the forest is still open to sunlight. Some pools already hold water from autumn and winter.
On suitable mild, rainy nights, pool-breeding amphibians move toward breeding water. Eggs hatch and aquatic young begin growing.
Evaporation, plant water use, and falling water tables can lower the pool. Young amphibians must complete their aquatic development before it dries.
A dry hollow may hold dark, matted leaves and fallen twigs. Some small organisms persist as resistant eggs or resting stages; amphibians shelter elsewhere in the surrounding landscape.
Wood frogs and several mole salamanders are characteristic seasonal-pool breeders in the Northeast and Midwest. The species present depend on location; not every pool contains every species.
Freshwater fairy shrimp are crustaceans, not insects. Many seasonal-pool invertebrates survive dry periods through resistant eggs or other resting stages.
Fallen leaves support microbes and small consumers. Those organisms help sustain larger predators, including aquatic insects and salamander larvae.
Predaceous diving beetles, other aquatic insects, salamander larvae, and visiting wildlife can eat eggs or young animals. Seasonal pools still have active food webs.
These simplified life cycles show why both the pool and its surrounding land are essential. Development rates vary with species, temperature, food, and water conditions.
A race before the water disappears: a dry phase helps maintain the habitat, but a pool that dries too early may not allow that year’s young to complete development.
Repeated drying limits permanent fish populations. This reduces an important source of predation on amphibian eggs and larvae, while favoring organisms adapted to temporary water.
Water lasts through the year and can support animals whose young need longer aquatic development. Fish may be present, although not every permanent pond contains fish.
Neither habitat is “better.” They support different communities. Deepening a seasonal pool or stocking it with fish can remove the very conditions that make it valuable.
Keep the seasonal basin and its natural water supply intact. A pool still matters when it is dry.
Leaf litter, fallen wood, and sheltered soil provide places for adults and juveniles to feed, hide, and overwinter.
Animals need ways to move between breeding pools and terrestrial habitat. Roads, cleared areas, and development can interrupt those movements.
Watch from the edge, use binoculars or a camera, and leave eggs and animals in place. Keep pets out of the water and avoid trampling the basin.
Look for the whole story: revisit through the seasons, listen for spring calls, and notice shallow depressions with matted leaves. A single photo or one dry summer visit cannot establish a pool’s full water cycle.
A shaded woodland pool may have a basin covered mostly in fallen leaves, with few living plants. Where light and water conditions allow, wetland plants grow along its edges or in the basin as water recedes.
These four native plants are associated with Midwest seasonal pools and other wet habitats. None is exclusive to vernal pools, and their presence alone does not identify one. Brian’s photographs below illustrate the plants, rather than confirmed vernal-pool locations.
Onoclea sensibilis
Moist woodland and pool margins can support this fern.
Symplocarpus foetidus
An early-season plant of wet woods and seepage areas; it can occur around seasonal pools.
Caltha palustris
Spring flowers brighten wet ground around some pools and other wooded wetlands.
Impatiens capensis
A summer wildflower of moist, rich ground, including suitable pool margins.
Plant associations: Michigan Natural Features Inventory — Vernal Pool.
Blue-spotted salamanders (Ambystoma laterale) are closely associated with Wisconsin’s seasonal woodland pools. They use breeding water for eggs and aquatic young, while the surrounding woodland provides habitat on land.
This is a different species from the spotted salamander illustrated above. Both help tell the story of animals that need water and woods.
This photograph shows the animal; it does not establish the water cycle of the place where it was photographed. Learn more from UW–Milwaukee Field Station.
This frog appears to be a green frog; the raised fold behind its eye and along its back is a useful clue. Green frog tadpoles often overwinter in water, so longer-lasting ponds are important breeding habitat.
Wood frogs and spotted salamanders illustrate a different strategy: their young can develop in seasonal breeding pools before moving onto land.
A photograph alone cannot establish whether this pond dries seasonally. Learn about green frogs from Wisconsin DNR.
Many familiar examples are woodland depressions filled by precipitation, snowmelt, or groundwater. Amphibian breeding, leaf litter, and surrounding forest are central to their story.
Seasonal pools also occur in open landscapes with wet winters and dry summers. Specialized plants can flower as water recedes, and some animals wait out the dry season as resistant eggs.
The name covers more than one regional setting. A California wildflower pool need not resemble a shaded Wisconsin woodland pool.
Brian’s two posters bring together a broad wetland wildlife community. Some featured animals depend strongly on seasonal water; others are visitors or use several wetland types.
A nature-art scene celebrating wetland wildlife.
An educational wildlife scene with species to discover.
This overview focuses on habitat ecology rather than identifying or delineating a wetland from a single visit. Illustrations are interpretive; the plant and amphibian photographs are Brian Woolman’s field photography.