Vernal pools are ephemeral wetlands which provide a unique habitat for a variety of forest and wetland organisms. They fill in the fall and winter from rising groundwater, runoff from rain or melting snow or the seasonal inundation of flooding rivers. They hold water for a few months in the spring and summer and then are generally dry by late summer. Some pools are semi-permanent, not completely drying but becoming shallow, warm, and deficient in dissolved oxygen as the summer progresses. Because of dry or low water conditions, winter freezing or other conditions, vernal pools are free from breeding populations of fish. These fish-free yet temporary waters are the required habitat for numerous amphibian and invertebrate species which have evolved to take advantage of relative safety of vernal pools. Some of these animal species require vernal pools for their life cycle and are found nowhere else. We refer to these as the "obligate species". "Facultative species" use vernal pools as well as a variety of other wetland habitats for their various life activities such as feeding, drinking, or reproduction.
In New England, the obligate vernal pool species include the wood frog (Rana sylvatica); four species of mole salamander: the spotted salamander (Ambystoma maculatum), the blue-spotted salamander (A. laterale), the Jefferson salamander (A. jeffersonianum), and the marbled salamander (A. opacum); and three species of fairy shrimp (Eubrancipus sp.) Since it is difficult to distinguish among the different types of fairy shrimp, most non-specialists just refer to them as "fairy shrimp". Two additional amphibians might be considered vernal pool species, yet they would not be defined as indicator species. The spadefoot toad (Scaphiopus holbrookii ) and the four-toed salamander (Hemidactylium scutatum) both breed in temporary pools. Those utilized by the spadefoot may last only a few weeks and dry much more quickly than waters we think of as vernal pools. The four-toed salamander lays its eggs in moss clumps overhanging pools into which the hatching larvae fall and then develop. These pools are often quite small (a few square feet) and included in larger forested wetlands.
One of the more fascinating aspects of vernal pool observation is the sudden appearance of animal life in the just thawed waters of a spring pool. Since the pools dry periodically, organisms which utilize them have evolved strategies to take advantage of the temporary abundance of water and food without actually remaining in the pool itself. Wood frogs and mole salamanders live in the adjacent forest but lay their eggs in the pool. Other frogs overwinter elsewhere (forests or wetlands) and hop to the pool to exploit it as a feeding resource for themselves and their offspring. Turtles walk to the pool from nearby permanent wetlands to feed and leave as the water recedes. Fairy shrimp hatch from eggs left on the dry pool bottom. Fingernail clams emerge from their mysterious survival in the damp mud below the dried leaves. Adult insects come and go. Some arrive to lay their eggs, their young feeding upon the abundant life of the pool. Others come just to feed on other species.
Although fairy shrimp seem to appear suddenly in a recently-filled spring pool, they may be active in the water under the ice of a pool which filled during the winter. When the pool thaws, fairy shrimp, if present, are active and visible. These animals have hatched from eggs which remained on the pool bottom through its dry period. Fairy shrimp produce two types of eggs. One hatches immediately to produce a current generation. The other, laid at the end of the growth season, drops to the pool bottom and must dry before being submerged and then hatching. The adults swim "upside down" in the water column, their rhythmically beating legs propelling them about the pool as they feed on microorganisms and detritus. They are about one inch long and vary in color from gold to greenish, depending upon their diet. Since all fairy shrimp in a pool are likely to have the same food resources, those within a given pool are generally of uniform color. Since fairy shrimp require a wet period and a dry period for their life cycle, they are considered a vernal pool indicator species or "obligate species".
The most noticeable spring activity for a vernal pool begins when the mole salamanders migrate to the vernal pool. "Big Night" as it has been dubbed, is the night of the first warm rains of spring after the ground has thawed. It is on this might that the mole salamanders and wood frogs, often accompanied by toads, spring peepers, leopard frogs and pickerel frogs, emerge from their winter hibernation and journey to the vernal pool for breeding. Wood frogs and mole salamanders are animals of the forest. They spend their lives in upland territory and venture to the pool only to breed. The signal to return to the pool is a heavy rain extending into the evening, air temperature in the upper 40's (F) or higher, and thawed ground. The amphibians may need to cross snow drifts and the pool may still retain ice, but they journey considerable distances to return to their breeding pool. After mating and egg laying, they reverse the voyage on another rainy night and spend the rest of the year in their forest habitat. Their young, having developed in the vernal pool, follow a few months later.
