Vernal Pool

When you hang out with naturalists too much you get inducted into a society of words that sound ordinary but are precise, scientific terms. Like vernal pool — a pond that exists in the spring and dries up the rest of the year through a process they (the naturalists) call evapotranspiration. We know what that is. Apparently the vernal pool hails from California where they transpire a lot and ours aren’t the real thing, we have the wrong climate so they are often also filled in the fall. But we use the term here nonetheless. Important because all sorts of amphibians lay their eggs in them.

The vernal pool has obligate species and indicator species. The former species breed in vernal pools 99% of the time. The latter are the same but seen from the pool’s point of view. They tell you it’s the real thing. To sum it up, a vernal pool is a temporary but cyclically occurring piece of water on which certain species depend entirely for their existence. No vernal pool, no species. Wood frog and fairy shrimp being two of them.

That is kind of a scary thing if you think of it. Plainfield is of course rife with vernal pools. What makes it so likely here is the glacial till or ledge right under the soil. In other words, we have some soil, but you don’t need to go far down until you hit a layer water won’t go through. Or your shovel for that mater. I can assure you, digging 4 feet down for footings when the till is at 2 ain’t for sissies such as myself. In any case, the water likes to lie on top of that layer when there’s a bit of a depression. Add a more or less continuous supply of water and you have a perched wetland, which has its own obligate species I am sure and that’s a different story.

But if your water supply is only available as a result of spring runoff and high ground water levels, and it dries up but not too fast you may have a certifiable vernal pool if you send the right photos with the right arrows to the right people.

Which is why I am witnessing a drama unfolding. Last week I was poking about some springs with my friend the stream hopper and we found a depression caused by an infernal machine that had been poking around right around there as well. In it, masses of frog eggs tied underwater to sticks and stones by mama frog so they would not pop up out of the water too early. Gelatinous masses with dotted with dark heart op-art so far not having gown the tail of the tadpole. But even without the tails, I knew them as my pals to follow in their development.

I’ve had a soft spot for frogs ever since Saskia de Jong and I were collecting them in buckets sometimes getting them to jump across our fingers but most of the time just to see how many we could get. One time we had 17 and were on our way to quite a record but we were called for dinner. Dinner was pork chops on the BBQ by the fire and pretty special to us — someone must’ve been to America, no self-respecting Dutch family had a BBQ at that time — so we stuck our bucket in her dad’s boat for safekeeping until we could go back to our frog hunt, thinking that they would not be able to jump out.

Of course the frogs did seek their freedom during dinner except they could not get off the boat of which the sides were tilted inward, it being a traditional sort of Dutch fishing vessel in vogue for the entirety of the nineteenth century well into the twentieth, made to prevent water from sloshing in and fish from flopping themselves out but pretty effective against frogs escaping also. And that of course occasioned much shrieking and searching and cursing as the whole family returned in the pitch dark to go to bed.

But that isn’t the drama. The drama is that it hasn’t been raining a whole lot and I returned this weekend to see how my pals were doing and found the pool dried up entirely. No sign of frog eggs. I guess I just have to accept that nature makes redundancies so that even as some vernal pools dry up frogs make it in others. Except these frogs were fooled by a pool that shouldn’t have been there in the first place, a temporary aberration caused by a careless machine that dried up quickly for lack of intact till or ledge.

But I was hooked on frogs now so I went to see the vernal pool behind the house of my friend the naturalist and who told me he has not just frog eggs but also spotted salamander spermatophores. (That’s’ the model where dad leaves the sperm behind tacked loosely to leaves on the bottom so it won’t float off, and mom comes by to pick it up. Drive-in-fertilization as it were.) Little fuzz men and less defined than the frog eggs, I’ve been trying to photograph them but they aren’t as happy to sit for portraits.

Today I went back to this pool number two to see how we were doing on tadpole development only to find that it has shrunk considerably and some of the sticks with egg masses are poking up out of the water. No rain in sight. I have a call into my pal to get him to don big boots and move the sticks into deeper water for mama frog. Not that I am a sentimentalist or anything like that. He says the jello will protect them. More on that later.

The thing that keeps sticking in my mind is that this is such an amazing system: drive-in fertilization and temporary nurseries where the young-uns can’t get too used to staying home because they dry up. Such a tricky balance with water in and water out. Gotta get it just right though, not too wet and not too dry, and there are not too many of these left in California in favor of farms and houses with lawns. I hope we do better in the long term but here, too, the bulldozer eats 40 acres a day sez Audubon.

The whole story begs the question of course: of what are humans the obligate species and of what are they the indicator species?

7 thoughts on “Vernal Pool

  1. I agree with Hayley. I too am watching. In our cellar hole. Last year the numbers were astounding. I notice the partying ceases and returns with rising and falling temperatures.

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  2. Dear Pleun,
    You are egg-obsessed. Yesterday it was Easter eggs. Great pics (including the challenging spermatophores), and some new info re California style VP’s. The critters in VP’s are generally pretty tolerant of drying conditions. The Fairy Shrimp go into a state of hibernation in the dry season – the Marbled Salamander actually returns to the dry VP to mate and lay it’s eggs.

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    1. Thank you all. Obsessed, yes I admit it. And I am always happy to learn. Going to have a look today to see what happened to Jacques’ high and dry frog eggs….

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  3. Best reading and looking I’ve had in the 5 weeks or so since I was last here. Thank you. Glad to be home and at my own PC with access to your miracles. – CF

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