Nevada Fish & Wildlife Office
Pacific Southwest Region

Desert Tortoise Recovery

 

Monitoring

 

In order to demonstrate that tortoise populations are recovering adequately, it is important to use the best available data from the best available survey methods to describe the long term population trends throughout the Mojave and Colorado deserts. Line distance sampling has been adopted as the best technique by the group of federal, state, and local agency partners that are working toward recovery of the desert tortoise. Distance sampling generates a detection function based on the perpendicular distance of objects (tortoises) from theExamining Tortoise center of a transect. Since tortoises are cryptic and harder to see the farther they are from a person, the detection function provides a correction for the number of tortoises occurring per kilometer walked along the transect. Because tortoises may also be hidden in burrows or deep bushes, even when close to an observer, further correction is provided by estimating the proportion of tortoises that are visible during the monitoring period. Tortoises equipped with radio transmitters are monitored each day to assess the proportion of encounters when they would be deep in a burrow or thick vegetation and not visible. The encounter rate and the two correction factors provide an estimate of the overall population density within each monitoring area.

Long-term monitoring areas were selected because they are managed to recover the desert tortoise in the Mojave and Colorado Deserts. These tortoise conservation areas are managed under designations such as Desert Wildlife Management Areas, USFWS critical habitat, and BLM Areas of Critical Environmental Concern, or they occur on national parklands or national wildlife refuges.

Distance sampling and visibility data have been collected in every year since 2001, except in 2006, in California, Nevada, Arizona, and Utah. These data are used to develop annual density estimates, reported in publications of the USFWS Desert Tortoise Recovery Office.

 
  • 2012 Desert Tortoise Monitoring Handbook
        Introduction 23Mar2012
        Chapter 1 Distance Sampling and Desert Tortoises 11Feb2010
        Chapter 2 Desert Tortoise Handling 21Feb2012
        Chapter 3 Compass and Navigational GPS 21Feb12
        Chapter 4 Electronic Equipment 22Feb2012
        Chapter 5 Line Distance Protocols 22Feb12
        Chapter 6 Radio Telemetry and G0 Protocols 1Mar2011
        Chapter 7 Data Collection and Verification 25Feb2012 (.7 MB PDF)
        Appendix 1 Annotated Datasheets 23Feb2012
         
         
         
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2012 Annual Report (1.2 MB PDF)
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2011 Annual Report (1.2 MB PDF)
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2010 Annual Report (1.1 MB PDF)
        Erratum: Table 7 was corrected and replaced on 11 June 2013.
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2008 and 2009 Reporting (1.7 MB PDF)
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2007 Annual Report (2.8 MB PDF)
     
  • Range-wide Monitoring of the Mojave Population of the Desert Tortoise: 2001-2005 Summary Report (USFWS - 2006) (4.78 MB PDF)
           
     
  • Anderson DR and KP Burnham. 1996. A monitoring program for the desert tortoise. Report to the Desert Tortoise Management Oversight Group and US Fish and Wildlife Service, Reno, NV. 15 pp.(.645 MB PDF) New Document
    Last updated: January 24, 2014