Persistent drought linked to local decline of desert tortoise

Agassiz's desert tortoise faces a difficult future under drought conditions
Agassiz’s desert tortoise faces a difficult future under drought conditions The population of the long-lived Agassiz’s desert tortoise is dwindling in the Sonoran Desert of California as their environment shifts to a warmer and dryer climate. 

Agassiz’s desert tortoise is listed as vulnerable under the IUCN Red List and has increasingly suffered in recent years from habitat destruction, respiratory diseases and human interference. Now a local population in the Sonoran Desert of California is facing an even greater challenge: changes in their environment due to climate change.

The tortoise is a remarkable creature which can live for over 50 years. During its long lifetime, it must endure a variable climate, including freezing winters and periods of drought. During hot years, it survives by escaping into burrows or rock shelters which help to keep it cool and reduces water loss.

But new research published in Biological Conservation sheds light for the first time on the difficulties that such a long-lived species can face if drought conditions continue over consecutive years. The authors followed the fortunes of a population of desert tortoises in an area of the Joshua Tree National Park in California for over three decades. Multiple surveys revealed that the population had a relatively high survival rate up until 1996. Since then however, the adult population has declined greatly, in tandem with persistent droughts.

The tortoises rely on desert plants for food, the growth of which is highly dependent on precipitation levels in winter. If a drought continues for even 2-3 years, these food plants become less available and the tortoises no longer have access to sufficient amounts of food to sustain the entire population. This ultimately leads to an increase in the death rate for adults. These findings were confirmed by a survey of dead tortoises in 2012 which found that most of the reptiles had indeed perished from starvation and dehydration.

There is still a chance that the tortoise population may yet recover, as it did after the dry years in the 1960s and 1970s. However, such a recovery is only feasible if the droughts are followed by periods of greater precipitation. Unfortunately, under current climate scenarios, the duration and frequency of droughts in the Sonoran Desert are predicted to increase. With few options currently available to managers to improve the situation, the future looks bleak for the survival of the tortoises in this region.

Reference: Jeffrey E. Lovich, Charles B. Yackulic, Jerry Freilich, Mickey Agha, Meaghan Austin, Katherine P. Meyer, Terence R. Arundel, Jered Hansen, Michael S. Vamstad, & Stephanie A. Root (2014). Climatic variation and tortoise survival: Has a desert species met its match? Biological Conservation, 169, 214-224 DOI: 10.1016/j.biocon.2013.09.027

Image: Michael Tuma


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