Desert Tortoise Populations in the Mojave Desert
and a Half-Century of Military Training Activities


U.S. Army-CERL, P.O. Box 9005, Champaign, IL 61826-9005, USA

Current address:  Embry-Riddle Aeronautical University, 3200 Willow Creek Road, Prescott, AZ 86301, USA

        ABSTRACT:  The history of the United States military in the Mojave Desert is summarized, and a detailed case study of the effects of landscape-scale military training activities on the desert tortoise in the central Mojave is presented.
        Thirty-five years of Army training activities (occurring between 1940 and 1983) have fragmented tortoise populations at Fort Irwin, the Army’s National Training Center (NTC).  Recent data have demonstrated that tortoise population densities in areas that are not used or lightly used by tactical vehicles are stable, even after six years of intensive war games at the NTC.  These areas of the fort include installation boundaries, high rugged bajadas along mountain ranges, Goldstone (off-limits to training), a multi-purpose range complex (a live-fire range off-limits to maneuvering vehicles), and areas between the cantonment area (housing and associated infrastructure) and the actual training ranges.  Tortoise densities have declined significantly in those portions of Fort Irwin used as training ranges for tactical vehicle maneuvers.
        Two novel approaches are demonstrated to analyze, interpret, and visualize desert tortoise distribution and density patterns on a landscape scale.  A statistical analysis and inference strategy was developed that is independent of the usual statistical assumptions.  Desert tortoise distribution and density data were spatially modeled on a landscape scale with an analytical interpolation and smoothing technique refined in our laboratory —“Smoothing Thin-Plate Splines with Tension.”  This technique may have promising applications in ecology and conservation biology for modeling distribution and density patterns of populations or genetic structure and species-habitat relationships on landscape scales, and further research is being conducted.

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