Vernal Pools
Vernal Pool Animals
Vernal pools are unique wetland habitats where some of the state’s most recognizable
reptiles and amphibians can be found. Vernal pools are small, shallow wetlands that
do not have a permanent inlet or outlet of water flow. They fill in the fall or
spring when rain or snowmelt drains into shallow depressions, and can retain water due
to non-porous soils. Vernal pools only hold water for part of the year and experience
a drying phase every year or every few years, usually in late summer.
Because their aquatic habitats are temporary, animals that depend on seasonal
pools are adapted for both aquatic and terrestrial habitats at different life stages.
These animals also benefit from the dry phase, because it prevents year-round
water-dependent animals like fish from living in the pools. Fish prey heavily
on eggs and larvae, and without seasonal pools some species would not be able
to compete and reproduce. Although there are obvious challenges for an animal using
an aquatic environment that disappears for part of the year, the benefit is a habitat
free from predation by fish.
Mixed shrub herb, Mt. Cydonia. Credit: Betsy Leppo
Marbled salamander (Ambystoma opacum) Credit: Jack Ray
Indicator Species
Fairy shrimp (Eubranchipus vernalis). Credit: Jack Ray
Vernal pools support wildlife that would not be successful in permanent waters.
Animals that require temporary aquatic habitats for reproduction and development of their
young are called vernal pool indicator species. Some sources refer to them as vernal pool obligates.
Pennsylvania’s large and secretive mole salamanders are all vernal pool indicators, along with
two other frogs and several species of small freshwater crustaceans. These animals use
seasonal pools almost exclusively during some stage of their life cycle.
Pennsylvania’s Vernal Pool Indicator Animals
Vernal pool indicators have developed different strategies for coping with the periodic drying of their wetland habitats. Some amphibian species
travel to vernal pools to lay their eggs shortly after the first spring rains. Other species, such as fairy shrimp and clam shrimp, leave eggs
in the bottom of the pool that can withstand drying out in the summer and freezing in the winter. Young vernal pool invertebrates and amphibians
(most are called larvae) must grow quickly once they hatch from the egg in the spring. As spring turns to summer, water evaporates and the pool
gradually shrinks in size until it disappears. The larvae must transform into terrestrial adults before the pool dries up.
Eighty five percent of vernal pool amphibians return each year to breed in the pond where they were born (Colburn, 2004). They will bypass
other pools that provide suitable habitat and cross obstacles such as roads and other forms of human disturbance in order to return to the
pool of their birth. This fidelity by individual amphibians to a particular pool is an important consideration when determining how to protect
a species as a whole.
Facultative Species
A diagnostic ‘X’ crosses the back of a spring peeper. Credit: Charlie Eichelberger
Many animals take advantage of the resources vernal pools provide, but do not require them for survival. These ‘facultative species’
may breed in seasonal pools, or simply use them as a place to forage for food and find shelter.
Facultative species have physical or behavioral adaptations that allow them to successfully utilize seasonal pools but they can also
survive in permanent wetland habitats. Some examples include the red spotted newt, northern spring peeper, American toad, wood turtle, and spotted turtle.
After the Pool Dries
Adult and recently metamorphosed invertebrates and amphibians will leave the vernal pool and head into the surrounding landscape.
Although they need seasonal pools to reproduce and for their young to grow, adults spend the summer, fall, and winter in the uplands
around the pools where they find food, shelter, and overwintering sites.
While their exact habitat needs vary, all vernal pool species benefit when a pool and its surrounding uplands (500-1,000 feet or more) are naturally vegetated and have a minimum of human disturbances.
Woody debris on the ground creates foraging sites where amphibians can find food and also provides protection from the heat of summer and cold of winter. Credit: Betsy Leppo
Adult spotted salamander (Ambystoma maculatum). Credit: Charlie Eichelberger