For the spotted salamander, reproduction begins when the males arrive at the pool and join in a congress. The males nudge each, rub noses against bodies, dive and surface, and swim in a salamander maelstrom throughout the evening. This "Liebespiel" (love play) results in the deposition of spermatophores on the pool bottom. Individual males maximize their chances of reproductive success by producing up to 80 spermatophores, many of which are deposited on top of those of other males. A female spotted salamander picks up a spermatophore with her cloaca and her eggs are then internally fertilized. The eggs are laid within few days in a gelatinous mass attached to a stick or emergent vegetation in water of the pool. Each female usually deposits her eggs in a single mass. Females do not always breed each year although they may still return to the pool. Males may have breeding success beyond their numbers because of the quantity and placement of their spermatophores. Thus, it may be difficult to estimate a population of spotted salamanders from account of their egg masses. One study which counted every salamander entering a pool and the total number of egg masses, suggests that each egg mass represents 5 adult spotted salamanders in the population.
Blue-spotted salamanders and Jefferson salamanders also venture to vernal pools in early spring to breed arriving before or at the same time as the spotted salamander. These two species seem to have crossed sometime during the last glaciation and the populations found in New England are likely hybrid populations which resemble either the blue-spotted or Jefferson salamander. Many biologists just refer to them collectively as the Jeffesonianum complex. In any case, the slender, chocolate colored with light flecks, Jefferson salamander is found west of the Connecticut River. The blue flecked blue-spotted salamander seems to be found in the eastern areas.
These animals enter the pools on rainy nights and go through a mating dance in which the male nuzzles the female snout to snout. He deposits a spermatophore which she picks up in her cloaca. The eggs are internally fertilized and she lays clumps of 1 to 30 eggs which she positions on vegetation in the pool. Sometimes the eggs are just dropped on the pool bottom. On rainy nights following mating, the various species of mole salamander return to their forest habitat, leaving their eggs to develop on their own.
The remaining New England mole salamander, the marbled salamander, has completed its breeding well before the pools thaw in spring. This species enters the dry pools in the fall and goes through a one-on-one mating ritual similar to that of the Jeffersonianum complex. The female lays about 50 eggs in a depression or under debris in the pool and remains with the eggs until rains submerge them. By spring, the larvae are about 2 inches in size.
While the salamanders silently trudge to pools to breed, the wood frog is a vocal amphibian which announces its presence for all to hear. Raucous choruses of males signal their presence and readiness to mate. Wood frogs are "explosive breeders" in the amphibian world. These woodland animals spend the winter buried in the litter of the forest floor where they actually freeze solid protected from cell destruction by a naturally produced antifreeze compound. Within a few weeks of thawing from the winter freeze, they venture to vernal pools where the males commence a "quacking" chorus. Females are attracted to the males. The males grasp the females in amplexus and fertilize the eggs as they are released. Each female produces one egg mass which is attached to vegetation in a communal cluster of masses produced by numerous females. Most egg masses within a vernal pool are located in the same area.
As spring continues, facultative species including other amphibians, some reptiles, and numerous invertebrates venture to the pool to feed or breed. But what brings them to this temporary wetland, little more than a wicked big puddle, which might be dry in a few months? Two things. The abundant leaf litter on the pool bottom from the forest trees provides an energy and nutrient source which can support a dense animal population and there are no fish to devastate that population. Fish are such efficient predators that many animal species risk the early drying of a vernal pool in order to exploit its rich waters which are free of fish predation. Certainly there are other predators present, particularly insects, but none with the voracious appetites and predatory efficiency of fish.
The food chain within the vernal pool begins with forest leaves which drop or blow into the pool in fall. Bacteria and fungi begin the decay process and themselves become food for slightly larger micro-organisms such as daphnia, copepods, and rotifers. A variety of insect larvae feed on leaves shredding them as they forage. The caddisfly larva both nibbles on leaves and utilizes leaf fragments or small sticks to build a cryptic case within which it resides. Other insects, such as water boatman, amphibious snails and small crustaceans, such as the Aescelus, feed on leaves and other plant material, reproduce and become abundant. When the frog tadpoles hatch, them become constant feeders on leaves, other plant material and algae of the pool. The proliferation of animal life feeding on plants is hunted by the developing larvae or nymphs of the predaceous diving beetle, fishfly, dragonfly, damselfly, water scorpion as well as the adults of some of these and other species. These predators also hunt each other.
