Vernal pools derive their name from the Latin vernus meaning spring. Perhaps this is the season when they were first noticed, with their showy wildflower displays. They are actually intermittent bodies of water; wet during a portion of the year but bone-dry during the balance of the year. Their sizes vary from only a meter or two in diameter to a few large vernal lakes exceeding 200 surface acres. But they all hold in common a wet and dry or “boom and bust” ecological economy.
They can form primarily in regions of Mediterranean climates which have cool, wet winters and hot, dry summers. While five major regions of the world share this climate type, only a few places produce “classic” vernal pools. Such pools are found in southern Oregon, in Baja California, Mexico, in the Cape region of South Africa, and, most spectacularly, in California.
While different geo-types occur, vernal pools share in common the inability to percolate water downward. Impervious layers of lava, hard-pan or bedrock hold the winter’s rainwater in the pool. Instead of trickling downward through the pool’s bottom, the pool’s water must evaporate upwards. Depending upon the weather this might take a few weeks or a few months. As the pool dries, imagine the changes that occur. Beginning cool and clear, the pool slowly becomes warmer and more turbid. The pool’s circumference shrinks over time, reducing its total volume. If you’re a growing animal, it gets crowded in there (and dirty too)! Earlier levels of oxygen are high, diminishing to low. Change after change in the pool creates a dynamic system unequalled in time or space anywhere else in California.
From what appeared to be a lifeless and desiccated in the summer and fall months emerges a veritable circus train of life. In orderly succession pioneer species are replaced by seral stages until, finally, climax ecosystems develop. Without food, none of the pool’s critters could survive, so the all-important algae spring forth from their dormant spores. The algal bloom is grazed by myriad zooplankton and other invertebrates. Microscopic crustaceans like copepods give way to larger relatives like seed shrimp, clam shrimp, fairy shrimp and even “monster” tadpole shrimp. Planaria and snails cruise the pool’s bottom and even the underside of the water surface for food. Amphibians make way to the pools to chorus and mate; and leave their fertilized eggs to hatch into thousands of tadpoles of tree frogs and toads. In some pools spadefoots emerge from their summer slumber in the dried mud below while tiger salamanders join in the short-lived game of life. Birds don’t miss the show either as they move in for the freshly-stocked grocery shelves of food. Stilts, avocets, ducks, killdeer, plovers, egrets, herons and other waterfowl abound.
Wildflower enthusiasts make regular pilgrimages to vernal pools to bask in artists’ palettes of color. Blooming in concentric rings (as the pool’s circumference shrinks) are meadow foams, popcorns, Downingias, goldfields, yellow-carpets, Johnny-tucks, and scores of botanical beauties. Many vernal pool endemics have evolved in such pools and some of California’s rarest plants are associated only with these unique waterholes.
Think of it! What was once absolutely lifeless will soon be teeming with complex food-webs! Strategies for surviving these ephemeral ecosystems include migration, hibernation, dormancy, seeds, spores, cysts and eggs. Tolerance of desiccation and outrageously high temperatures are often required of those who stay.
No wonder vernal pools are such biological treasures. They are strongholds of genetic information. They are discrete, complex, and highly evolved systems. They are essentially our very own “Galapagos Islands” only in reverse! What important secrets lie within these age-old biological libraries? Answers to our own survival? Tolerance to drought? To stress? Unfortunately they lie in the path of bulldozers; of development. Can we rescue the few remaining pools to guarantee ourselves a future of beauty and intrigue – and a future of answers?
Written by Joe Medeiros, Sierra College Professor of Botany