The Species

The pick-up truck comes to a screeching halt. Biologist Michelle Thompson steps out and walks toward a pond situated within the coastal chaparral Californians are so used to. She is alone, seemingly for miles. Yet, just over the hills lies Silicon Valley and the rest of the San Francisco Bay Area. This small protected land is one of the last refuges of the endangered San Francisco garter snake and it serves as Thompson’s study site.

This subspecies of the Common Garter Snake prefers a habitat of densely vegetated ponds with some minor upland components. Cattails, spike rushes, and bulrushes surrounding the pond edges are used for cover. The snakes use upland components for estivation during the dry summer months when they crawl into rodent burrows. San Francisco garter snakes spend the majority of their time, however, at secluded ponds where they hunt their primary prey, the threatened California Red-legged Frog.

It’s tough to persist when your principal food source is also low in numbers. So why are both of these species threatened by extinction? The answer is simple. Their wetland habitat is lost due to urban, commercial, and agricultural development. On top of that, when there is water available, they often fall prey to the much bigger bullfrog, an invasive species introduced to the west coast by humans.


“Luckily, no bullfrogs are found at these ponds”, Thompson states, now arriving at one of her drift fence funnel traps. These are basic set-ups, consisting of a solid board fence hammered into the ground, at the end of which are small cages. When a snake encounters these barriers, it moves along them and is subsequently funneled into the trap. Thompson opens the lid. This one, however, is empty. She checks the twenty-four drift fences located on this 224 hectare reserve twice per day. This ensures that the snakes are not in the traps for too long.

Ten more traps, still nothing. Then, finally, Michelle pulls out a sub-adult San Francisco garter snake. It becomes immediately apparent why snake lovers call this species the most beautiful in North America. The red and turquoise stripes along its body are saturated and the black stripe running between them provides a stark contrast. One might think that these colors indicate that the snakes is incredibly venomous, yet it is completely harmless.

Now the real work begins for Thompson. She takes the snake’s measurements, weighs it, and sexes it. This one is a healthy looking female. Thompson first checks if the snake has been trapped before. By tagging all of the individuals she catches with PIT tags, basically giving each snake a unique barcode, can she determine abundance and survival rates. This female was tagged only a few months ago, but has grown in size since then. A good sign.

This research has been going on for two years and it appears that this population of San Francisco garter snakes is slightly increasing in numbers. In fact this area may be the stronghold for the whole population. Since this species is indeed only found in San Mateo County in California, not much suitable habitat remains.


Luckily there are a couple of organizations that are concerning themselves with the well-being of this striking reptile. The Peninsula Open Space Trust (POST) is actively managing land and purchasing more acreage for the San Francisco garter snake. Their goal is to maintain sufficient habitat for the species — and the unique ecosystem of which it is an integral part — to survive. Additionally, the San Francisco Zoo is conducting a captive breeding program with the goal of hopefully releasing some snakes back into the wild.

The future of the San Francisco garter snake is unclear, with less than an estimated 5000 individuals left. Only with the increased knowledge gained by research like Thompson’s can organizations and local government make informed decisions that can help the survival of this species. The general public can also help by promoting land management practices that maintain or create marshes and ponds (or build their own). They can minimize the use of pesticide, herbicide, and fertilizer by buying organic foods when possible. And finally, by properly disposing of garbage to help control raccoon populations. “We can all play a part in helping these guys”, says Thompson.