During my tenure at Geauga Park District, I fielded many phone calls from the public searching for answers to their nature-related questions. The subjects were wide-ranging and included topics such as how to discern a certain sound heard during the night or how to identify a colorful caterpillar discovered on their property. Some people wanted anecdotes on how to keep a bird from constantly smacking against their window, or advice on what kind of plants to establish in a pollinator garden. Yet, of all these queries, one particular phone call haunts me still.
One morning after a warm, rainy night in March, a distraught homeowner phoned, stating that she was mystified by high numbers of salamanders and frogs that had been wandering aimlessly around the base of her house overnight and falling into her window wells. It seemed as if the amphibians were everywhere, moving about in a rather confused state. My heart sunk and sadly I let her know that her house had been built on top of the site of a vernal pool.
Vernal pools
The word “vernal” means spring. So, it only makes sense that these aptly named habitats make their appearance as a result of the spring rains and snowmelt. Located in shallow woodland depressions that lack any flow of water in or out, vernal pools are self-contained, retaining water due to soils that harbor non-porous qualities. These small bodies, usually less than an acre in size and no more than a few feet deep, typically become completely dry by mid to late summer, disappearing from view and often going unnoticed.
It is this drying out that is the key factor to a vernal pool’s success at harboring its unique ecosystem. The annual loss of water prevents predators, mainly fish, from becoming established and preying upon the eggs and larvae of the denizens which depend upon the dynamics of the pool for their success.
Although there are loads of species that can be found utilizing vernal pools, there is a select group of animals that depend on them exclusively for their breeding success. These animals, known as indicator, or obligate species, have become adapted to exploiting this temporary habitat and include wood frogs, several species of large salamanders belonging to a group known as mole salamanders and a tiny crustacean called a fairy shrimp.
Mating season
Here in northeast Ohio, as early as January, when the ice begins to retreat from around the edges of the pool, a species of mole salamander, gray in color with blue specks and up to 6 inches in length, known as a Jefferson (Ambystoma jeffersonianum), begins its annual trek into its natal pool to breed. Males commence the migration from the surrounding forest, slipping into the pool between the edge of the ice and the shore to await the arrival of the females. As the girls begin to wade in, they are greeted by the males who perform an intricate underwater dance in an effort to lure them to an area where small packages of their spermatophore have been carefully deposited on the submerged leaves. Once a female has chosen a particular male as her mate, she will straddle his particular packets, absorbing them into her system where internal fertilization of her eggs will occur. Submerged branches provide the perfect sites for the attachment of her gelatinous egg masses.
As the first warm March rains dampen the forest floor, a second wave of salamanders begin their nocturnal march into the pools. As the Jefferson salamanders, their breeding complete, begin their exit, they cross paths with waves of spotted salamanders (Ambystoma maculatum) up to a whopping 9 inches in length, heading in the opposite direction to take their turn. During this magical event, hundreds of salamanders can be observed in a single night as they migrate in silence through the woods, across roads, over parking lots, etc. While spotted salamanders inundate the pools, performing the same ritualistic dances for their potential mates as described above, they are joined by wood frogs (highlighted in my Feb 29 article). This frog, another obligate species, with its overwhelming cacophony of ducklike vocalizations, adds to the melee. After a few days, all amphibians begin their departure in a return to their forest haunts for the remainder of the year. In order for the eggs that they have so carefully deposited to be successful, the larvae must develop quickly and mature before the pool begins to dry out during the summer months. Today, many vernal pools are drying up too soon, wiping out complete generations of amphibians that never had a chance for success.
If you remember hosting tiny, shrimplike organisms known as a Sea Monkeys as a kid, then you are already familiar with another vernal pool indicator. The fairy shrimp, a small, translucent, pastel-colored crustacean topping out at 1.5 inches in length, is uniquely adapted to life in the pool. If you stare down into the water, you can spot these graceful organisms as they swim upside-down, using their legs to create currents that help them to extract particles from the water, a method known as filter feeding. Fairy shrimp produce eggs known as cysts. When the vernal pool becomes dry, these cysts enter a state of dormancy that can survive many years. During this time, they have an amazing ability to withstand a host of harsh conditions including frosts, droughts and UV radiation only to be successfully revived when the pool fills again.
Drying up
Obligate species, the salamanders and wood frogs, have a special connection to “their” vernal pool. Before their eggs even hatch, they imprint on the chemical makeup of that particular pool’s water. This will be their sole breeding destination for the rest of their lives. Considering that some of the mole salamander species live to be 20+ years of age, many generations come to rely on this single depression in the forest to perpetuate their species. When a vernal pool is destroyed, hundreds of amphibians will wander in confusion, searching for something they will never again find. Their dedication and hard work extinguished forever.
I doubt that the woman who called me on that March morning had any idea that her house had been built atop a vernal pool. Construction no doubt occurred during the summer months when the pool was dry and workers’ eyes had failed to notice the slight depression and other telltale signs that the pool existed when they chose to bulldoze the site on which to build. Sadly, this is the fate of many vernal pools. As entire populations of amphibians, generations and generations, disappear from existence, we are reminded of the plight that many species are facing. We must try harder.
