Habitat Influences on Desert Tortoise Translocation Success

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

Zoological Society of San Diego dba San Diego Zoo Wildlife Alliance

Recipient

Escondido, CA

Recipient Location

38th

Senate District

75th

Assembly District

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$489,656

Amount Spent

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Active

Project Status

Project Update

The project hatched the first cohort of 63 hatchling tortoises at Edwards Air Force Base (EAFB). This number is sufficient for the 18-month old group to be released in 2020. Unfortunately, there was complete nest failure at the Cadiz site. Researchers tried to overcome this setback by attempting to collect a larger number of eggs in 2019 so that half could be reared indoors to reach the size the 18-month cohort would have achieved, but this was unsuccessful. Investigating the cause of the nest failure should lead to improved guidance for head-starting. The team is collaborating with a complementary project (EPC-16-038). The covid-19 pandemic also prevented access to EAFB, blocking the research team from releasing tortoises there in spring 2020. A no-cost time extension is being processed to allow adequate time to track this set of tortoises.

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 is determining 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 are also studying 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 will generate quantitative scientific information about the most cost-effective husbandry and release methods during and following a head-start program for this threatened species.

Project Benefits

This project will increase the effectiveness of conservation actions designed to mitigate renewable energy impacts on Mojave Desert tortoises. This will be achieved by evaluating 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 will also be tested experimentally where possible.

Lower Costs

Lower Costs

This project is expected to lower costs by making head-start mitigation more cost-effective, reducing obstacles to future renewable energy deployment. It is important to determine the best practice methods for head-starting juvenile tortoises, including reducing time spent in captivity and increasing survival in the wild after release. This will minimize mitigation costs and maximize the contributions of these actions to desert tortoise recovery.

Environmental & Public Health

Environmental & Public Health

New scientific knowledge on minimum size required at release will guide release site selection and management in the future, while improving survivorship, resource needs, and translocation practices. A better understanding of the habitat features that allow tortoises to thrive can also be applied to guidance for wildlife agencies about which land parcels to protect and set more meaningful restoration targets. Implementation of more effective mitigation practices may increase the probability of de-listing the desert tortoise from the endangered species list in the future. Lessons from the 2018 nest failure will provide important guidance for future mitigation efforts.

Key Project Members

Project Member

Ron Swaisgood

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