The Abundance of Vernal Pools

9689by David Lukas, co-author of Sierra Nevada Natural History

Unless you are a soil scientist how often do you think about soil? Not often I’d guess, and you’d probably never imagine that California has an official “state soil”!

In fact, soil is the basis for all life on land. Deriving from local geology and weather conditions, soils are a complex and biologically rich mix of minerals and microorganisms that uniquely reflect each particular region in the state.

Of all the soil types in California, the oldest may be the San Joaquin soils of the Central Valley – California’s official and most famous soil type. Formed on alluvial plains left as Pleistocene seas retreated from the Central Valley, San Joaquin soils have a distinctive hummocky topography and an impervious hardpan just under the surface that limits root growth and restricts the percolation of water.

The result is a perfect example of how life adapts to soil. The hummocky surface collects water in small shallow pools during winter storms and the hardpan prevents the water from dissipating until it evaporates in the summer sun, creating what are called vernal pools.

While much of the Central Valley has been overwhelmed by invasive species, the unique vernal pool environment remains not only remarkably pristine but is also home for many rare and endemic species.

With the arrival of hard rains in December, vernal pools begin to fill and come to life. Threatened California tiger salamanders (Ambystoma californense) lumber out of rodent burrows and head for the nearest pools to breed. The delta green ground beetle, first described in 1878 then rediscovered in 1974, appears during the winter around the Jepson Praire Preserve and nowhere else in the world.

And most curious of all may be the highly adapted fairy shrimp that emerge from cysts buried in the mud. Looking like the 220 million year old crabs they are related to, these seemingly prehistoric creatures are restricted to specific vernal pools. The endangered conservancy fairy shrimp (Branchinecta conservatio), for instance, is found at only eight sites in the Central Valley.

Like all fairy shrimp, the giant fairy shrimp (B. gigas) which grows up to six inches long, is able to complete its entire life cycle in the short window of time before the shallow pools dry up – a necessary trait in this distinctive habitat.

Vernal pools, however, are best know for their flamboyant wildflower displays, especially for the way in which colorful flowers form concentric rings around each pool as it dries up. Wildflower enthusiasts come from far and wide to see species like white meadowfoam (Limnanthes alba), dwarf dowingia (Dowingia pusilla), and goldfields (Lasthenia californica), whose lives like the shrimp are fleeting and ephemeral.

Even if you seldom think about soils, it is not hard to imagine the impact that plows and agriculture have on these fragile ecosystems. California’s vernal pools have nearly all been plowed under or developed, and what is left is disappearing at a rapid clip.