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Vernal Pools

Just what IS a "vernal pool"?

Vernal (Latin vernalis, equivalent to vern - of spring) pools are seasonally available small bodies of water used by various amphibians (salamanders, frogs, toads) and some invertebrate species to breed and reproduce. Because these pools are temporary, they are devoid of predatory fish thus making these pools ideal nurseries for the eggs and young of these amphibian species. As the summer sun dries the vernal pools, amphibian tadpoles rapidly develop to their adult stage and hop or walk away to begin the terrestrial part of their lives. They will return to the location of the vernal pool of their birth when it is their time to reproduce the following spring.

Here's a comprehensive article on vernal pools by Steven M. Roble, Ph.D. that appeared in a past edition of Massachusetts Wildlife, the quarterly magazine of the Massachusetts Division of Fisheries and Wildlife:

Life in the Fleeting Waters

Scattered throughout the Commonwealth are many small, temporary pools of water that are often overlooked or regarded as insignificant and expendable. They rarely exceed 150 feet in width and three feet in depth. Some are only a few yards across. Because of their small size and ephemeral nature, they often fall victim to the casual pass of the bulldozer. Indeed, many of these pools have been destroyed during the past several decades to make room for development projects of all types, including roadways, subdivisions, office buildings and industrial parks. Many of the remaining pools have become favored dumping grounds for old tires, scrap wood and metal, appliances and even discarded automobiles. The pools also suffer from the belief that they pose a public health hazard (and should therefore be eliminated) because some species of mosquitos use them as breeding grounds.

An appreciation of the beauty and critical ecological role of these pools, variously known as vernal, spring or autumnal pools, can only be gained with some close scrutiny. Where the average person may see nothing more than a "big puddle", the discerning eye will quickly discover intricate complexity and significant value. Despite their small size, vernal pools support a diverse biological community ranging from millions of microscopic organisms, such as protozoans, rotifers, copepods and water fleas, to many amphibians and reptiles. A wide array of insects, including predaceous diving beetles, water scorpions, water boatmen and backswimmers, as well as the immature stages of caddisflies, mayflies, damselflies, dragonflies, and indeed, some mosquitos, can be found in vernal pools. Among the most beautiful of the invertebrate animals that inhabit these pools are the exquisite, almost transparent fairy shrimps. The beauty and grace exhibited by these small, upside-down swimming crustaceans is almost magical.

Vernal pools may be found almost anyplace where small depressions, swales or kettle holes collect runoff or intercept seasonally high groundwater tables. Although many are located in wooded habitats, they can also be found in areas such as meadows, sand flats and river floodplains. The word "vernal" means spring, and indeed, most vernal pools are filled by spring rains and snowmelt. One unfortunate consequence of this source of water is that vernal pools are more affected by acid precipitation than other, more permanent bodies of water. Acidic waters may prevent hatching, cause developmental abnormalities or retard the growth rates of the amphibian embryos and larvae that are often found in the pools. Aluminum and other toxic metals leached from surrounding soils may also have an adverse effect on the development of amphibian embryos.

Breeding Migrations

Vernal pools are perhaps best known as the destination of the spectacular mass migrations of mole salamanders. These species, which include the spring-breeding Spotted, Jefferson and Blue-spotted Salamanders, are so named because, like moles, they spend most of their lives underground in burrows. They are seldom seen except during breeding season, although they may occasionally be found under stones or rotting logs, more rarely in damp basements. On the first warm, rainy nights of spring, these secretive animals emerge to cross forests, fields and roads en route to their traditional breeding sites. Many are killed by vehicles when they attempt to cross roads. The installation of salamander tunnels (such as the internationally famous one in Amherst) or the temporary closure of roads along migration pathways can reduce these unnatural deaths significantly. Local breeding populations of mole salamanders usually number in the dozens or low hundreds, although a few are known to approach a thousand individuals! Some mole salamander populations contain unusual, nearly all-female hybrid forms of the Jefferson and Blue-spotted Salamanders. Commonly known as Silvery and Tremblay's Salamanders, these individuals possess three or even four sets of chromosomes, rather than the normal complement of two sets.

Wood Frogs are highly synchronous breeders that also migrate to vernal pools in early spring. If weather conditions are right, almost every frog in a local population will arrive at the vernal pool within a matter of hours, and mating is often completed within one or two days. Breeding populations of Wood Frogs may number in the hundreds or thousands. Unlike the salamanders, which conduct their courtship activities in silence, male Wood Frogs announce their presence and availability to prospective mates with duck-like quacking calls. Many inexperienced observers have peered into vernal pools in early spring, searching in vain for nonexistant ducks! The extreme brevity of the mating season combined with a shortage of females at the pools usually results in many desparate male Wood Frogs attempting to clasp anything that even remotely resembles a mate, including other single males, paired frogs, mole salamanders or even beer cans!

