Science Spotlight

CDFW scientists conducted a groundbreaking survey of lizards across the entire Mojave Desert. CDFW graphic.

One might say that a groundbreaking new study conducted by two CDFW scientists and their research partners provides a leap forward in lizard research.

Dr. Brett Furnas, a senior environmental scientist with CDFW’s Wildlife Investigations Lab, is the lead author on a paper entitled Hierarchical distance sampling to estimate population sizes of common lizards across a desert ecoregion. The co-authors on this paper are Scott Newton and Griffin Capehart, both formerly contractors for the Wildlife Branch at CDFW, and Dr. Cameron Barrows with the Center for Conservation Biology at the University of California, Riverside.

According to Furnas, monitoring programs that survey many wildlife species at the same time across large geographic regions are important for informing conservation decisions, but reptiles are often missing from these efforts because they are difficult to survey. Therefore, the researchers applied a new distance-sampling approach to more accurately count lizards across the entire Mojave Desert within California (25,803 square miles in total).

“As far as we know, this is the first time a lizard population has been accurately enumerated over such a large area of desert,” said Furnas.

Visual surveys of lizards were conducted in 2016 along quarter-mile long transects at 229 widely dispersed sites throughout the desert. The surveys were repeated several times during the same month, which allowed CDFW scientists to correct for lizards that were missed during any one visit due to hot weather and other factors. The researchers validated their results by comparing them against a different set of surveys conducted by Barrows over a much smaller area at Joshua Tree National Park.

After using advanced statistical models to extrapolate survey results across the entire desert, CDFW estimated 82 million lizards for the three most common species of lizards across an area amounting to 16 percent of the total land area of the state. The population numbers reflect an average of 3,170 lizards per square mile.

“Having a good measure of population size for any species is important because it allows us to make more effective conservation decisions when we know how abundant a species is, what habitats is uses, and whether it is increasing or declining in numbers,” Furnas said. “These are often the first questions decision-makers want answers to.”

This is especially important in the deserts of California and the Southwestern United States, which are already experiencing severe increases in temperature and reductions in rainfall due to climate change. There is concern that these increases in temperature may already be exceeding the physiological limits of some lizard species, thereby increasing their risk of extinction. 

“Lizards and other reptiles are particularly sensitive to temperature, in part because they are ‘ectothermic,’” explained Furnas. “Unlike mammals, reptiles cannot use their metabolism to regulate body temperature; instead they may need to take shelter on very hot days, which may limit the time they can spend foraging for food.”

In addition to demonstrating the value of a new method for monitoring reptiles, the study was able to map the distribution of lizards throughout the Mojave Desert and show how population levels and the behavior of lizards vary with differences in vegetation cover, human land use, and temperature. Of the three species studied, Western Whiptail (Aspidoscelis tigris) and Common-side Blotched Lizard (Uta stansburiana) were more abundant in cooler places found at higher elevations or where there was greater vegetation cover. On the other hand, Zebra-tailed Lizard (Callisaurus draconoides) was more tolerant of high temperatures, but was the most sensitive to human development and disturbance.
CDFW Photo and Graphic. Top Photo: This common side-blotched lizard (Uta stansburiana) was surveyed at Joshua Tree National Park. CDFW Photo by Cameron Barrow.

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Media Contacts:
Kyle Orr, CDFW Communications, (916) 322-8958
Brett Furnas, CDFW Wildlife Investigations Laboratory, (530) 227-3998  

Does the Western pond turtle (Actinemys marmorata), a freshwater species native to the Pacific Coast, hold secrets to survive climate change and adapt to rising sea levels? CDFW biologists want to know and have partnered with UC Davis and the Department of Water Resources to conduct a long-term study in Solano County’s Suisun Marsh to better understand the aquatic reptiles.

Officially, the Western pond turtle is a Species of Special Concern in California because of declining populations brought about by habitat loss, degradation and competition from that pet store favorite – the non-native, red-eared slider. The pet slider turtles are often released into the environment by their owners after outgrowing or outliving their welcome. They also outgrow and out-compete the medium-sized western pond turtles for food and critical basking spots. Western pond turtle populations are faring even worse in Oregon and Washington.

And yet in the Suisun Marsh, with its brackish water and high salinity, the Western pond turtle appears to be thriving. The Suisun Marsh, ironically, may now be home to one of the strongest populations of Western pond turtles on the West Coast.

“It’s just a really unique population in a place where we didn’t expect to see a freshwater species,” said Mickey Agha, the UC Davis Ph.D. student leading the university’s turtle research with Dr. Brian Todd, an associate professor in the UC Davis Department of Wildlife, Fish and Conservation Biology.

As if to underscore the point, researchers this summer collected a turtle with a barnacle attached to its shell – a testament to the marine-like environment to which the Suisun Marsh turtles have adapted.

Researchers also have been impressed with the age, health and size of the individual turtles. At 1 ½ to 2 pounds and with an upper shell that stretches up to 8 inches in length, researchers are discovering some of the largest Western pond turtles ever recorded in California.

“Looking at the ones we’ve collected, we’re seeing a lot of healthy turtles in good body condition,” said Environmental Scientist Melissa Riley, who is leading CDFW’s efforts.

The research began in the summer of 2016 with scientists trying to get a basic sense of turtle population numbers. The turtles are trapped in baited, floating hoop nets, their size, weight and age recorded. Before being released, each turtle is marked by filing a unique pattern of small notches along the edges of the upper shell. More than 125 turtles have been recorded in the project’s database.

Turtle trapping is taking place on three sites at the Suisun Marsh in and around the Grizzly Island Wildlife Area. Biologists are particularly interested in turtles at the Hill Slough Wildlife Area along Grizzly Island Road as 500 acres there will soon be restored to tidal marshland. Biologists plan to affix tiny, solar-powered, GPS tracking devices to some of the turtles to study their movements and see how they respond to the increasingly saltwater environment at Hill Slough and other parts of the marsh.

“That’s one of the many questions we have,” Agha said. “If sea level rise occurs, what happens to these turtles?”