Joint Study Looks Deep Into Fragile Ecology of City’s Canyons

At first glance, the Spruce Street canyon appears to be a thriving natural preserve in the heart of urban San Diego, with its stately eucalyptus trees, occasional skunks and raccoons, and songbirds.

But closer inspection reveals that the 10 1/2-acre canyon in Hillcrest is devoid of almost all native plants, bird and mammal species that were found there before development rimmed and shrank the canyon with homes and businesses beginning early in this century.

No more than 10% of the canyon’s native chaparral remains, and California quail, coyotes, and voles--among other chaparral animal species--have long ago disappeared. In their place have come ice plant, starlings, Norwegian rats and other animals that can coexist or thrive with human habitation.

What happened to Spruce Street canyon has taken place in numerous other urban canyons throughout San Diego, and is under way even in larger, suburban canyons in the north city area, which only recently has been subjected to development pressures. The speed at which native species disappear depends on the size of the canyon and remaining chaparral, and the number of years that urban growth has isolated the area.

“Despite how these canyons look, they are sick and dying habitats,” said Michael Soule, who is heading a joint UC San Diego-San Diego State University investigation of the city’s canyons. “They are not true natural habitats any longer but rather open space, just an area without homes.

“This is a serious study on consequences of the fragmentation of the area’s canyon system, something that has happened throughout coastal chaparral canyons in Southern California.”

The county’s Fish and Game Advisory Commission has lent $3,000 to the project.

For Soule, the project arose from a marriage between his professional training as a biologist and his memories of animal-watching as a child growing up around the canyons of San Diego’s Kensington and Point Loma neighborhoods.

“San Diego is unusual in having a series of canyons that branch much like a river system,” Soule said. The canyons once interconnected with each other and fed into various river bottoms, such as Chollas and Tecolote creeks and the San Diego River. Even as development isolated individual canyons from each other with houses, dirt fill and roads, urban planners noted with satisfaction that the canyons still provided open space and served to define one neighborhood from another.

But Soule suspected that they were becoming merely sterile vestiges of their natural diversity, based on theories concerning fragmented habitats developed by ecologists over the past 20 years.

The anecdotal memories of longtime canyon rim dwellers added to Soule’s suspicion. Residents told of jack rabbits, coyotes, road runners and other native chaparral species gradually becoming fewer and then completely disappearing. Road runners--so common in Florida Canyon through the 1960s--are no longer seen at all, Soule noted as one example.

Through surveys of residents and historical data, Soule and his colleagues drew up a list of nine native bird species and 10 native mammals that were known to live in undisturbed coastal chaparral canyons. They include: the California quail, the official state bird; road runner; cactus wren; wrentit; bobcat; deer mice; pocket mice, and coyotes.

The researchers then put a group of graduate and undergraduate UCSD students to work surveying 37 canyons, from half an acre to 200 acres in size and from two to 80 years in isolation. The canyons range from Auburn Canyon in East San Diego to Talbot Canyon in Point Loma to Pottery Canyon in La Jolla.

The students first trained with tapes to recognize the birds’ distinctive songs and then set out to listen for them. The students visited each canyon three or four times until certain that they knew whether or not the birds still lived there.

In many of the canyons long subjected to urban pressures, no more than two or three native birds species have been found. Those areas include the Spruce, Laurel and Talbot canyons, as well as Poinsettia Canyon in Loma Portal, and the 32nd Street and Zena canyons in the Chollas area. “They’re moribund,” Soule said. Other canyons only more recently isolated, including Alta La Jolla in the Muirlands area and Edison in Clairemont, still can support a majority of the bird species.

“The first to disappear are the quail, the road runners, and the California thrasher, either from predation or loss of breeding opportunities--those that are really intolerant to development,” Soule said. “Residents tell us that the canyons gradually get quieter and quieter as the wrentit, the black-gnatcatcher and others later go. The diversity gets less and less.”

The mammal survey involved students placing small traps in the canyons--100 traps at a time on several occasions--to try to catch brush rabbits, jack rabbits, wood rats and other small native animals. The traps are placed in late afternoon with any animals caught overnight released the next morning after being recorded on tally sheets. Larger animals are sighted either visually or through indirect evidence of dens or food remnants.

“It’s clear that a canyon begins to lose some species soon after development begins,” Soule said. “The rate depends on the the amount of habitat and the years of isolation. It’s just like the longer a lifeboat is on water, the fewer the people who remain.”

For example, native birds and mammals will continue to survive in Tecolote Canyon for another 100 years or more, Soule said, because of its tremendous size, its links to neighboring finger canyons and substantial remaining chaparral. But in smaller canyons, the wildlife may be gone in as little as 20 years.

In Syracuse Canyon, near Standley Park in University City, a street and chain link fence dividing it from the larger San Clemente Canyon have already resulted in a loss of road runners and quail. The fence has prevented coyotes from entering the small canyon and provided Soule with a prime example of what happens when coyotes disappear.

“In 17 canyons where we have found no coyotes, we have also found no quail and, in general, fewer numbers of bird species compared to (canyons still with) coyotes,” Soule said. “This suggests that coyotes have something to do with the maintenance of bird life.

“Our tentative hypothesis is that coyotes control the population of house cats (that get loose in canyons), since there are a large number. The cats can survive along with coyotes but once coyotes go due to loss of habitat, the cats are free (to go after birds) without any predators on them.

“Coyotes and other animals are poor dispersers. They cannot easily get from one island (habitat) to another if it means crossing roads and (housing tracts) in traveling.”

The destruction of chaparral and its replacement by palms, eucalyptus or ice plant hastens the loss of animal species, but has been popular because of the unwarranted bad name the native shrub has earned as a fire danger, Soule said. Rather than willy-nilly eliminating as much chaparral as possible, Soule believes the valuable water-holding ground cover can be maintained, together with buffers of more fire-resistant native plants to protect homes.

Soule sees the fragmentation process as particularly invidious because the deterioration of habitat proceeds almost invisibly.

“But now that we have identified it, people have to decide how far they want to go in sterilizing the environment,” he said. “I think the preservation of native wildlife and plants is a major part of what makes San Diego unique, and a desirable place to live, in giving joy to life here.

“Sure, there are compromises. You don’t want mountain lions and grizzly bears around. But you don’t want an all-concrete city with a dome on it, either. And if people want wildlife, then coyotes, for example, are essential to maintain it.”

There are strategies for slowing down canyon isolation, particularly in developing urban areas, Soule said. And some of the UCSD students are working with county land-use planners as the first step in getting information from the project into the hands of decision-makers.

“They are the bridge, from academia to developers,” Soule said. “And in areas that are just developing, it should be much easier to have a little extra design that could serve the function of protecting wildlife. It takes relatively little effort right at the start.”

Soule pointed to Rancho Penasquitos as one area where many opportunities still exist for developers to link canyons with corridors of chaparral to avoid canyon fragmentation.

“Corridors between canyons, maybe even just 50 to 100 feet wide, would be enough to keep contiguous open space and maintain a better, healthier habitat,” he said. “You already have the concept of corridors linking neighborhoods for bike and horse riding.”

Soule admits that older canyons cannot be helped in major ways, short of tearing down homes to provide natural corridors between individual canyons. But even in those canyons, exotic plants could be replaced by chaparral and native shrubs, and some animals could be reintroduced under a nature management program.