Speaking of Nature: Dive into vernal pools
A vernal pool at the High Ledges in Shelburne. Note the bare leaves surrounding the water that denote the original size of this quickly shrinking pool. Also note the fallen branch in the middle of the water.
(BIll Danielson photo)
A mass of eggs clings to a branch above the water. These eggs were originally laid underwater, but they were exposed as the water disappeared.
(Bill Danielson photo)
A close inspection of the eggs reveals the tiny, legless body of a salamander larva inside an egg. Note the frilled gills near the back of the larva's head.
(Bill Danielson photo)
We are generally aware of the locations of the permanent ponds in our neighborhoods, but in springtime there are also temporary pools that will appear in the woods. These pools, called vernal pools because of their association with springtime, are the result of rain and melting snow that collect in depressions in the forest floor.
The combination of a long, snowy winter and a cool, rainy spring (so far) has resulted in a plentiful supply of water for vernal pools, but it may be difficult to find these pools because they appear and disappear without a sound. However, it is precisely at this time of year that the sounds of singing frogs might give the locations of some of these pools away.
You might be exploring a forest that you’ve visited many times before, and suddenly there is that telltale sound coming from the woods. It might be wood frogs, or spring peepers, or possibly even an American toad singing on a cloudy day, but whatever the sound is, it just might be interesting enough to lure you off the path and into the unknown.
As you take your first steps onto the crunchy leaves where no trail goes your mind might conjure up images of dwarves and hobbits wandering off the path and into the gloom of Mirkwood Forest. Undaunted, you push on with the heart of an intrepid explorer beating in your chest. You don’t know what awaits you, but you plunge headlong into adventure! And then you reach a small puddle of water in the woods and realize that you really have quite an imagination.
Vernal pools can be almost any size, but in my experience most of them are small, shallow, and filled with crystal clear water. Another feature common to every vernal pool I have ever seen is the fact that their bottoms are lined with dead leaves. Believe it or not, the leaves are the fuel source that drives these little ecosystems.
Dead leaves contain a vast supply of organic material that can be food for the right kinds of organisms. Decomposers like bacteria and fungi can eat the leaves, the decomposers can then provide food for a diverse community of microscopic insects and invertebrates, which in turn provide food for larger animals. In this sort of situation the “winner” is the largest predator and in a vernal pool, where there are no fish to contend with, even a small animal can be the largest predator. This is exactly the situation that makes a vernal pool so attractive to an amphibian like a frog or salamander — that rare chance for a small animal to be the big boy on the block.
The temporary nature of these pools might help them to go unnoticed by humans, but the local amphibians surely have their locations burned into their brains. Thus, year after year, the adults will head for the very pools where they themselves may have started their lives. Frogs and toads are nice because they make so much noise, but salamanders are also very active in these pools. For all of them, however, the trick is speed.
There is no time to dilly-dally when vernal pools are concerned. Frogs, toads, and salamanders have to locate the pools, find a mate and reproduce post haste. To expedite the process, the females will form large numbers of unfertilized eggs inside their bodies before they even reach the pools. This is why female frogs are generally larger than males; larger bodies equal larger numbers of eggs.
By the time the females arrive at a pool, the males are already waiting for them. There may be some selection by the females, but a host of aggressive males will definitely speed up the process. A male will get onto the female’s back, grab her around the abdomen and hold on for everything he’s worth. Then, with the male onboard, the female will select a spot to deposit her eggs and then extrude them out into a mass while the male fertilizes them all at once. And then that’s it. The parents abandon their offspring.
Once the eggs are laid the clock starts running. The wager has been made and chance takes over. The very characteristic that made the pool so attractive now becomes the thing that will make it so dangerous. With every passing day the water in the pool will diminish. At the same time, the eggs must hatch and the larvae must mature. Legs must replace tails. Lungs must replace gills. If the water disappears too quickly, an entire generation can be lost.
This is exactly the kind of disaster I encountered a few years ago up at High Ledges in Shelburne. On a beautiful day in the middle of May, I was on my way up to the overlook when I discovered a vernal pool that must surely make its annual appearance by the side of the road. It was dry and the water was already dropping quickly, which had exposed many egg masses that had originally been laid under its surface.
This presented me with a wonderful opportunity to get a backlit view of what was inside the eggs, and in this particular instance it turned out to be baby salamanders. They seemed to be thriving, despite the fact that many of them were already out of the water, so as soon as my photos were secured I snapped the branch they were anchored to and allowed them to sink back down into the water. I’m just an old softie, I guess.
But that was no guarantee of success. Back in 1998, when I worked as an interpreter for the Massachusetts state parks, I was out exploring the ridgeline trail that runs along the length of the Holyoke Range State Park. I can’t remember if it was late May, or early June, but I do remember how surprised I was when I discovered the shrinking remains of a vernal pool near the top of one of the mountains.
As I walked down to the edge of the pool, I could see its surface start to ripple as little creatures in the water tried to get away. The problem was that the water was so shallow there was nowhere for them to go. Hundreds of little pollywogs were clinging to life, but they were doomed. The area surrounding this remnant of water was littered with the dead bodies of thousands of pollywogs that had already seen their portion of the pool sink back down into the ground. This is the risk of depending on a temporary resource. You are completely at the mercy of the weather in any given year.
But this past winter was long, cold, and snowy and this spring has been cool and rainy … so far. The vernal pools in our area must certainly be filled with water and are hopefully filled with activity. Songs will be sung, eggs will be laid and little amphibians will hatch out and swim through the pools in search of tiny prey.
Meanwhile, shrews and snakes will patrol the shorelines of these small pools. Such high concentrations of little frogs and salamanders are sure to attract the attention of animals that will find them tasty. Even if they are the largest predators in the pools, they are still nothing more than delicious little morsels when they are forced to leave the water. The lucky ones will survive, grow to maturity, find a safe place to spend the winter, and then return next year to perpetuate their respective species. And all of this is right out there for you to see if you’re just willing to go on a little adventure.
Bill Danielson has worked as a naturalist for 16 years. In that time, he has been a national park ranger, a wildlife biologist and a field researcher. He currently works as a high school chemistry and biology teacher. His Speaking of Nature column runs weekly in The Recorder, except for the first Thursday of each month, which is when his Kids and Critters column for young readers appears. To contact Bill, or to learn more about his writing, visit www.speakingofnature.com