If Pennsylvania’s tree frogs and marble salamanders were the size of caribou or bison, this would be one of the most spectacular – and obvious – wildlife spectacles around.
Instead, it’s an annual migration in miniature that goes largely unnoticed.
Each year, generally after dark in spring on warm, rainy nights – critical for animals with moist skin – frogs, salamander and toads that spend the winter below the leaf litter on the forest floor or even underground begin traveling. Some might go only 100 yards; others, perhaps a mile.
Their destination: temporary puddles, or what are officially known as vernal pools.
The importance of vernal pools
Some Pennsylvania amphibians can lay eggs in any waterway. Spring peepers, those tiny but vocal frogs that sing so often and loudly, are one example. American toads are another.
But some can’t. Six species – marbled, spotted, Jefferson and blue-footed salamanders, wood frogs and Eastern spadefoot toads, along with two kinds of crustaceans, fairy and claim shrimp – only reproduce in vernal pools, which the Pennsylvania Natural Heritage Program describes as a “unique type of wetland habitat.”
“They are typically small, shallow, ephemeral water bodies, and unlike a pond or a lake, they have no permanent inlet or outlet,” it says. “They are filled each spring by rain and snow melt, then dry up for a period of time during the summer.”
Something else they don’t have is fish, said Quinn Heist, an educator at Memorial Lake and Swatara state parks in Lebanon County.
“Because what likes to eat amphibian eggs? Fish,” Heist said. “So, if these species lay eggs in pools that dry up in the summertime, they eliminate one of their potential predators.”
Of course, waters too temporary to hold fish are, well, temporary. So eggs laid in spring must hatch, and the resulting young must disperse into the surrounding forest, before pools dry up.
“It’s a race against time,” said Chris Urban, chief of the natural diversity section for the Pennsylvania Fish and Boat Commission.
Vernal pool conservation challenges
Vernal pools exist in lots of places, in lots of sizes. According to the U.S. Environmental Protection Agency, they can pop up anywhere there’s a depression in the ground, so long as the underlying geology and climate are right.
Here in Pennsylvania, they’re commonly found in floodplains, poorly drained valley bottoms and lowlands, and the glaciated northeast and northwest corners of the state, among other places, said Betsy Leppo, an invertebrate zoologist with the Pennsylvania Natural Heritage Program, in a recent vernal pool conservation and management webinar.
They come in varying sizes, she added.
“Vernal pools in Pennsylvania range from little leaky puddles that you can jump across to really large wetlands where you can really fill your hip waders,” Leppo said.
Those who explore Lebanon County can expect to find some, if they look.
“We have quite a few vernal pools in Swatara,” said Heist, who leads programs in spring to explore them, starting from Sam Siding Trailhead. “Just the geology of the area, it’s pretty good for supporting shallow ponds.
“Are they always obvious? Maybe not. But if you’re looking for them, you can definitely find them.”
Memorial Lake State Park has vernal pools, too, he added, if not as many.
Finding them is a often matter of looking for clues. Downed trees are sometimes one, Heist said. A vernal pool might form when water fills the hole left by exposed roots, for example. At Swatara specifically, some of the old canal remnants serve as manmade vernal pools. They’re worth investigating, he added.
Listening can lead you to vernal pools, too. Spring peepers are often a dead giveaway to a pool’s location, Heist said.
Life and ecology in vernal pools
Everywhere they exist, for however long that is each year, they’re important pieces of a larger ecosystem. Leppo said vernal pools perform the same essential ecological services as all wetlands, from flood control to water purification. But they also “exert an oversized effect on the surrounding landscape.”
Hundreds of migratory and resident bird species use vernal pools each year, along with a similar variety of insects, from midges to caddisflies. Unique and rare plants also coexist with vernal pools in spots. And in Pennsylvania, Leppo said, spotted, wood and box turtles – all classified as “species of greatest conservation need,” meaning they’re not endangered yet, but in need of help if they’re to avoid that dubious distinction – frequent vernal pools for food and water.
