2 water snake species threaten to displace native serpents

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Two types of non-native water snakes have gained a foothold in the Sacramento region, and scientists fear their spread could imperil endangered native species.

A thriving population of the common water snake and the southern water snake now call the area home.

The more worrisome of the two is the common water snake, a species spread across the Eastern U.S. but rarely found west of the Rocky Mountains, said Jonathan Rose, a doctoral candidate in ecology at UC Davis. Rose is lead author of a new UC Davis study on the pervasiveness of the two invasive water snake species in the region.

The recently published study identified regions with climates suitable for growing water snake populations. It found water snakes are well suited to the habitat traditionally occupied by the giant garter snake — a species now on the state’s threatened species list.

The common water snake was first identified in the Sacramento area in 2007.

“The common water snake eats the same prey and has the same habitat as the giant garter snake,” Rose said.

The giant garter snake, unlike the more common garter snake, relies on water habitat. This brings it into direct competition with the invasive water snake for food. “It has never come into contact or had to compete with another snake like this,” Rose said.

The giant garter snake has disappeared from 98 percent of its former local habitat. Habitat loss remains the greatest threat to its survival.

Today, giant garter snakes exist in small scattered populations in the Sacramento Valley. The species depends on flooded rice fields and irrigation canals.

Like most invasive species, the common water snake reproduces rapidly. A large female can produce up to 50 offspring a year, Rose said.

The giant garter snake matures at a later stage, giving the common water snake an advantage in reproduction, he said.

Water snakes also pose a risk for the giant garter snake’s prey. These include many native fish and amphibians, including the red-legged frog — a species on the state’s endangered list, Rose said.

Common water snakes are now thriving in a stream near a walking and cycling trail near a local high school, said Brian Todd, professor with UC Davis’ department of wildlife, fish and conservation biology.

A second population was found recently closer to the school, Todd said.

The total population there is estimated at 300 snakes.

“These are dense populations,” Todd said. “The number of animals is greater than what is found for the same species on the East Coast.”

The southern water snake has been found in a series of lakes and ponds south of Lake Folsom. Those populations do not seem as robust as the common water snake population, Todd said.

He said researchers don’t know how the snakes got to either suburb. He suspects some of them might have been kept as pets and released into the wild. The common water snake and southern water snake are nonvenomous.

The state Department of Fish and Wildlife bars individuals from owning common water snakes as pets.

To give an idea of the damage the water snakes could cause, Todd cited the problems created by the Burmese python in the Everglades region of Florida. The species has become common there. Authorities are now trying to control its spread by allowing bounty hunting by certified biologists, Todd said.