Habitat Influences on Desert Tortoise Translocation Success

Determining whether head-started tortoises can be released sooner in ideal habitat conditions

Mojave desert tortoise with tracking device

Grid Decarbonization and Decentralization Variable Renewable Generation

Zoological Society of San Diego dba San Diego Zoo Wildlife Alliance

Recipient

Escondido, CA

Recipient Location

40th

Senate District

76th

Assembly District

beenhere

$499,605

Amount Spent

closed

Completed

Project Status

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Project Result

The project has been completed, and a final report was submitted to the CEC. This project investigated how to shorten the rearing period (and lower costs) by finding more favorable habitats to release juvenile tortoises. To generalize results, researchers studied:
- Two regions of the Mojave Desert
- Two habitat types (Yucca woodland and creosote scrub) and several habitat characteristics
- Two age groups

Once released into the wild, young tortoises preferred to settle in habitat with more shrubs, less bare ground, sandier soils, and more burrows available. Because of the large number of failures with the tracking devices, it was not possible to prove the survival benefit of yucca woodland habitat over creosote scrub. Results were shared with the US Fish and Wildlife Service at their Mojave Desert Tortoise Coordination Meeting as well as other presentations. The benefits include potential reduction in mitigation costs if rearing times can be reduced with similar positive outcomes in tortoise survival.

The Issue

With planned expansions in the California deserts, renewable energy projects potentially represent another risk factor for the threatened Mojave Desert tortoise. Head-start methods have been identified as a potential recovery tool and mitigation action for the desert tortoise, but current methods that rely on more than 4 years in captivity are expensive and therefore may be impractical.

Project Innovation

The project team determined if habitat characteristics of the release sites can improve survival rates of smaller juveniles to the point that they are equivalent to the rates of the animals that were released only after reaching the desired size target. Careful measurement of resources that may help tortoises avoid predation or meet nutritional requirements will allow investigators to differentiate excellent habitat from merely adequate habitat. They also studied the effects of outdoor rearing on juvenile desert tortoise behavior and health, and size-age relationships to survival in the wild upon their release. The project generated quantitative scientific information about the most cost-effective husbandry and release methods during and following a head-start program for this threatened species.

Project Goals

Evaluate Best Practice Husbandry for Effective Desert Tortoise Headstarting.

Enhance Understanding of Environmental and Habitat Factors to Improve Post-Release Success.

Project Benefits

This project can increase the effectiveness of conservation actions designed to mitigate renewable energy impacts on Mojave Desert tortoises. The project evaluated the relative effectiveness of head-start and release methods in an experimental framework. Improvements to the effectiveness of habitat management to encourage natural recruitment of juveniles were tested experimentally where possible.

Affordability

This project is expected to make head-start mitigation more cost-effective, reducing obstacles to future renewable energy deployment. It is important to determine the best practice methods to minimize mitigation costs.

Environmental Sustainability

New scientific knowledge on minimum size required at release and habitat features will guide release site selection and management in the future, while improving survivorship, resource needs, and translocation practices.