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, they provide an excellent example of how the importance of becoming familiar with native habitats and efforts to preserve them can violently collide with the desire to develop property by individuals unfamiliar with their value.
Although typically located in flat grassy areas, 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.
A vernal pool in San Diego County.
To the untrained eye, Southern California 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. In fact, they are relatively unattractive to the average individual unfamiliar with them. 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 the 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 since the invasion of exotic weeds. These weeds’ intolerance to standing water prevents them from overwhelming pool areas as they have many other open spaces.
San Diego Mesa Mint.
Each pool has its own, special combination of plants; changing from year to year as environmental conditions vary. A prolific species one year may be absent the next. The pools are ephemeral, as are their patterns. 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. Each species’ place in the vernal pool assemblage is determined by its own adaptive tolerance to dehydration and ability to obtain required moisture through its roots. One of the most interesting is the diminutive, San Diego mesa mint (Pogogyne abramsii). When in bloom, this tiny plant bursts forth with dozens of snapdragon-like flowers, highlighting the pool bed with what seems like a light purple fog. Its sparkling leaves add to the pageantry by filling the air with a rich, minty fragrance noticeable hundreds of feet away, not unlike breaking open a dinner mint. This display is truly one of the best-kept secrets of the chaparral. While thousands travel elsewhere to view springtime flower displays, mesa mint and other vernal pool wildflowers quietly celebrate their ever-changing beauty, each pool displaying a different assortment of color and fragrance. The pool’s animal life is equally impressive with the most obvious representatives being spadefoot toads (Spea hammondii) and their favorite food, the San Diego fairy shrimp (Branchinecta sandiegonensis).
Spadefoot toad tadpoles. Notice the albino form on the left.
Adult spadefoot toad. Notice the cat-like eyes. These toads spend the dry season in burrows. Photo by Bill Howell.
Edith Purer 1948
San Diego County vernal pools were largely ignored in the early 1900’s. 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 San Diego’s first female professional ecologist. She was also the consummate citizen naturalist.
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." 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, 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 were 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 the 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. Today, you can view the remaining pool at the end of a cul-de-sac on Hillery Drive. Under the crowded gaze of multistory homes, 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.”
The last of 67 vernal pools behind an iron fence.
The monument marking the site of a different kind of Legacy.
Additional information concerning the preservation of vernal pools in Southern California can be found on the