Habitat Influences on Desert Tortoise Translocation Success

The solar energy industry and the Mojave desert tortoise (Gopherus agassizii) share a preference for the sunny desert regions of southeastern California. The tortoise is listed as threatened by the U.S. Fish and Wildlife Service and the state of California, which impose a rigid permitting process on the solar industry and other development. The release of immature tortoises following captive rearing, or headstarting, is recommended as a recovery tool to mitigate population declines, although this can be costly and require four to ten years in captivity. Therefore, identifying optimal headstart methods is key for species recovery.

This research aimed to understand the role of headstarting methods and careful selection of habitat characteristics at the release site on post-release growth, movement, and survival of juvenile tortoises in hopes of shortening the captive rearing phase. This research identified key microhabitat characteristics that are important for juvenile tortoise resource selection and began to identify relationships between habitat variables and post-release movement and survival in translocated juvenile tortoises.

Findings suggest that tortoises preferentially select habitat with a greater density of burrows, shrubs, and the small sand mounds that form under shrubs. The researchers do not yet have evidence for any significant effects on survival of these habitat features or of age at release (one versus two years). Severe drought following release inhibited juvenile tortoise growth as no forage was available and may have contributed to mortalities. Due to the slow life history of the Mojave desert tortoise, impacts of age at release and habitat on demographic variables may take more time to manifest. The findings do suggest that home-range sizes and travel distances are larger for two-year-olds in comparison to one-year-olds. These findings could have important implications for carrying capacity, release site selection, target densities of translocations, and other factors pertinent to tortoise conservation.