Window on Nature
By Lowell & Kaye Christie, F47246
On a recent drive into California’s Central Valley, we saw our first sign of spring. Although readers who live in the northern parts of the United States might think we are a little premature, what we saw was our favorite vernal pool coming to life. We know we’ll have to take advantage of the opportunity to see these interesting wetlands over the next few months, because vernal pools, by their very nature, have a short life span.
What is a vernal pool? Well, the definition depends entirely upon where you live. The term originally was used to identify small, temporary wetland areas found in Mediterranean-type climates, such as California. In our locale, these miniature environments burst into bloom each spring (“vernal” means spring) after the short rainy season fills depressions. The new moisture encourages the previous year’s seeds, eggs, and cysts to awaken from their winter sleep.
We were in Red Rock Country, on the Colorado Plateau, when we discovered our first vernal pools. However, at that time we called them potholes and ephemeral pools. We wondered how a tiny depression in a rock, holding perhaps a gallon of water that might evaporate by the end of the week, had its own resident colony of fairy shrimp (seen with a magnifying glass).
Vernal pools exist in the eastern part of the United States, but not all of them occur in the spring. We remember our amusement upon seeing a roadside sign that read “Salamander Crossing.” It wasn’t a joke. In parts of the East, many folks recognized that one of the best signs of spring is the annual migration of wood frogs and salamanders to their local vernal pools, where they lay eggs that must develop into adults and escape before the water dries up.
The definition that was developed to encompass all of these similar environments is based on the inhabitants of the pools rather than the pools themselves. Although many creatures make temporary use of the pools, most can survive elsewhere. Others, called “obligate species,” depend upon the temporary pools to survive. They must live or breed in vernal pools. If one of these species is found in a small body of water, by definition it is a vernal pool.
Since vernal pools are dry for part of the year, they cannot support fish species, a prime predator of young amphibians. This is why some amphibians seek out these pools to lay their eggs. An example is the wood frog, an inhabitant of upland forests. In early spring the frogs lay their eggs in the safety of a vernal pool and then hop back to their home in the woods. The eggs develop into tadpoles, the tadpoles grow into frogs, and the young frogs leave the vernal pool before it dries up. One information source we came upon indicated that breeding wood frogs can be used to define a vernal pool. Salamanders, another obligate species, follow the same general procedure.
But the most interesting inhabitants of vernal pools may be the ones that don’t escape and have somehow adapted to the absence of water. Since they depend upon water for life, what do they do when the water evaporates? Either they have a method for resisting desiccation, or they have at least one stage in their life cycle in which they can tolerate drying out.
Examples of resisting desiccation are found in some small snail species. As the water disappears, they burrow into the soil and seal their trap door (operculum) while waiting for better times. When the wet season returns, they resume their normal lifestyle. Snails, being snails, don’t have to hurry their development. The dry season is just a period when a snail’s pace is even slower than usual.
At the opposite extreme are the critters that must rush through their lives, because they can resist desiccation in only one stage. They usually survive as eggs or cysts (a capsule-like sac that contains an organism during its dormant or larval stage). This is called cryptobiosis, and in extreme cases some organisms can survive after the loss of up to 92 percent of their water content. If you think that’s incredible, consider this. Scientists in the late 1970s glued the cysts of brine shrimp to the outside of a spacecraft. They then recovered and hatched viable shrimp after the spaceflight. Although the experiment probably was intended to explore the effects of radiation exposure in space, it showed that these organisms can survive “” and continue their life cycle “” in the most extreme conditions.
Species with eggs or cysts that survive the dry times have an amazing ability to recover. According to a Canyonlands National Park publication, some organisms that live in potholes can become rehydrated and fully functional in as little as half an hour. This can be critical when their watery habitat may evaporate almost as fast as it fills. Some of these species have a life cycle as short as 10 days, which means that one generation can hatch, develop, and lay eggs for the next generation during a typical human vacation. That’s called living on the fast track. But what if it rains just enough for the organism to hatch, but the temporary pool dries out in a week?
Fortunately, just because some cysts respond quickly to water doesn’t mean that they all do. Imagine what would happen if they all hatched at the slightest shower. The entire population could potentially be wiped out if no more rain followed. Interestingly, some seem less sensitive to the application of water and may go through multiple cycles before hatching.
Many of the inhabitants of vernal pools require a magnifying glass to appreciate, but much of our enjoyment comes from the floral displays that sprout around the pools. The longest-lasting vernal pools are situated above hardpan surface that prevents water from sinking directly into the ground. And since a vernal pool, by definition, has no water outlet, it shrinks slowly by evaporation. As the pool becomes smaller, circles of wildflowers, such as yellow carpet (blennosperma), meadowfoam (limnanthes), goldfields (lasthenia), and sky blues (downingia) follow over the moist ground.
This wondrous explosion of flowers is perhaps best described in a quote from John Muir describing California’s Central Valley in the spring of 1869: “Sauntering in any direction, my feet would brush about a hundred flowers with every step … as if I were wading in liquid gold.”