Sarah Doyle | Geology Reports

USGS2025

Spatially explicit demographics of Mojave Desert Tortoises on a demography plot in California, USA

Obtaining reliable estimates of demographic parameters is critical to effective wildlife conservation and management. Densities of Mojave Desert Tortoises ( Gopherus agassizii ) were historically derived from capture–mark–recapture surveys on small, often strategically placed demography plots, or demographic study areas, that also provided information on demographic composition and vital rates. After protection was afforded to Desert Tortoises under the US Endangered Species Act in 1990, monitoring shifted mostly to line-distance sampling across broad areas for estimating densities of primarily adult tortoises to inform long-term population trends. However, that approach is incapable of providing data about other demographic characteristics important to population growth and viability. We surveyed a previously unsampled demography plot in the western Mojave Desert, California, USA, during 2022 and applied spatial capture–recapture (SCR) models to estimate spatially explicit Desert Tortoise density and sex-by-size class compositions. Directly accounting for spatiotemporally varying survey effort in SCR models via hazard-based adjustment reduced the estimated detection rate by 91% and increased the estimated density by 17%. Estimated spatial mean Desert Tortoise density across a 2.53-km 2 area was 18.53 tortoises/km 2 (95% confidence interval [CI] = 12.36–27.77). The SCR model–estimated size class ratio was skewed toward prereproductive tortoises (64% prereproductive; 36% adults), whereas the adult sex ratio was female biased (61% females; 39% males). Those ratios corresponded to densities of 11.86 prereproductive tortoises/km 2 (95% CI = 7.91–17.77), 4.08 adult female tortoises/km 2 (95% CI = 2.72–6.11), and 2.59 adult male tortoises/km 2 (95% CI = 1.73–3.89). Estimated tortoise density and demographic composition collectively support a high potential for population growth. Our study provides an illustrative example of using SCR models to directly estimate spatially explicit local Desert Tortoise densities and demographic composition that can be used for long-term monitoring and comparisons with other demography plots to inform conservation.

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USGS2024

Desert Tortoise translocation plan for the U.S. Department of the Army National Training Center and Fort Irwin Western Training Area

The U.S. Department of the Army proposes to commence military activity at the Fort Irwin National Training Center within the Western Training Area (WTA) and to translocate Mojave Desert tortoises ( Gopherus agassizii ; hereafter tortoise) that will be affected to the Western Training Area Translocation Site (WTATS). This desert tortoise translocation plan provides a timeline of activities, actions for which permits may be required, and guidelines for assessing the short-term and long-term success of this desert tortoise translocation. Importantly, the monitoring projects described are designed to document the ultimate effects of the Army's translocation action (not just inform future translocations elsewhere). Results from the translocation, corresponding monitoring, and research projects will inform future translocations throughout the Mojave Desert for expanding human development. The plan has three main objectives: (1) provide guidelines to achieve a safe, humane, and successful translocation of tortoises from the WTA, with minimal effect to resident desert tortoises at sites where translocated animals are released (recipient sites); (2) study translocated, resident, and reference animals (tortoises living near translocation areas but whose home ranges do not overlap those of translocated or resident tortoises) to learn as much as possible about the ecology, conservation, and management of the desert tortoise; and (3) define best management practices for successful translocation and provide metrics to evaluate success over multiple time scales, which we identify for the short- and long-term. The procedures to plan, implement, monitor, and study translocation of tortoises were written using terms and conditions outlined in the U.S. Fish and Wildlife Service Biological Opinion 2021 that described effects of the expansion of the military base boundary, as well as recommendations provided in the Desert Tortoise Recovery Plan (and 5-year review). We provide guidance on appropriate translocation timing and procedures, as well as on how tortoise ecology and habitat can best be studied to further knowledge on tortoise translocation. The plan provides analysis for landscape tortoise density and abundance estimates, suitable sites for translocation of tortoises, and short- and long-term metrics that are addressed and measured by specific monitoring and research projects that can be used to assess the success of translocation activities.

California↗