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Study: Grazing, Vehicles Bad For Desert Tortoises

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A baby desert tortoise | Photo: Joshua Tree National Park/Flickr/Creative Commons License
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A new study of desert tortoise populations in the western Mojave Desert shows that the reptiles fare better when they're protected from off-road vehicles and grazing livestock. The study, with veteran U.S. Geological Survey tortoise biologist Kristin Berry as lead author, shows that tortoises in an area where neither vehicles nor livestock have been permitted thrive in greater numbers, and with lower death rates, than their counterparts in unprotected areas.

That may seem like a bit of a no-brainer, but the issue of livestock grazing and tortoises achieved national prominence earlier this year, when Nevada rancher Cliven Bundy hit the news for resisting confiscation of about 900 cattle he was grazing illegally on critical habitat for the tortoise in southern Nevada. Supporters of the rancher suggested that cattle actually benefit desert tortoises by processing desert plants into an easier-to-digest form -- in other words, suggesting that torts thrive on a diet of cow flops.

Few herpetologists take that idea seriously, and now Berry et al's study adds more data to the pile, supporting the idea that the best way to protect tortoises is to leave their habitat intact and protected.

In the study, published in the December 2014 issue of Herpetological Monographs, Berry and her USGS colleagues Lisa Lyren, and Julie Yee, along with Tracy Bailey of the Desert Tortoise Council, studied 240 plots of about an acre and a half each in the western Mojave Desert between the town of Mojave and city of Ridgecrest. A third of the study sites were in the Desert Tortoise Research Natural Area northeast of California City, which has been fenced off and thus protected from livestock and vehicle traffic since 1980.

Another third were in critical habitat for the tortoise in the same region. Since critical habitat for the federally Threatened tortoise was designated in 1994, parts of the critical habitat area studied by Berry and her colleagues have periodically been closed to vehicle traffic and grazing. However, that protection has been temporary and incomplete, and the land in question has also been grazed by domestic sheep and used as the site of off-road events with more than 10,000 vehicles participating.

The final third of the study plots were on private land in the California City-Randsburg area that have not been protected from vehicles, grazing, or other uses potentially detrimental to the tortoise.

On all 240 plots the team counted live tortoises they encountered, as well as tortoise sign (mainly burrows and scat) and remains of dead tortoises. They also tallied evidence of human activity such as tire tracks, sheep dung and other evidence of grazing, lead shot or other and trash, as well as evidence of ravens and mammalian predators of tortoises.

The results showed a distinct difference between the long-protected habitat in the Desert Tortoise Research Natural Area and either the critical habitat or unprotected private lands. Twelve of the 17 live tortoises found on all 240 study plots were found within the Natural Area, with two on critical habitat plots and three on private lands. The fully protected area had a fewer dead tortoises for each live one than the critical habitat area, with 22 remains of tortoises in the protected sites (1.8 dead tortoises per live tortoise) compared to 23 dead tortoises on the critical habitat sites, which works out to almost 12 dead tortoises for each live one.

The authors suggest that both sheep grazing and off-road vehicle use may harm tortoise populations not only directly through trampling tortoises and collapsing burrows, but by reducing the amount of seasonal food available to the tortoises. Off-road vehicles do so by crushing plants before they can set seed, changing soil structure and introducing exotic grasses and herbs that compete with native vegetation but provide less nutritive value to the tortoises. Sheep do all of the above as well, and they also eat those valuable tortoise food plants, which means the tortoises can't.

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