Sonoran Desert Tortoise study by Meyer et al. (2010)

A key study on Sonoran Desert Tortoises began  when Dr. Walt Meyer discovered the charred shells of Sonoran Desert Tortoises that jojoba bean pickers camping on his grazing allotment had eaten. Meyer began monitoring the tortoises.

Twenty years later, the U.S. Fish and Wildlife Service found that listing of the species as endangered or threatened may be warranted. Upon learning this finding, the Winkelman Natural Resource Conservation District called the U.S. Fish and Wildlife Service into coordination on the tortoise and entered this peer-reviewed report, “An Eighteen Year Study of Population Dynamics, Diet and Health of the Sonoran Desert Tortoise (Gopherus agassizii) in the San Pedro Valley of Southern Arizona,” into the Administrative Record.

Based in part on the Meyer et al. study the  U.S. Fish and Wildlife Service ruled that managed grazing, which includes all grazing on federal lands  in the United States, is not a likely threat to the species. The agency nevertheless ruled that the unmanaged grazing in Mexico, which includes massive plantings of invasive, fire-prone African buffelgrass, is a likely threat to the species.

An excerpt from the report:

This study began in 1980 and continued through 1997, although data continued to be collected when tortoises were encountered in the study area after 1997. Limited data was collected as late as November 2009. The objectives of the study were to: 1) find and identify as many tortoises as possible within a defined study area which encompassed several Land Resource Areas (vegetation types); 2) determine the distribution of tortoises within the study area; 3) calculate an estimate of the number of tortoises within the study area; 4) compare tortoise data within four different livestock grazing systems; 5) determine the ratio between the number of adult males, adult females and juveniles (sub-adults); 6) study the diets of desert tortoises using visual observations of foraging tortoises and microhistological analyses of tortoise fecal material; and 7) determine the health of the tortoise population within the study area.

Click here to read the Meyer (2010) study.