Episode 328: Vernal Pools

Photo courtesy of the Chesapeake Bay Program

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Transcript 4/20/24:

It’s tricky business traversing the forest this time of year. What once seemed like open woods only a few weeks ago full of hummocks and mounds now seem to be shallow, sylvan ponds studded with small, duffy islands. Welcome to The Nature of Phenology where we share the cycles and seasons of the outdoors. I’m your host, Hazel Stark.

Ask a group of ten people what their favorite signs of spring are and you’ll get ten different answers. Flowers blooming, warm sunshine, birds singing, water running, buds swelling, new leaves growing—there are so many signs of spring in a place like Maine with four boldly different seasons that there is no shortage of options. I certainly cannot pick a favorite spring sign, myself. It depends on what week you ask me. In early March, I anticipate the return of turkey vultures. Shortly after, I listen for the songs of winter wrens and song sparrows. But there’s one spring sign that I find is the most common favorite around here: the chorus of a couple different frogs, one of which only exists due to the presence of vernal pools.

In the verdant forest after the vernal equinox, you may stumble upon a pool of water with no inlet or outlet. These vernal pools exist only in the spring due to the influx of liquid water the forest receives from snowmelt and spring rains. They form in depressions in the forest floor and are large enough to last for a few months, but small enough that they tend to dry out by mid-summer. As a result, vernal pools have no fish in them. This lack of a key predator population makes them a perfect place for amphibians to breed.

There are three amphibian species in Maine that rely on vernal pools for breeding: the spotted salamander, blue-spotted salamander, and wood frog. You have probably seen two of these species in your wanderings. Spotted salamanders are large, up to ten inches long, and have bright yellow spots on a dark background. I found one under a rock in the woods behind my childhood home when I was a kid and named it Sally, of course, and visited it daily for a part of one summer. I have only come across a blue-spotted salamander once, in a woodpile a few years ago. These salamanders are also dark and large, but are mottled with flecks of pale blue. But I encounter wood frogs far more often than these two salamanders, and that’s largely because this time of year, I hear them before I see them [wood frog sound].

Hearing that unique quacking sound of the wood frog is a good clue that there is a vernal pool nearby and spring has surely arrived. When I hear wood frogs, I know that the melodious chorus of spring peepers is soon to follow. These brown to tan wood frogs have a characteristic dark stripe extending from the nose to behind the eye, almost appearing as an eye mask, so they are easy to identify by sight as well as sound. If you do hear a wood frog chorus, approach the area quietly, because as soon as they notice you, the whole group will get quiet as if you’ve shut off a switch.

As days get longer, the sun gets warmer, and the ground thaws, wood frogs and spotted and blue salamanders emerge from their winter hibernacula and head to a vernal pool to breed—this mass amphibian migration occurs primarily on what are called “Big Nights,” which are warm, rainy spring nights. Once they’ve reached their vernal pool, they lay masses of eggs without risk of fish eating them, though other predators are likely to try—like great blue herons, raccoons, and even dragonfly larvae. One way to differentiate between wood frog and spotted salamander egg masses is that wood frog eggs look like a lumpy collection of marbles stuck together while spotted salamander egg masses are surrounded in a thick, smooth jelly.

So the next time you’re outside, you could head into a forest near you to listen for the quacking chorus of wood frogs that indicates a vernal pool is nearby. And if there’s a warm, rainy night in the forecast, you could connect with a local conservation organization to see if there are any Big Night events happening where volunteers are needed to help amphibians cross our busy roadways unharmed.

You can download this episode and find a link to the transcript, photos, information about podcasting and more by visiting archives.weru.org. Wood frog sound courtesy of wildearthvoices.org. Thanks for listening and please join us next week for another dive into The Nature of Phenology.

References:

Vernal Pools: A Significant Wildlife Habitat (fact sheet), Bur. of Land & Water Quality, Maine Department of Environmental Protection. (2009, April). http://Www.maine.gov. https://www.maine.gov/dep/land/nrpa/vernalpools/fs-vernal_pools_intro.html