J. Mark Peaden | USGS Science

USGS2019

Effects of short-term, outdoor head-starting on growth and survival in the mojave desert tortoise (gopherus agassizii)

The combination of life-history traits that makes some turtle species vulnerable to population declines also limits their ability to recover even after threats have been addressed. Because juvenile turtle survival is typically lower than adult survival, head-starting, the process of rearing juveniles through one of their most vulnerable periods, may be a useful recovery tool. We evaluated short-term, outdoor head-starting in Mojave Desert Tortoises ( Gopherus agassizii ) by comparing growth and survival among three treatments: (1) juveniles reared in outdoor predator-resistant enclosures and receiving low (LOW) or (2) high levels of rain supplementation (HIGH); and (3) free-ranging animals released 0-18 mo after hatching (FIELD). Juveniles from the HIGH treatment had higher annual growth (12.7 mm midline carapace length [MCL] per year) than juveniles from the LOW or FIELD treatments (10.7 mm). Annual growth also varied among years, presumably due to variation in rainfall. Annual survival was high (0.94 ± 0.01) for both LOW and HIGH treatments; MCL at hatching had a weak positive effect on survival probability (effect size: 0.42 ± 0.35). Annual survival of FIELD animals averaged 0.48 ± 0.09. There was no effect of size at release (40.8-61.5 mm MCL) on post-release survival of FIELD animals, suggesting that the greatest benefit of short-term outdoor head-starting is increasing survival during the head-start period. Although releasing at larger sizes (100 mm MCL) has been recommended, slow growth in tortoises would require extended outdoor head-starting periods. Indoor rearing, which has been successfully implemented with other turtle species, may increase growth rates of juvenile Desert Tortoises and warrants future study as a conservation technique.

Herpetological Conservation and Biology↗

USGS2016

Habitat selection by juvenile Mojave Desert tortoises

Growing pressure to develop public lands for renewable energy production places several protected species at increased risk of habitat loss. One example is the Mojave desert tortoise ( Gopherus agassizii ), a species often at the center of conflicts over public land development. For this species and others on public lands, a better understanding of their habitat needs can help minimize negative impacts and facilitate protection or restoration of habitat. We used radio-telemetry to track 46 neonate and juvenile tortoises in the Eastern Mojave Desert, California, USA, to quantify habitat at tortoise locations and paired random points to assess habitat selection. Tortoise locations near burrows were more likely to be under canopy cover and had greater coverage of perennial plants (especially creosote [ Larrea tridentata ]), more coverage by washes, a greater number of small-mammal burrows, and fewer white bursage ( Ambrosia dumosa ) than random points. Active tortoise locations away from burrows were closer to washes and perennial plants than were random points. Our results can help planners locate juvenile tortoises and avoid impacts to habitat critical for this life stage. Additionally, our results provide targets for habitat protection and restoration and suggest that diverse and abundant small-mammal populations and the availability of creosote bush are vital for juvenile desert tortoises in the Eastern Mojave Desert.

California↗