Pacific chorus frogs make urban comeback


  • A Pacific chorus frog, at home in an industrial drainage ditch.

    Rob Schell
 

As dusk fell one spring evening in 2003, a small group of volunteers crawled along a creek bank, searching among tall grasses, under piles of decaying garbage and in stagnant puddles for gelatinous clutches of eggs.

The Port of San Francisco was about to build a new bridge over Islais Creek Channel on the city's southern waterfront, and time had nearly run out for its Pacific chorus frog colony. Urban development and predation by invasive birds and fish had wiped out most of the city's frogs. Before backhoes carved away one of their last bits of habitat, volunteers spirited as many frogs as they could to an unassuming marshy pit tucked between warehouses, freeways and parking lots.

Nearly eight years later, this ad hoc sanctuary is teeming with frogs. The exact location is a carefully kept secret, to avoid visits from wildlife collectors and the curious, who might cause inadvertent harm or introduce disease. Only a handful of herpetologists, who prefer to remain anonymous, know just where to find the colony. They visit it regularly to monitor water quality, measure egg production and tend to native plants. For the most part, they work under the radar: without permits to collect and transplant frogs, and outside the auspices of any formal program or organization.

Why so much trouble for a few slimy amphibians? Although Pacific chorus frogs inhabit marshy puddles all along the West Coast, Islais Creek Channel is thought to have been the last refuge of one of San Francisco's oldest gene lines, so its frogs are particularly well suited to the city's peculiar microclimates, foliage, predators and prey. That means they can act as bio-indicators for local ecological restoration projects, connecting the aquatic and terrestrial food webs to reveal the health of an ecosystem with their presence -- or lack thereof.

For a few years, the new habitat was a lonely island in an industrial wasteland. As tractor-trailers rumbled past, a thin trickle of water from a nearby hillside was the frogs' primary lifeline.

But now, thanks to guerrilla preservation efforts and amphibian vigor, the frogs are on the brink of an unexpected comeback. Their camouflage and foraging abilities, as well as homing instincts that scientists have yet to fully understand, make them a particularly resilient species. And as the city has converted former industrial areas into green space, chorus frogs have been surprisingly adept at establishing pathways -- sometimes through gutters and sewers -- to new habitat.

Nobody knows, or is willing to acknowledge, whether they've had human help along the way. Regardless, the frogs' territories are expanding. So far, they've found their way into parks, marshes, even backyards. Finding them is easy: Come March, cartoon-like ribbits resonate on street corners in the Mission district alongside the sizzle of food carts and whine of electric buses.

Frogs are calling in the Presidio, too. The former military base at the foot of Golden Gate Bridge is now a national park, and it is undergoing a comprehensive ecological restoration. Although a recent wildlife survey failed to find proof that chorus frogs had moved in, Presidio biologist Mark Frey has heard them singing, and thinks it's only a matter of time.

"I think the long-term opportunity for the frogs to occur (in the Presidio) is because of the restoration, at least in part,"  Frey says.

Though much of the frogs' resurgence seems to be an accidental byproduct of open-space restoration, it's possible their return could have been even more widespread had it been part of a more robust, officially sanctioned effort.

"Amphibians are disappearing all over the planet, but they were able to survive here," says David Erickson, a neighborhood organizer who worked for years to restore the habitat around Islais Creek. "This is a miracle."