Mole salamanders and Wood Frogs often breed before the ice has completely left their pools, but their eggs are very tolerant of the cold. The gelatinous mass which surrounds the eggs is several degrees warmer than the surrounding water, hastening embryonic development and providing protection against the lethal effects of freezing. Furthermore, most female Wood Frogs lay their eggs in one limited area of the vernal pool. This communal "nesting" behavior enhances the insulating properties of the individual egg masses.

Advantage and Risk

A significant physical attribute of vernal pools is that they periodically become completely dry. This has profound effects on their ecology. Perhaps the most important consequence of a periodic lack of water is that vernal pools cannot support fish populations. Since fish readily devour the eggs and larvae of most amphibians, the absence of fish is crucial to the successful breeding of several species of amphibians in Massachusetts. The breeding strategies of these amphibians ...rely on vernal pools and almost complete avoidance of permanent ponds (which usually contain fish). These species, somtimes called "obligate" vernal pool-breeding species, include rare, state-listed species such as the Blue-spotted, Jefferson and Marbled Salamanders, as well as more common species like the Spotted Salamander and Wood Frog.

In exchange for the benefit of not being subjected to fish predation, breeding adults of all these species annually risk the possibility that the pools will dry up before their aquatic larvae or tadpoles can transform into terrestrial juveniles. The gelatinous mass which surrounds and insulates the eggs, however, can also protect the devloping embryos from desiccation. The eggs of Wood Frogs and Spotted Salamanders can survive temporary stranding (caused by receding water levels) for up to one week. When the egg masses of many Spotted Salamanders are clustered together in a communal "nesting" area, as sometimes occurs, the embryos in the inner masses of the cluster are further protected and can survive for a month without water! Subsequent rains may submerge these egg masses, allowing the larvae to survive. In hotter and drier years, larval amphibians which have escaped predation, disease and other sources of mortality may face near certain death when they become stranded in the remnant puddles of the shrinking pool. Subsequent rains may save them by restoring water levels, or only grant them a temporary reprieve. Stranded amphibian larvae are easy prey for all kinds of birds, which are quick to take advantage of the grand feast that a drying vernal pool presents.

Many or all of the offspring produced by "obligate" vernal pool amphibian species in a given year may die before transforming. However, despite complete or near complete reproductive failure in some years, these amphibian populations persist because, in most years, their offspring successfully transform before the pools dry up. Those pools which consistently retain water long enough to permit growth...are favored as breeding sites over other pools that typically persist for shorter periods. Over thousands of years, these amphibians have evolved a decided preference for the most reliable pools, and they return to them faithfully. The sensory mechanisms that guide their migrations to these pools every year are not well understood.

Each female mole salamander, depending on her species and body size, produces between 60 and 500 eggs during each breeding cycle. Individual adults return to the breeding pool every year, or in some cases only every second or third year. Studies have shown that 75 to 90 percent of the mole salamanders that are present at the breeding pool in one year survive to the following year. In contrast, Wood Frogs typically produce 300 to 1500 eggs, but are much shorter lived than the salamanders. Most adult Wood Frogs have only one opportunity to breed in their lives. Only about 10 percent survive until the following spring.

All four species of fairy shrimp that are found in Massachusetts, including the rare Intricate Fairy Shrimp, spend their entire lives in vernal pool habitats. Fairy shrimp normally move slowly through the water using a constant beating motion of their eleven pairs of feathery legs. However,when pursued by a potential predator, they will take evasive action and move in rapid spurts. Although such maneuvers may enable some of them to evade invertebrate predators, they are usually ineffective against fish. Fairy shrimp are largely confined to vernal pools because of their inability to withstand fish predation. They survive the periodic absence of water by producing drought-resistant eggs (that can remain viable for up to 20 years!). When the eggs are flooded, whether in fall or spring, a new generation of fairy shrimp emerges. Like "obligate" amphibian species, the continued existence of local populations of these fascinating creatures hinges on the continued presence of vernal pools.

Seasonal Progression

Many amphibian species which can breed in permanent bodies of water such as ponds, marshes, swamps and streams, also utilize vernal pools for breeding purposes. These "facultative" vernal pool-breeding species include the Spring Peeper, Gray Treefrog, Green Frog, American and Fowler's Toads, Red-spotted Newt and the rare Four-toed Salamander. The seasonal progression of amphibian species that breed in vernal pools throughout the Commonwealth is impressive: Blue-spotted and Jefferson Salamanders in late March or early April; Wood Frogs, Spring Peepers and Spotted Salamanders several days to several weeks later; American Toads, Fowler's Toads, Gray Treefrogs and Four-toed Salamanders in late spring; and Green Frogs in summer. Unlike the other mole salamander species in Massachusetts, the rare Marbled Salamander breeds in the fall and lays its eggs on land in dry or partially wet vernal pool depressions. The female remains with her eggs until they are flooded by rising water levels. This species and the four-toed Salamander are the only vernal pool-breeding amphibians in Massachusetts that exhibit parental care behavior. After hatching, the larvae of the Marbled Salamander overwinter in the pool.

