A long-term study of desert tortoises in Joshua Tree National Park indicates that drought and climate change are behind a decline in their population, according to the U.S. Geological Survey.

The report, based on studies from 1978-2012 conducted in a square-mile study plot in the Sonoran Desert in the park, is one of only a few long-term published studies to examine desert tortoise populations and their response to climate change, USGS officials said in a release.

The study's results support earlier predictions that parts of the Sonoran Desert area may become unsuitable for continued tortoise survival, based on previously published climate models projecting warmer and dryer conditions in the future, the release added.

The study revolved around a population of threatened Agassiz's desert tortoises. It spanned about 1.4 tortoise generations, which is important when studying species with long life spans. The full report, published in the journal Biological Conservation, is available online.

"If drought duration increases under a warming climate scenario, our results suggest there could be wider and more significant impacts on Agassiz's desert tortoise populations in the lower elevation areas of the Sonoran Desert in California," said USGS scientist Jeff Lovich. "This information should be useful to resource managers as they address drought effects on tortoises."

The USGS study found that desert tortoise survival rates are highly dependent on climate events, particularly the duration of droughts. Results show that after three years of drought, the tortoise population evaluated in this study decreased, the USGS reported.

"Estimated adult population size declined greatly from 1996-2012, which was concurrent with persistent drought in the area. The postures and positions of a majority of dead tortoises found in 2012 were consistent with death by dehydration and starvation. Dry conditions also result in decreased tortoise reproduction at the study site," the release said.

Mr. Lovich said that some live and many dead tortoises found in 2012 showed signs of predation or scavenging by mammalian carnivores. During periods of drought when germination of annual food plants fails or is weak, small mammal populations that feed on these plants decline and carnivores appear to shift their diet to tortoises, the USGS said. Scientists found evidence of this prey switching by desert carnivores, such as coyotes.

The Agassiz's desert tortoise is protected under the U.S. Endangered Species Act as threatened in portions of Arizona, California, Nevada, and Utah where populations appear to be declining rangewide, with few possible exceptions. This study was funded by the U.S. Bureau of Land Management with support from the Sonoran Desert Rapid Ecological Assessment program to better understand the role of climate and tortoise ecology in the region.