In short, there are a lot of different things eating and being eaten in, around and on the way to and from vernal pools.
“They’re a good example of a keystone habitat,” Leppo added. “Without vernal pools, Pennsylvania’s forests would look and function very differently and some species would disappear altogether.”
Urgency in protection and awareness
Yet they’re in trouble. Pennsylvania once had 1.127 million acres of wetlands, Leppo said. Now it’s got about 500,000 – a 56 percent decrease – with the rest having been drained, filled and/or converted to other uses. That’s bad news considering that, across the Mid-Atlantic region stretching roughly from New York south through Pennsylvania almost to Virginia, 26 percent of all threatened and endangered amphibians depend on vernal pools.
But conserving vernal pools is not as simple as just protecting low, wet spots. Most of the species that rely on them to reproduce live the majority of their lives in the surrounding forest, Heist noted. They return to pools – often the very one they were born in – only for a short period each year.
“That’s why important not only to preserve vernal pools, but the area immediately around vernal pools,” he said.
Traditionally, not everyone’s understood the role vernal pools play in larger ecosystems. But when exposed to them and the “vernal pool soup” of life they contain, as Leppo describes it, that can change, Heist said. That’s why he leads people to see them each year, and encourages people to explore on their own, even now, before this year’s crop of vernal pools dries up for another season.
Just be respectful of any you find, he added. They’re precious places, all the more valuable for the short annual duration of their live-giving window.
“Each vernal pool, it’s a crazy, life-packed system,” Heist said. “It’s kind of cool. Life’s not easy in general, but especially in a vernal pool.”
Vernal pools
Just how soon vernal pools dry up each year is weather dependent. Some springs and early summers are wetter, some drier. But as a general rule, they begin losing water around the time trees leaf out, said Chris Urban, chief of the natural diversity section for the Pennsylvania Fish and Boat Commission.
So if you want to find any this year, don’t wait. Go looking now.
And if you find one, consider registering it.
The Pennsylvania Natural Heritage Program – a partnership of the state’s natural resources agencies, the Western Pennsylvania Conservancy and U.S. Fish and Wildlife Service – serves to “gather and provide information on the location and status of important ecological resources.” That includes plants, vertebrates, invertebrates, ecological communities and geologic features – and vernal pools.
Citizen-science volunteers can help by contributing to the program’s “vernal pool registry,” which is a listing of pools across the state and the animals using them.
Details on getting involved are at the Pennsylvania Natural Heritage Program website.
Likewise, there’s a volunteer effort underway to document the presence, abundance and range of herps – amphibians and reptiles – across the state. The Pennsylvania Amphibian and Reptile Survey has collected more than 200,000 observations since its launch in 2013, more than 800 this year alone at last count.
“Very little information about Pennsylvania’s amphibians and reptiles has been collected through the years when compared to other groups of organisms,” the site reads. “This is unfortunate as herps are important indicators of the health of our natural places and the very presence of certain species can tell us a lot about an area.”
Volunteers create an account and record what they see, with data recorded in a map broken down into blocks and quads.
Visitors to the website can also search the map to see what others are encountering by county. In Lebanon County, that’s included spotted salamanders, wood frogs and red-spotted newts so far in 2024.
Finally, to learn more about the creatures that rely on vernal poles exclusively to reproduce, visit the heritage program’s website on vernal pool animals.
What might you learn? How about this. Wood frogs, the most common indicator of vernal pools in Pennsylvania, spend their winters above ground, tucked only under leaves, in what is essentially a lifeless state. According to the Pennsylvania Natural Heritage Program’s Betsy Leppo, falling temperatures lead them to create an internal antifreeze that “prevents their cells from bursting when they freeze.
“But in the spring,” she said, “they gently thaw out again and their heart starts beating again and they return to their natal vernal pool to breed.”
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