Posted by Janet Harriett on Mar.10, 2010

Photo Credit: Gary Stolz/ US Fish and Wildlife Service

Although garter snakes as a group are the most common reptile in the U.S., a few of the 13 recognized subspecies of garter snakes are endangered and require conservation plans to manage their populations. The most common pressure threatening garter snake populations is habitat loss. Snakes will live in close proximity to humans, but they need suitable habitat for their semi-aquatic lifestyle, which requires a sunny area for basking and a year-round wet area like a pond, marsh or stream edge. They also need prey, and some subspecies are more picky about dinner than others.

Living in close proximity to humans presents other survival dangers for threatened garter snake subspecies. Roads that warm up before the surrounding ground make good basking areas, until the snake becomes roadkill. Living in suburban yards puts snakes at risk of being run over by lawnmowers. Conversion of snake habitat to agricultural purposes may expose the snakes to agricultural chemicals or alter the surrounding wetlands to reduce prey. Rare garter snakes living in vacant urban lots are susceptible to trapping to be kept as pets, either by the person catching them or for sale on the illicit pet market. Trade in threatened and endangered snakes is for the most part prohibited, but collectors may flout the rules to acquire a rare specimen. While being kept as a pet may actually extend the individual snake’s lifespan, it removes the animal from the breeding population. The one snake may live, but at the expense of producing dozens of offspring. Particularly for these endangered garter snakes, keeping a diverse breeding population in a safe habitat is essential for their subspecies survival.

San Francisco Garter Snake

The San Francisco Garter Snake has been on the Endangered Species List since 1967. The snakes’ propensity to flee into water when disturbed makes an accurate count difficult, but ecologists estimate the population at only 1,000-2,000 snakes in California’s San Mateo and Santa Cruz counties, with habitat loss from development the major threat pressuring the population. Expanding agricultural and urban land use eats into the snakes’ native habitat.

San Francisco Garter Snakes prey preferentially on California red-legged frogs, which are themselves a registered threatened species, setting up a real-world example of the conservation quandary, “What do you do when one endangered species eats another?” Conservation plans for the frogs and snakes are largely independent of one another; however, both rely significantly on reducing habitat destruction, so what helps one species may help the other.

Butler’s Garter Snake

Butler’s Garter Snake is on the short side for a garter snake, usually less than two feet long, and ranges from central Ohio through Indiana, Michigan and Ontario, Canada. While their range extends farther than the San Francisco Garter Snake, the actual populations are isolated from each other within the range. Isolated populations lose genetic diversity and gradually become weaker.

Compounding the Butler’s garter snake’s habitat isolation, the individual snakes don’t move very far. Most are found within about 175 feet of water, in wetlands or fields that remain reliably moist. When a wetland gets cut off from adjacent swamps or streams by being drained or filled in for development, the snake population in that particular wetland will likely stay isolated.

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Posted under Living, Nature and Environment.