Vernal Migration: Ephemeral Pools

March 10, 2015  •  Leave a Comment

In mid to late March, or in the case of a severe winter, early April, the first signs of spring can be witnessed stirring in shallow woodland pools filled with amphibians, reptiles, and even freshwater shrimp. These creatures appear at least a couple weeks prior to the wildflowers that we often view as the harbingers of spring. Usually we will know the season has arrived by sound, rather than sight. After the first warm rain, amphibians roused from their hibernation in the mud and detritus of the forest floor gather together in copious quantities in temporary ponds that have appeared with the addition of meltwater from winter snow and ice. The small ponds, usually not larger than a backyard swimming pool, go by many names, such as vernal, woodland, or ephemeral pools, generally with the latter being most appropriate. Within the basins a sonorous and usually deafening hymn can be heard going round the clock for several weeks. A walk in the quiet solitude of a gray-brown forest is often quickly interrupted upon approach of one of these pools, quickening the pulse, and giving proof that the landscape is in the process of being rebooted, just as our sluggish minds and bodies are after being confined these past long, cold months.

Without a doubt, the first, and most prolific life to be found, is the wood frog. This species has a wide distribution in the U.S., occupying most of the east coast north of Georgia and continuing westward to Minnesota. Frogs also inhabit a majority of Canada and Alaska. On a cold day just before a warming rain, these pools are empty, aside from the innumerable leaves and branches lining the bottom, and are nothing extraordinary to behold. After the water begins to penetrate the leaf litter and sink into the ground, the frogs burst from their hiding places in unison during the night if the ambient temperature spikes to at least 40° F. They appear in the thousands or millions in a rain swept area. Drivers will have to be mindful of the roads during these times as the frogs recklessly journey across the pavement in migration to their watery mating grounds, often those in which they themselves were born.  The following day a return visit to the pools morphs into a lively spectacle, complete with a cacophony of sound that resembles a mix begin the quacking of a flock of ducks and the buzzing of a bee hive—the mating call produced by males.

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Wood Frogs

Wood frogs are able to make a speedy exit from the confines of their winter hibernacula. Rarely do they burrow into the earth more than a few inches. They frequently lie just below the leaf litter in a zone that offers protection from the physical elements, such as snow, harsh winter winds, and predators, but doesn’t quite ensure adequate insulation from the penetrating cold. The frigid weather that would easily kill most amphibian species doesn’t seem to bother the wood frogs—their bodies are actually capable of freezing solid during the winter and thawing out in the spring, the result of special proteins that keep cells from being damaged by ice crystals.

At a short distance from the pools the deafening calls drown out all other forest sounds; but upon reaching the borders the noise instantly ceases giving way to an uneasy silence, only interrupted periodically by the occasional splash from individuals too closely approached. In some of the larger ephemeral pools it may be difficult to spot the wood frogs, with most blending into the inky waters; some females being the exception, cloaked instead in a ruddy pink. A thorough scan of the water may reveal a male statically floating atop the water, with all appendages spread out, similarly to one pinned to a dissection board. Their eyes are tightly fixed on your position for the duration of your stay. Only those males who have a locked onto a female and are in competition for the privilege to mate seem to be too enraptured on their goal to pay any notice to a nosy human. Some splashing might attract your attention, where a bloated female being harassed by multiple males is spotted, all trying desperately to attach themselves to her body—it’s not uncommon to see 4 or 5 in pursuit of the same female. In the process of them trying to clasp themselves to her, the flailing of their legs propels the lot around the pond, as if a motorized boat. In such cases, it’s quite easy to walk up to them without the frogs scrambling away, and, if you so choose, pluck one from the water. In this state mating becomes more important than endeavoring to avoid predators.

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Wood Frogs in Amplexus

Each female lays about 1,000 eggs, all congealed into one solid jelly-like mass that is usually attached to some sort of aquatic vegetation or detritus. The black dots seen within the cluster are individual eggs, which, within a month, will hatch and morph into tadpoles.  Egg masses are rarely solitary; normally all females of the pond group them together in only one or two areas, forming vast clusters, or aggregates that can be quite thick, reaching from the bottom of the pool to the surface. As the weather warms, algal blooms within the ephemeral pool usually coat the eggs, making the floating mats resemble pond scum. This is a rudimentary cloaking device that keeps the eggs hidden from sight, and ensures the slimy, green masses remain an unsavory meal choice for any animal that can peer through the deception.

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Wood Frog Egg Masses

If all goes well and the pools remain filled with water, within two months the diminutive tadpoles will fully transform into terrestrial adults. Once mature, they will exit their natal ponds and join their parents in the forest until the following spring, when the end of winter rains gently nudge them out of their hiding places and encourage the frogs to take part in the cyclical vernal migration.

Wood frogs may be the most visible species to utilize ephemeral pools, but they are far from the only ones that rely on them for breeding. Numerous salamander and newt species, decked out with vibrant colors and abstract mottling also journey to these places to mate. Spotted, blue-spotted, tiger, marbled, and redback salamanders, with their aptly descriptive names, can be found in these localized pools for a brief duration. The red-spotted newts, the plebian dwellers of these environs, are usually the only amphibian species aside from the frogs that are somewhat easily viewed. Most of their salamander cousins are rather elusive and are rarely seen, with their eggs being the only trace giving hint of their presence. Each species’ egg mass differs in shape, coloration, and number of eggs contained within. Identifying species by egg clusters alone is normally how biologists are able determine the amount of biodiversity within a given area.

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Red-spotted Newt

Along with frogs and salamanders, the occasional turtle can be seen roaming about the pools before they dry up; various aquatic insects plying the surface and some traversing the depths swarm throughout; and the fairy shrimp, a crustacean similar to “sea-monkeys” (brine shrimp) know of no other home. In short, these pools though small and transient occupy a significant niche in the landscape. The size of these water bodies, however, often leads to their demise. Individuals who lack a knowledge of the importance of ephemeral pools too often view them as nothing more than mosquito breeding grounds. They fail to get close enough to them in body and mindset to witness the beauty and biologically rich array of life within, and thus, frequently set out to rid their property of these bits of so-called swampland. Unless unusually significant in some way, most pools garner no legal protection, being well under the required wetland size of 12.4 acres (5 hectares) to qualify for protective status in New York. Ephemeral pools, as their name suggests, may be just that—ephemeral—as they are quickly vanishing from our forests, a result of continued sprawl and apathetic attitudes towards the environment.

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Spotted Salamander


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