The Earth presents challenges, and life responds. No matter what the extremes, living things find a way to tolerate whatever levels of temperature, rainfall, sunshine, wind, slope, and soil chemistry a habitat may provide. Occasionally, even a generally moderate region will feature a certain corner that presents difficulty as well as opportunity, and plants and animals find a way to make it work for them. The Mediterranean climate is not known for its harshness. Its narrow temperature range and moderate rainfall make it attractive to both humans and a wide variety of flora and fauna suited to the relatively easy life. But this climatic zone can also produce some regular difficulties, and the occasionally rigorous test. In these particular spots, sufficient rainfall and the proper soil profile can combine to provide a unique tableau called a vernal pool.
Vernal pools are ephemeral habitats that occur during years of decent rainfall in areas where the soil profile includes a clay pan or other impermeable layer. Unable to percolate deep into the soil, standing water accumulates in relatively flat areas that feature shallow depressions. The pools can be from several feet across, to a number of acres in size. Vernal pools occur in a number of places around the world, commonly in Mediterranean climates, although not exclusively. California is a part of the world relatively rich with them, and seems to be the only place with a large collection of species entirely dependent on that habitat. The pools form in many parts of the state, especially along its coastal terraces, in the Central Valley, and in weathered volcanic areas. These settings frequently develop the clay soil layers that will hold rainwater in pools at the surface. Once the water is thus situated, it diminishes mainly by evaporation. With the end of the rainy season desiccation begins, and these sites become dry, mudcracked land, full of yellowed, dead vegetation, appearing to be entirely unremarkable. Because vernal pools feature both a flooded stage and longer dry periods that can extend for years in drought events, the plants that can live there must be especially adaptable. The group of plants that might grow in a close-by, better drained landscape cannot tolerate both the flooding and the dryness, leaving the space more available for specially adapted vernal pool plants and animals.
The pools develop largely as closed systems, fed by rainwater falling in the immediate vicinity rather than by sources flowing in from a distance. As such, nutrients necessary for plant growth are limited to what is already there on the surface. As plants grow in this water that is usually no more than few inches deep, they must be able to deal with the variable water temperatures and pH levels throughout the day and night, affected by sun intensity and photosynthetic activity. Those qualities of active-living flexibility impress, but are nothing when compared to what these flora must endure once spring fades into summer and green turns to brown. Seeds and spores, once produced, must find a safe home under the detritus of the dying vegetation, or partially buried in the dried mud. There they may sit for not only the duration of one summer, fall, and into winter, but perhaps years of bleak dryness. And yet they persist, and when the rain comes again, these stored reproductive entitites, collectively known as propagules, spring to life to create another round of inundated greenery.
This process, while remarkable for the plant life, is astonishing when considering that an ecosystem of animals also goes through a similar cycle. Amphibians such as the California tree frog, western spadefoot toad, and various salamanders use vernal pools as temporary safe havens in which to lay their eggs. Their tadpoles must make quick progress to grow legs and be ready for a terrestrial adult life in the time a vernal pool exists. Even more remarkable are the fairy shrimp. Having been around for some 400 million years, they are considered one of the oldest crustacean groups, and their lifecycle is astounding. As their eggs, or cysts, come to life with the winter rains, a number of different fairy shrimp species get to eating, growing, molting, and growing some more. Most are from a few millimeters to a few centimeters in length, and reach maturity in one to three weeks. Time is of the essence, for they must reproduce while there is still water available. This gives them only a few months at best, after which time they die in the dryness. But buried in the drying mud are the cysts of the next generation. They are prolific—in one East African vernal pool they were 9,600 to the square meter. Fairy shrimp need those numbers because they may have to endure both blistering heat and freezing cold not only for one round of seasons, but for years. Or decades. Maybe even centuries. Their cysts have been experimentally subjected to temperatures from well over 250 degrees to near absolute zero, and even sent into the vacuum of space…after which trials they hatched. Though fairy shrimp would seem to be forever tethered to one small location for generations, the cysts can actually secure transportation in the stomachs of birds who unknowingly scoop up the microscopic eggs while feeding. They can withstand digestive juices, and find themselves deposited in a distant vernal pool, there to wait and then bring a new population to a new location. Perhaps even more ingenious in the fairy shrimp life strategy is that not all the cysts burst into life with the onset of rain. Some stay dormant, which saves the species if the first rains are just a brief, weak showing. An insufficiently filled and short-lived vernal pool will see its overeager inhabitants fall short of reproducing. By holding back, the cysts that remain await possibly more abundant rainfall, averting doom for that pool’s population.
Other animals such as isopods, copepods, and various insects also inhabit the vernal pools, and their complex interdependencies and natural histories are full of other tales of amazing adaptation. Like so many of nature’s settings, the scene mixes incredible fragility and mind-boggling toughness and adaptability. Left to their own natural cycles, vernal pools would persist forever, marching on through both common yearly cycles and the harshest of climatic swings. Sadly, however, vernal pools cannot sustain themselves if their clay pan is broken. Once the soil is sufficiently disturbed, falling rain will simply sink into the deep earth as it does most everywhere. Due to rampant development, roadbuilding, agriculture, and overgrazing, California has lost 95 percent or more of its original vernal pool habitat, and such places are endangered in other places around the world. In fact, in the region of the Mediterranean Sea itself, vernal pool habitat is rare despite having the weather and soil types that should support them. Thousands of years of grazing may have wiped away such mini-environments there. The Mediterranean climate is where everyone wants to be—there’s just too much development in these zones, and it’s hard to drum up concern over tiny plants and animals that may only be visible for a very few of months the year, if that. Still, the fabulous adaptability of vernal pool biota should inspire us to preserve the natural wonder of their unique abilities.
Vernal pools can be found in many parts of California. The Benchmark California Road and Recreation Atlas can guide you all over the state with gorgeous shaded relief mapping. Get one from Maps.com.
caption: A San Diego County vernal pool soaked with winter rains. As the pool evaporates, a sequence of different plant species encircling the water will rise in turn. Many grow nowhere else.
source: Flickr: Joanna Gilkeson/USFWS (CC by 2.0 Generic)
caption: A ranger and biologist search for fairy shrimp, an animal that can survive the potentially long dry periods between the rains that fill the pools.
source: Flickr: Joanna Gilkeson/USFWS (CC by 2.0 Generic)
caption: A vernal pool depression in summer or drought is barely recognizable. Hard to believe that life is in there, just waiting for water.
source: Camp Pendleton USMC website: Lance Corporal Asia Sorenson (Public domain)
caption: Fairy shrimp swim on their backs. Ducks love them, but by living in vernal pools, they are safe from predation by fish. Habitat loss is their main worry—many species are threatened or endangered.
source: Wikimedia Commons: Heide Couch/USAF (Public domain)
caption: Various amphibians, like the rare tiger salamander, depend upon vernal pools as a place to lay their eggs, and provide a home for hatchlings to develop.
source: Travis Air Force Base website: Heide Couch/USAF (Public domain)