EndangerBus Feature: San Francisco Garter Snake

December 23, 2010

Red-legged frog
California red-legged frogs are a key food for the critically endangered San Francisco garter snake. Unfortunately, the frogs are themselves threatened by habitat loss and other problems. Photo: KQED Quest

Reptile expert Robert Stebbins calls the San Francisco garter snake “one of the most beautiful serpents in North America.” The patterns of color that dazzle biologists and snake lovers also serve as camouflage in its native habitat: the open marshes, stream banks, grasslands, and vernal pools of the San Francisco Peninsula.

Among the green, gold, and maroon shades of marsh and pond plants, the snake hunts mostly amphibian prey, especially Pacific tree frogs and California red-legged frogs (the latter are themselves a threatened species, though not because of the snakes).

The snakes follow the seasonal cycle of their food. From early spring through mid-summer, they live near the freshwater ponds where amphibians breed and mature. In late summer and fall they follow the amphibians to nearby uplands, where they hibernate in rodent burrows or dense grass, sometimes emerging on sunny days to bask. The San Francisco garter snake can swim, too, gliding across water with the same sinuous motion it uses on land.

The San Francisco garter snake’s known range is the San Francisco Peninsula from Lake Merced south to Año Nuevo. Today, fewer than a half-dozen breeding populations remain, some on unprotected land. It’s estimated that there are just 1,000 to 2,000 San Francisco garter snakes left in the wild.

The greatest threats to those snakes’ survival are development and destructive land management practices, which reduce not only the snake’s habitat but also the numbers of its red-legged frog prey.

Culverts and rip-rap on stream edges keep the animals from getting to the water, and road-building, housing, and chemical use contribute other stresses to these complex ecosystems. Both the federal government and the state of California listed the San Francisco garter snake as endangered in the late 1960s and early 1970s. Unfortunately, the snakes tend to live in open land with lovely views, places we humans also like to settle, and loss of contiguous habitat has isolated populations of snakes that used to intermingle.

The history of the snake’s habitat near Pacifica illustrates the challenges to recovery. With the help of the Pacifica Land Trust, the Coastal Conservancy, and the Trust for Public Land, Mori Point was added to the Golden Gate National Recreation Area in 2000. Staff and volunteers with the Golden Gate National Parks Conservancy created several protected ponds edged with native marsh plants, restored native grasslands near the ponds, and built trails and boardwalks to lower visitor impacts. Both frogs and snakes have moved into the new ponds at the 110-acre park and seem to be thriving there. In 2005 the San Francisco Zoo joined the effort with a public education program including classroom visits and on-site presentations with live San Francisco garter snakes to introduce citizens to these beautiful animals.

But Mori Point sits between the Sharp Park Golf course and the former Rockaway Quarry. The quarry owner’s proposed 87-acre luxury oceanfront development was thwarted by two public referenda, but the area could still be built over as the owner seeks another developer for the land.

Just north of Mori Point is more prime garter snake habitat at Sharp Park Golf Course, built on land donated to San Francisco in the early 1900s. Over the years, course construction, flooding, and pumping of the site’s natural lagoon have killed thousands of red-legged frog eggs and adults. The site’s San Francisco garter snake population has followed the frog in decline. In 2008 only two snakes could be found here. A coalition of more than 20 environmental and community groups is lobbying the city of San Francisco to close the golf course and partner with the National Park Service to restore habitat for the frog and snake, but a decision seems unlikely in the near future.

With only a few disconnected San Francisco garter snake populations left and their food sources in decline, the future of the species is tenuous at best. Learning about and protecting the snake’s remaining habitat and focusing human development away from these areas will go a long way toward sustaining the future of this most beautiful and vulnerable of North American serpents.

San Francisco garter snake by Natalie McNear, Flickr.
San Francisco garter snake, Natalie McNear, Flickr

Learn more about the San Francisco garter snake:

Bay Nature Feature: Mori’s Story

Bay Nature Video: Mori Point Restoration

Wild Equity Institute’s Restore Sharp Park campaign

Center for Biological Diversity on SF garter snakes

SF Weekly Feature: Bleeding Green at Sharp Park

San Francisco Zoo Garter Snake page

California Reptiles and Amphibians – San Francisco Garter Snakes

This article was part of Bay Nature’s 2011 EndangerBus project.

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