In recent years, California's Agassiz's desert tortoise population has been decimated by shootings, residential and commercial development, vehicle traffic, respiratory disease and predation by ravens, dogs and coyotes.
Now, dwindling populations of the reptiles with scruffy carapaces and skin as tough as rhino hide are facing an even greater threat: longer droughts spurred by climate change in their Sonoran Desert kingdom of arroyos and burrows, according to a new U.S. Geological Survey study.
Drought conditions are linked to declines in a population of desert tortoises in a square-mile study plot in Joshua Tree National Park, according to the study published in the online journal Biological Conservation.
The study, one of the few to examine a desert tortoise population's response to climate change, surveyed about 1.4 generations of the species scientists know as Gopherus agassizii.
"The last time the climate of the Earth jumped as rapidly as it seems to be now was about 55 million years ago — and that was a five-degree increase over thousands of years," Jeff Lovich, lead researcher of the USGS team, said in an interview. "The changes we are seeing now are virtually unprecedented, and they are occurring in a desert landscape fragmented by development and roads."
"The desert tortoise is surviving in the current landscape by its toenails," he said.
"Although the animal's name suggests that it is well adapted to desert conditions, it is not," Lovich said. "Prior to 7,000 to 10,000 years ago, the region was cooler and wetter with lakes fringed with Joshua trees and junipers. That's the landscape that dominated the evolutionary history of the so-called desert tortoise."
"It probably evolved its burrowing trait to escape predators," he said. "Later, burrows became a critical means of escaping the climate extremes."
"So, this animal has 'accepted' — not adapted to — desert conditions," he said. "Our study shows that its survival can be seriously compromised after two to three years of drought."
Desert tortoise survival rates in the study plot were found to be highly dependent on climate events, particularly the duration of droughts. For example, the study said, the adult population of about 175 to 200 tortoises declined to about 25 tortoises from 1996 to 2012, a period concurrent with drought conditions in the area.
The postures and positions of a majority of dead tortoises found in 2012, it said, were consistent with death by dehydration and starvation.
Some live and many dead tortoises found in 2012 showed signs of predation or scavenging by carnivores.
The problem, Lovich said, may be linked to drought conditions, which killed off annual plants and triggered a crash in populations of rodents that eat them. As a result, coyotes, which normally thrive on kangaroo rats and rabbits, turned to the lumbering tortoises for sustenance.
The study acknowledges that the tortoise population survived through the 1960s and 1970s, which appeared to be relatively dry years, then increased dramatically during later periods of greater precipitation.
"However, if drought duration and frequency increase," it said, "they will likely have wider and more significant impacts on Agassiz's desert tortoise survivorship, particularly in the low Sonoran Desert portion of their range in California, and it will be difficult or impossible for resource managers to mitigate their effects."
The study of the desert tortoise, which is protected under the U.S. Endangered Species Act as threatened in portions of Arizona, California, Nevada and Utah, was funded by the U.S. Bureau of Land Management.