A 35-year study of desert tortoises in Joshua Tree National Park underscores something that most desert biologists already strongly suspected: climate change will likely be a big problem for the southernmost populations of the desert tortoise.
The study, published in the January issue of the peer-reviewed journal Biological Conservation, tracked populations of the desert tortoise (Gopherus agassizii) on a square-mile plot in the Pinto Basin, in the eastern portion of Joshua Tree National Park.
The long-term study, covering the 35-year period from 1978 to 2012, showed that tortoises in the area declined sharply in numbers during drought periods. And as climate change is expected to bring longer, drier droughts to the desert, the results don't bode well for the long-term survival of the desert tortoise species.
The desert tortoise is protected as a Threatened species under the federal Endangered Species Act. According to the study, climate change may well make some parts of the Sonoran Desert unsuitable for desert tortoises in the future.
"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, the study's lead author.
Though desert tortoises are an iconic species for the California desert, the late tortoise biologist David Morafka hypothesized a decade ago that the reptiles aren't particularly well-adapted to desert conditions. Instead, suggested Morafka, the tortoise possessed adaptations to earlier environments that allowed the species to just barely survive in the desert.
Among those adaptations was the ability to store water for long periods. Desert tortoises can go for years without ever drinking liquid water as long as they have an annual flush of vegetation to eat for a few weeks. The tortoises get moisture from the plants they eat, and they can store the better part of a year's supply of water in their bladder, where they slowly absorb it as needed.
(That's why you should never bother a tortoise you might happen to meet in the desert: startling a tortoise can prompt it to empty its bladder, potentially dooming it to death from dehydration.)
When drought years mean sparse spring vegetation, tortoises will obviously have trouble stocking up on water for the rest of the year. And indeed, Lovich and his colleagues found that tortoises on the Pinto Basin study plot declined sharply in number in the latter part of the study period, as droughts intensified in the California section of the Sonoran Desert. Though survival among the plot's tortoises was good from 1978 until the late 1990s, the torts' numbers crashed in subsequent years -- and survivorship rates paralleled average precipitation in the area.
Compounding the pressure drought puts on tortoises is the fact that small mammals such as rodents and rabbits also rely on that same spring vegetation for food. If the spring salad and seed crop fails, fewer small mammals survive. That means that predators such as coyotes that usually eat small mammals turn to tortoises as an alternative food source.
All this was borne out in 2012, the final year of the study, which was a significant drought year in the park. In the words of the study's abstract:
The postures and positions of a majority of dead tortoises found in 2012 were consistent with death by dehydration and starvation. Some live and many dead tortoises found in 2012 showed signs of predation or scavenging by mammalian carnivores. Coyote (Canis latrans) scats and other evidence from the site confirmed their role as tortoise predators and scavengers.
Our fossil fuel use has committed the planet to a significant overall warming, so the southern end of the desert tortoise's range is likely to become inhospitable to the species in the next century. Ironically, the centerpiece of the Obama administration's effort to address climate change may prove the final injury to the species, as tortoise populations migrating northward to cooler, moister parts of the desert will find their migration corridors blocked by large desert renewable energy facilities.