VERNAL POOLS

Liquid Sapphires of the Chaparral

One of the flash points in efforts to preserve threatened natural resources has been the small, seasonal collections of water called vernal pools. Because of their homely appearance when dry, they provide an excellent example of how the importance of becoming familiar with native habitats and efforts to preserve them can collide violently with the desire to develop property by individuals unfamiliar with their value.
Vernal pools in San Diego County can be found between stands of chamise chaparral on flat mesa tops. Decorated with round cobbles from an ancient river, these unique habitats are unlike any other vernal pools in the world.
To the untrained eye, southern California's vernal pools are easily overlooked. They can be anywhere from the size of a car tire to a football field. In San Diego County they are rarely more than six inches deep when full. For most of the year, they appear as lifeless bare spots surrounded by chaparral or disturbed coastal sage scrub. During prolonged droughts, their desiccated condition can last several years. When sufficient rainfall does occur and their shallow basins fill, the pools seem naked, out of place. Perched high on mesa tops without feeding rivulets, the water source seems mysterious. The little pools look more like remnants of ancient dinosaur wallows than dynamic, living habitats.
Vernal pools experience an extreme version of the same climatic pressures characteristic of chaparral: winter rains followed by summer drought. The only organisms found in vernal pools are those capable of withstanding both weeks of inundation and months of mummifying aridity. Truly aquatic plants are excluded because pools are not wet long enough. Surrounding terrestrial vegetation is held back because it cannot tolerate prolonged saturation. In San Diego County, water can remain in the pools for forty-five days or more. This has been particularly important for the habitat’s persistence of non-native weeds and grasses. These weeds’ intolerance to standing water prevents them from overwhelming pool areas as they have many other open spaces.
For most of the year, the few, remaining vernal pools in San Diego County are dry, cobblestone-filled low spots on mesa tops.
The same pool after the winter rains, now a vernal pool, inundated by water and filled with life.

An Ephemeral Oasis of Biodiversity

Each individual vernal pool has its own, special combination of plants and animals that can vary from year to year with changing environmental conditions. A prolific species one year may be absent the next. The vernal pools are ephemeral (seasonal), as are their patterns - for each spring holds a new surprise. If winter rains fill the pools, sometime in late April or early May standing water disappears. This is when the specialized vernal pool plants produce a flush of growth and transform into more terrestrial forms. Each plant species place in the vernal pool assemblage is determined by its own adaptive tolerance to dehydration, and by its ability to obtain much-needed moisture through its roots.
A close-up of the San Diego Mesa Mint (Pogogyne abramsii), a state and federally listed Endangered plant species.
One of the most interesting plant species of Southern California's vernal pools is the state and federally Endangered San Diego mesa mint (Pogogyne abramsii). When in bloom, this tiny plant bursts forth with dozens of violet, trumpet-shaped flowers, highlighting the vernal pool beds in an amethyst-like fog. Its sparkling, hairy leaves add to the pageantry by filling the air with a rich, minty fragrance that's noticeable from hundreds of feet away. This display, though small in size, is truly one of the best-kept secrets of the chaparral. While thousands of people travel elsewhere to view springtime flower displays, mesa mint and other vernal pool wildflowers quietly celebrate their ever-changing beauty right in our backyard.
The pool’s animal life is equally impressive, and is represented by the Western spadefoot toad (Spea hammondii) and their favorite food, the federally-Endangered San Diego fairy shrimp (Branchinecta sandiegonensis).
Spadefoot toad tadpoles - notice how the specimen on the left displays albino traits, whereas the one on the right does not.
Downingia cuspidata.
A cat-eyed, adult Western spadefoot toad (Spea hammondii) at the edge of a vernal pool. These toads spend the dry season underground, in burrows. Photo: Bill Howell.
A group of San Diego fairy shrimp (Branchinecta sandiegonensis) swimming gracefully in the waters of a vernal pool. These tiny, yet extraordinary, specimens are federally-listed as Endangered.
San Diego coyote thistle or button-celery (Eryngium aristulatum var.parishii).
Dr. Edith Purer, of San Diego, CA, may be remembered as the pioneer of vernal pool research and preservation in southern California. (c. 1948).

To defend, or to destroy?

Throughout the early 1900s, San Diego County's vernal pools were largely ignored, if not forgotten. The only attention they received was from thirsty livestock and ranchers who filled them in with dirt while attempting to level their land. Edith A. Purer presented one of the first scientific papers fully describing the habitat to the Ecological Society of America in 1937. A science teacher at San Diego’s Hoover High School, Purer spent her summers studying the county’s natural history and became California’s first female professional ecologist. She was also the consummate citizen naturalist of the region, an early advocate for the California State Parks System, and was also known as a painter of natural landscapes.
Purer’s survey of San Diego’s Linda Vista Mesa described “thousands of pools filling the small depressions of the mesa, intercepted throughout by low, rounded hummocks" (mima mounds). The key words here are “thousands of pools.” Within forty years of Purer’s study none remained, having been filled in and covered over by the burgeoning growth of an expanding city. The remaining collections in the entire county would have disappeared as well if the scientific and environmental communities had not belatedly rediscovered them in the late 1970’s and publicly revealed the treasure trove of specialized life forms living there. Half the plant species growing within California’s vernal pools are found nowhere else on earth. This compares to 24% of all California plants being endemic, a remarkably large number itself. In a sudden explosion of interest, vernal pools were regarded as deserving protection and endemic species were listed as endangered or sensitive. Unfortunately, a few developers did what they could to avoid the new restrictions by bulldozing pools on their land before the laws took effect. Pools continue to be destroyed today despite their legal protection. The few remaining represent a tiny fragment of a once large network of ephemeral wetlands punctuating the chaparral like liquid sapphires.

