This subspecies of the common garter snake is found in scattered wetland areas on the San Francisco Peninsula from approximately the northern boundary of San Mateo County south along the eastern and western bases of the Santa Cruz Mountains, at least to the Upper Crystal Springs Reservoir, and along the Pacific coast south to Año Nuevo Point, and thence to Waddell Creek in Santa Cruz County. It is difficult to obtain reliable distribution information and population statistics for the San Francisco garter snake, because of the elusive nature of this reptile and the fact that much of the remaining suitable habitat is located on private property that has not been surveyed for the presence of the snake. This subspecies is extremely shy, difficult to locate and capture, and quick to flee to water or cover when disturbed. The U.S. Fish and Wildlife Service has stated that many locations that previously had healthy populations of garter snakes are now in decline due to land development pressure and the filling of wetlands in San Mateo County over the last sixty years. However, in many areas where it still occurs it is not rare, and is actually quite common and can be viewed with good success once its behavior is understood.
The snake’s preferred habitat is a densely vegetated pond near an open hillside where it can sun, feed, and find cover in rodent burrows; however, markedly less suitable habitat can be successfully used. Temporary ponds and other seasonal freshwater bodies are also appropriate. This subspecies avoids brackish marsh areas because its preferred prey, the California red-legged frog (Rana draytonii), cannot survive in saline water. Emergent and bankside vegetation such as cattails (Typha spp.), bulrushes (Scirpus spp.), and spike rushes (Juncus spp. and Eleocharis spp.) apparently are preferred and used for cover. The zone between stream and pond habitats and grasslands or bank sides is characteristically utilized for basking, while nearby dense vegetation or water often provide escape cover. The subspecies occasionally uses floating algal or rush mats, when available.
San Francisco garter snakes forage extensively in aquatic habitats. Adult snakes feed primarily on California red-legged frogs (Rana draytonii), which are federally listed as threatened. They may also feed on juvenile bullfrogs (Rana catesbeiana), but they are unable to consume adults; in fact, adult bullfrogs prey on juvenile garter snakes, and may be a contributing factor in the population decline of the San Francisco garter snake. Newborn and juvenile San Francisco garter snakes depend heavily upon Pacific treefrogs (Hyla regilla) as prey. If newly metamorphosed Pacific treefrogs are not available, the young garter snakes may not survive. San Francisco garter snakes are one of the few animals capable of ingesting the toxic California newt (Taricha torosa) without incurring sickness or death.
