Santa Clara Valley Chapter
Vernal pools form in shallow depressions of variable size (from less than 1 meter square to over 60 ha) that fill with water during winter rains. Water accumulates below the soil surface and above a layer of hardpan or other impermeable substance (such as volcanic rock) in a "seasonally perched water table." Water is prevented from percolating underground thereby leaving evaporation the only means of escape.
The depressions in which vernal pools form commonly occur between small hillocks known as "mima mounds" which are typically populated with grassland species. How mima mounds were formed is largely unknown and a subject of great debate. Theories abound describing the shrinking and swelling of clays; ground surface subsidence; differential weathering of the soil surface; and loss of structure leading to collapse basins. One of the theories given most credence at present is the "pocket gopher hypothesis": In areas with shallow soil that is saturated with water part of the year, high points are more desirable habitat for tunneling rodents such as pocket gophers. As the pocket gophers excavate their burrows, they push the soil toward these high points making them higher, consequently making the low points lower. Over time this, or a combination of the above processes, may account for the mounded topography around which vernal pools form.
Vernal pools are little known and no distribution map is complete. Most of the known vernal pools in California are found in alluvial valleys like the Great Central Valley, coastal terraces of Southern California, and volcanic mudflows in Tehama and Riverside Counties. In the South Bay, vernal pools have been found in the grassland near the fringes of the saltmarsh (or historic saltmarsh that has since been developed or used for agriculture). In February of 1992, a 255 acre parcel of vernal pool/grassland habitat in Fremont was purchased by the United States Fish and Wildlife Service for inclusion in the San Francisco Bay National Wildlife Refuge.
Most of the year vernal pools don't look like anything special, but the closer one looks , the more there is to see. Small green rosettes of the Callitriche or starwort float on the water's surface accompanied by gooey green marble-sized balls of Nostoc filaments -- a nitrogen fixing blue green alga. Beneath the surface swim fairy shrimp, a freshwater counterpart to the familiar brine shrimp that inhabit salt ponds around the bay. On the bottom of the pool scoot tadpole shrimp resembling miniature horseshoe crabs. These organisms depend on the ephemeral nature of vernal pools in order to complete their life histories. They have adapted to a habitat that is inundated with water during winter and spring, and completely desiccated the rest of the year. Many of the plants germinate with the first winter rains, grow while the pool is full of water, then flower and set seed as the water evaporates, forming concentric rings of showy blossoms around the margins of the shrinking pool.
The invertebrates have also evolved unique adaptations to their temporary habitat. Both the fairy and tadpole shrimps "overwinter" as eggs during the dry period, hatching as the pool fills with rainwater. Fairy shrimp hatch out early in the season presumably avoiding predation by the later appearing aquatic insects. Swimming upside-down they feed on algae, bacteria and small protozoans. Tadpole shrimp swim along the bottom of the pool, preying on the fairy shrimp and other invertebrates; as the pool dries, their lifeless carapaces crunch like last years leaves.
Larger, more mobile organisms such as birds are common visitors to vernal pools. While not dependent on these small temporary wetlands for their existence, birds may have played an important role in the distribution of plant species. They are probably the main dispersal agent for propagules too large to be carried by wind. The same or similar species of plants found growing in temporary pools in California and temperate South America (and nowhere in between -- a phenomenon known as amphitropical disjunction) is probably due to long distance migrants sowing seeds.
Vegetation, by far the most conspicuous component of vernal pools, has received the most attention. Exotic species such as wild oats and foxtails may inhabit grassland areas between pools, but they are not found in the pools themselves. The stringent conditions of inundation and desiccation are inhospitable to all but specifically adapted native species. In fact over 40% of the plant species found in vernal pools are endemic (i.e., not found anywhere else).
Vernal pool habitat has declined significantly with increasing urbanization. It is estimated that less than 10% of the historical vernal pools remain in the Central Valley. The primary threat to vernal pool preservation is development, which is currently regulated by the United States Army Corps of Engineers. Under the present regulations, wetlands that are smaller than one acre may be filled without notifying the Army Corps, and the filling of wetlands that are between one acre and 10 acres may not even require public notice.
The increasingly disjunct distribution of vernal pools compounds preservation difficulties and has led to significant loss of habitat for endemic species. Five species of invertebrates (four of fairy shrimp and one of tadpole shrimp) are candidates for listing as endangered species, as are three species of plants. Two plant species, the Loch Lomond coyote thistle and the San Diego mesa mint, have already been granted endangered status. Mitigation for loss of habitat has been attempted, but never proven successful. These efforts have been directed toward preserving plants, and have not yet addressed the invertebrate species.
The characteristics that make vernal pools special also make them one of the most endangered ecosystems in California. Their small size and ephemeral nature ensure that vernal pools go unnoticed for part of the year. Because no two pools are alike, their increasing fragmentation leads to loss of unique habitat, perhaps even entire species. Greater public awareness is needed to preserve these islands in the California landscape.
The best time to visit vernal pools is in the late spring (May to June) depending on the amount of rainfall during the previous winter. There are two vernal pool sites in the South Bay that are readily accessible, yet both may be destroyed by development soon. One is on Dumbarton Circle in Fremont (off of Paseo Padre Parkway) near the San Francisco Bay National Wildlife Refuge headquarters. Last spring this area was disced and is the future home of yet another light- industrial complex. The second pool is in Alviso on the east side of Disk Drive (off of Nortech Parkway). The Disk Drive pool has played host to some uncommon spring migrants (i.e., Ruff and Lesser Golden Plover) as well as a spectacular display of Downingia pulchella in late spring. This site has been disturbed by agricultural activities and is slated to become the new youth center for a local church.
References
This article was reprinted with permission from the San Francisco Bay Bird Observatory. The San Francisco Bay Bird Observatory is a non-profit membership organization dedicated to the protection, understanding and enhancement of native avifauna through research and education. General membership is $20.00 annually. If you would like to become a member or learn about volunteer opportunities, write to Pat Carlson, SFBBO, PO Box 247, Alviso, CA 95002, or call (408)946-6548.