Before development there were an estimated 28,500 acres of vernal pool habitat in San Diego County. Mesa tops, like the one where San Diego State University now rests (aerial photos below), were covered with so many pools that aerial photographs taken back in 1928 look like carpets textured with thousands of tiny, evenly spaced dots. Those are all gone now. When the county was last inventoried in 1986, only 7% of the original vernal pool habitat remained. Fewer than 2,400 pools existed in 2001. Of those surviving, some are temporarily protected in restricted areas like the Miramar Air Station or Camp Pendleton, but their futures are still uncertain; others remain vulnerable because they exist on private land.

In December 1999, owners of a protected vernal pool site off Arjons Drive, north of downtown San Diego in Mira Mesa, bulldozed a significant portion of the parcel, scrapping off native vegetation and filling in fragile pool basins. Destruction of a protected vernal pool site is a violation of state and federal Endangered Species Acts and would ordinarily be a clear signal for prosecution. However, in this situation, the City of San Diego issued a grading permit without first checking their files and completing a proper investigation. In response, the U.S. Fish and Wildlife Service and the California Department of Fish and Game coauthored a letter informing the city it violated state and federal regulations in addition to San Diego’s own municipal code for issuing permits. The pools had been protected in the 1980’s by an agreement approved by the Fish and Wildlife Service with the land’s former owner and that information was communicated during the sale via a signed letter from both real estate brokers involved. Responsible parties claimed ignorance and Michael Cafagna, co-owner of the property, denied any wrongdoing. In late 2002, San Diego County prosecutors quietly dropped the case.

“We looked carefully at all the players involved, looked at the liability and decided it didn’t meet the standard,” said Deputy District Attorney Karen Doty. “It didn’t rise to the level of prosecution and provable evidence.”

Even under the protection of government sanctioned conservation initiatives, protection of sensitive habitats is not guaranteed. Neither federal nor state Endangered Species Acts provide complete protection of species listed as endangered. Developers are frequently granted permits for the “taking” or killing of protected species. In an attempt to turn over the stewardship of endangered species and habitat preservation to local governments, Habitat Conservation Plans have been implemented. For example, the city of San Diego approved their Multiple Species Conservation Program (MSCP) in 1997, promoted it as a rational way to balance development with the preservation of the area’s remaining wild habitats.

As with many compromises involving competing interests, the MSCP has a significant number of open-ended definitions allowing agencies tremendous flexibility. Specifically, the section applying to vernal pools allows for continued destruction as long as everyone tries to minimize impacts. Section 3.3.3 states, “For vernal pools and narrow endemic species, the jurisdictions and other participants will specify measures in their subarea plans to ensure that impacts to these resources are avoided to the maximum extent possible” (MSCP 1998). The section is open to interpretation as it allows agencies to define the meaning of “maximum extent.” Shortly after the MSCP was approved, a major development was authorized in San Diego on Mira Mesa Blvd, next to Interstate 15. On site were 67 endangered vernal pools recognized by the plan as deserving protection. The developers, however, indicated they could not profitably build unless they were able to use the entire parcel. As a result, “maximum extent” was translated into a compromise favoring development over preservation. The consequence was that one vernal pool was salvaged when the apartment complex was built in the late 1990s. Today, you can view the remaining pool at the end of a cul-de-sac on Hillery Drive. Under the crowded gaze of multistory apartments, surrounded by black asphalt stands a stylish rod iron fence, enclosing the sole remnant of a once dynamic vernal pool complex. The sixty by forty-foot preserve is marked with a plaque and three park benches. The apartment complex that now covers the site where the vernal pool complex once existed is called “Legacy.”
Pictured above is the last remaining vernal pool of the 67 total pools that were destroyed by the development of the "Legacy" Apartment Complex, San Diego.
The monument marking the previous site of a lost Legacy.

The Loss of Vernal Pools

Aerial photograph taken in 1928 of the area now occupied by San Diego State University. The tiny dots are little round mounds (mima mounds), in-between which are vernal pools.
Same view today. San Diego State covers the exact location of the former cluster of vernal pools in the upper left-hand corner.
For more information on vernal pools and their preservation in southern California, check out the Chaparral Lands Conservancy website, which may be accessed by clicking their logo.