California Department of Fish & Game

The giant garter snake, once plentiful in the Central Valley, now is on the state's threatened species list and is experiencing new pressure from water snakes making California inroads.

Two water snake species establish colonies in suburban Sacramento, threaten to displace native snakes

Published: Thursday, Jun. 26, 2014 - 10:23 pm
Last Modified: Friday, Jun. 27, 2014 - 11:02 am

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 species exists in Roseville, and the southern water snake now calls Folsom 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 study, published Wednesday in the journal PLOS ONE, 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 once plentiful in the Central Valley but now on the state’s threatened species list.

The common water snake was first identified in Roseville 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 habitat in the San Joaquin Valley. Habitat loss remains the greatest threat to its survival.

Today, giant garter snakes exist in small scattered populations in the Sacramento Valley, including the Natomas basin. 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 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 not far from Woodcreek 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 high 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.”

In Folsom, 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.

A 2011 study found that the python may have caused a serious decline in some mammal populations in Florida’s Everglades National Park.

“Numbers of those species have declined to almost nothing since the establishment of the Burmese python,” said Todd.

Fortunately, Todd said, the invasion of both water snake species in the Sacramento region is thought to be in its infancy. Still, it’s important to take action quickly.

“Often it’s too late before we recognize what’s happened,” he said. “We want to do something before it becomes a significant issue and nothing can be done.”

The next step, he said, is using DNA technology to sample streams, lakes and other bodies of water to establish exactly how widespread the two invasive snake species have become.

Call The Bee’s Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.

Read more articles by Edward Ortiz

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