In California, researchers tracked desert tortoises in a protected area for 34 years; numbers fell from 589 to 47 by 2002, then rose to 73 by 2012

Imagine a species that can live for over 50 years, survives on a diet of cacti and wildflowers, and spends 95% of its life underground to avoid harsh sunlight from the Mojave Desert; only for 9 out of 10 individuals to disappear over roughly two decades. A population of desert tortoises living in a protected piece of land in California was hit by a 92% decline in numbers due to a number of factors. According to a study spanning 34 years, titled “The Catastrophic Decline of Tortoises at a Fenced Natural Area,” by USGS researcher Kristin Berry et al., published in the journal Wildlife Monographs, between 1979 and 2002, tortoise numbers in the Desert Tortoise Research Natural Area fell to just 47 individuals from an initial population of 589.

However, by 2012 numbers had increased to 73. This is an unfortunate but oddly positive tale. Those growing up in the US will likely know tortoises from books and pet tanks. However, the desert tortoise is an iconic animal in the Southwest of the USA. They are even the official state reptile of California and have survived for much longer than any civilization. To see them decline by half and then half again in a piece of land specifically set up to protect them is shocking.

A fence that didn't fully fix things


This area of the western Mojave was set up as a protected Research Natural Area in 1980, when the US Congress designated the land and protected it from off-road vehicles, livestock grazing, and mining equipment with a wildlife-permeable fence. This was intended to let the tortoises thrive. Within the protected area, tortoise numbers nonetheless fell by 90% by 2002. Outside the fence, where there were no protections, tortoise numbers fell by a shocking 95%.

Researchers found that the tortoises were suffering from a range of issues such as Upper Respiratory Tract Disease, predation by ravens (whose populations had soared due to man-made habitats such as power lines and landfill sites), and long periods of drought reducing wildflower numbers. The ecosystem could not recover quickly enough despite legal protection, as the pressures driving the decline: disease, drought, and predation, operated regardless of the fence, and the more severe devastation occurring just outside it likely still affected the protected population.

The effect of the fence, and its limits

It’s worth noting that the fence was not without effect: the 90% decline inside the fence was still notably better than the 95% decline outside it, suggesting the barrier slowed, but did not stop, the losses. In other words, the fence appears to have mitigated some of the damage from vehicles, grazing, and mining, but it could not shield tortoises from disease, drought, and increased predation, all threats that don’t respect a fence line.

The rebound

By the final year of the study, in 2012, numbers had recovered by 54%. Whilst this is an encouraging statistic, researchers do highlight that tortoises require 12 to 20 years to become sexually mature and therefore take a long time to recover. Looking at the overall population of desert tortoises across the Mojave region, they have continued to decline since 2012. Assessments of density have been used in federal recovery planning and show that most recovery units have continued to decline across the region in the last decade.

Why this matters beyond the desert

Whilst this may seem like a small conservation fact, it highlights that fencing off a problem such as a species, budget or mental health issue won’t actually help if the pressures causing the issues (disease, climate change, predation and human intrusion) are still happening. Long-term monitoring will be needed to help them recover. This is a frustrating reminder that some issues take a lot longer to recover from than they do to cause. We live in a world of instant solutions; however, this animal shows us that sometimes we need to be patient.