Salamander larvae are also carnivorous. When small they feed on daphnia and similar sized creatures. As they grow, their diet consists of whatever they can cram into their large mouths, including frog tadpoles and other salamander larvae. The marbled, blue-spotted and Jefferson larvae have the young of other salamander species as one of their major food items. The marbled larvae are large enough in mid-spring when the spotted and Jeffersonianum larvae start to hatch, that they can devour considerable numbers of the smaller hatchlings. Since the Jeffersonianum complex often lays eggs before the spotted salamanders, in pools without marbled salamanders, the blue-spotted and Jefferson larvae hatch early and are waiting to feed on the spotted larvae as they hatch. Even the spotted larvae are cannibalistic on their own species in times of overcrowding, as when the pool dries.
Turtles and snakes go to the pool to feed on the developing species. The spotted turtle likes to eat the egg masses of the spotted salamander as well as insects and developing larvae of amphibians and insects. Ribbon snakes catch larvae or emerging adults in the shallow vegetation at the pool edge. Owls swoop on amphibians on their nocturnal travels to and from the pool. Wading birds come to eat whatever is available. Raccoons grab anything within their reach.
The organisms developing in a vernal pool are in a race in which they must end their dependence on the pool's waters before those waters disappear. Whether they win or not, many still become food for others and pass the energy and nutrients from the pool back into the forest. Those that leave the pool may survive in the forest ecosystem to return to the pool to breed or they may be eaten by owls and other birds or some of the forest mammals such as raccoons and shrews. Those that die in the overheated shallows of a disappearing pool are scavenged by birds, mammals, reptiles, and insects and enter the forest ecosystem as units of energy. The abundance of amphibians and invertebrates which leave the pool, on their own on in the bellies of others, are a substantial amount of the animal tissue which is found in the adjacent forest and a significant food resource for the more obvious mammal and bird species.
The vernal pool is the hatchery of the forest. Its short period of intensive growth recycles forest leaves as animals. And these animals make up a significant portion of the wildlife of a forest. It is only because of the secretive nature of these animals and our own inattention that we do not realize their overall significance. For example, a moderate sized vernal pool might have several thousand wood frogs enter the pool for breeding and then return to the forest. Yet most people , even those who spend extensive time in the woods, might never encounter one of these woodland creatures. We might not be aware of their existence, but their total weight (biomass) could be greater than the biomass of all the birds, a more obvious group, in the same area of forest. Mole salamanders are seldom observed except on Big Night when hundreds might be seen entering a pool. Yet these animals live out their 20 years within a few thousand feet of that pool. How many have you seen?
When you find your vernal pool, treasure it, study it and protect it. That pool represents the cumulative evolution of a number of species to exploit a productive yet temporary habitat. Activity in the pool is carefully choreographed so that each species maximizes its own chances for survival. Organisms feed upon one another, yet sufficient numbers survive to maintain the population in future years. Years of drought or other adverse conditions which result in low survival rates might be followed by exceptional years which reestablish a population. A dry winter, late spring, or wet summer will favor the survival of different species of plant and animal. Over time, the variability of the climate determines the species which survive in a given pool. Life in the pool will continue while changing to reflect the conditions which modify it.
The vernal pool is dynamic, reflecting the competing lives of millions of organisms. At the same time, it seems to be a placid puddle drying in the intensifying sun of spring and summer. Do we treat the vernal pool as an important component of the upland ecosystem or a waste area to be filled for a tennis court or shopping mall? Do we watch salamanders continue their millennia old march to breeding pools or do we build a road around the pool and decimate the population with road traffic? Do we recognize the diversity of life in these ephemeral wetlands or do we poison them all in an attempt to attain a mosquito-free evening in summer?
Find a vernal pool and get to know it. Show its wonders to your child and your friends. Wade in its waters and observe the spectrum of its life. Watch a dragonfly defend its territory, listen to a treefrog call, and reflect on the finality of a flopping tadpole about to die on the mucky bottom of an evaporating pool. Visit the pool when it fills in the fall. Skate on it in the winter. Accompany the salamanders to it in the spring and observe their young emerging in early summer. Enjoy your vernal pool, its organisms, and its seasons of the year.
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