Vernal pools are important sources of food for many terrestrial vertebrates, especially during spring and early summer. Predators that are attracted to the pools for feeding purposes include Water, Ribbon and Garter Snakes; Spotted, Painted and Snapping Turtles; Green-backed and Great Blue Herons, raccoons, skunks and occasionally, mink. Various dabbling ducks - such as Mallards, Black Ducks, Blue-winged Teal and Wood Ducks - also feed in vernal pools. Whereas frog tadpoles are primarily vegetarians, salamander larvae are strictly carnivorous. Salamander larvae consume large quantities of microcrustaceans (copepods, water fleas) and aquatic insects, including mosquito larvae. Larger salamander larvae also prey on smaller salamander larvae and tadpoles...

Identification and Protection

Although vernal pools are most easliy identified when they contain water and amphibian larvae, a dry depression can often be confirmed as the site of a vernal pool by evidnece left behind by its former inhabitants. The most conspicuous types of evidence are the discarded cases of larval caddisflies which have long since transformed into flying adults, and the shells of tiny clams commonly known as pea or fingernail clams. The shells of amphibious, air-breathing snails also indicate the presence of a vernal pool, but their identification requires more expertise because of the possible occurrence of land snails in the vicinity of the pool.

An important component of the wildlife habitat amendment to the Massachusetts Wetlands Protection Act which became effective on November 1, 1987, is the involvement of the Division of Fisheries and Wildlife in "certifying" vernal pool habitat. The Division's Natural Heritage and Endangered Species Program has the task of reviewing documentation submitted in support of applications for vernal pool certification. Staff limitations unfortunately prevent the Program from locating and surveying the pools, but any citizen can submit an application for vernal pool certification by obtaining specified types of documentation (such as photographs and mapping information).

Certified vernal pools that fall within wetland resource areas (such as the 100-year floodplain, marshes and swamps) receive strong protection under the Wetlands Protection Act, regardless of their size. Uncertified pools remain susceptible to filling and other adverse impacts. The accelerated rate of development in the Commonwealth makes it imperative that vernal pools be certified and mapped in advance of any proposed development projects in an effort to steer such projects away from these important habitats.

Many vernal pools will not qualify for legal protection, despite being certified, because they are isolated depressions in upland areas which do not meet the minimum size requirements to qualify as isolated wetlands under the Wetlands Protection Act (11,000 cubic feet of water with an average depth of at least 6 inches). This is an unfortunate shortcoming of the jurisdiction of the Act, because upland vernal pools are clearly important to many species of wildlife. Furthermore, current protection of a buffer zone around certified "wetland" pools is limited to a maximim of 100 feet, all of which must also fall within a wetland resource area. This is inadequate because the areas that provide essential habitat for adult vernal pool-breeding amphibians during most of the year extend much farther from the pools and almost always include upland habitats. Mole salamanders are known to migrate up to several hundred yards to reach their breeding pools, and some Wood Frogs may travel even further.

For all "obligate" wildlife species - specialized amphibians, fairy shrimp, and a variety of invertebrates - vernal pools provide essential breeding and living habitat that is critical to the continued existence of their local populations. When vernal pools are filled or otherwise destroyed, local populations of all these species are in jeopardy of extinction unless other vernal pools are present nearby to serve as new breeding sites. Many of these other pools may have remained unused previously because they consistently dry up too quickly or are too acidic to permit normal growth and development. Clearly, the protection of vernal pools is an important step in the conservation of the many wildlife species which depend upon them for breeding habitat, but stronger measures are needed to protect adjacent upland habitat that is just as critical to their continued survival.

Vernal Pools Need Your Help

Unless a vernal pool is certified and duly entered into the computer banks of the MDFW's Natural Heritage and Endangered Species Program, it remains a "non-entity" under the law, commanding little or no legal protection. Furthermore, it is essential that vernal pools be certified before filling and development projects are proposed, or it may be too late to save them. Your help is urgently needed to protect these unique and vulnerable wetlands that are of such vital importance to the continued survival of many wildlife species. The Massachusetts Division of Fisheries and Wildlife encourages citizens of the Commonwealth to become involved in wildlife habitat protection through the process of vernal pool certification. All it takes is a small commitment of time and effort.

To find locations of state certified vernal pools in your community, contact your local Conservation Commission, usually located in your town or city hall.

Click here to view the number of certified vernal pools in your community.

Click here to find out how to state certify vernal pools in your community.

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The following photos are from a vernal pool education activity at a pool in the Southeastern Massachusetts Bioreserve:

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