Drought conditions are linked to declines in a protected desert tortoise population in part of the Sonoran Desert, according to a U.S. Geological Survey study.
The new report is one of only a few long-term published studies to examine desert tortoise populations and their response to climate change. 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.
Scientists surveyed a population of threatened Agassiz's desert tortoises from 1978 to 2012 on a square-mile study plot in the Sonoran Desert of Joshua Tree National Park, Calif. This long-term study includes 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."
Desert tortoise survival rates were found to be 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. 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.
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. 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 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.