Geology topics

P.A. Medica

Publications and source records attributed to P.A. Medica.

At least 19 recordsLinked to original sources

USGS2012

Does translocation influence physiological stress in the desert tortoise?

Wildlife translocation is increasingly used to mitigate disturbances to animals or habitat due to human activities, yet little is known about the extent to which translocating animals causes stress. To understand the relationship between physiological stress and translocation, we conducted a multiyear study (2007–2009) using a population of desert tortoises (Gopherus agassizii) near Fort Irwin, California. Blood samples were collected from adult tortoises in three treatment groups (resident, translocated and control) for 1 year prior to and 2 years after translocation. Samples were analyzed by radioimmunoassay for plasma total corticosterone (CORT), a glucocorticoid hormone commonly associated with stress responses in reptiles. CORT values were analyzed in relation to potential covariates (animal sex, date, behavior, treatment, handling time, air temperature, home-range size, precipitation and annual plant production) among seasons and years. CORT values in males were higher than in females, and values for both varied monthly throughout the activity season and among years. Year and sex were strong predictors of CORT, and translocation explained little in terms of CORT. Based on these results, we conclude that translocation does not elicit a physiological stress response in desert tortoises.

California↗

USGS2012

Translocation as a conservation tool for Agassiz's desert tortoises: survivorship, reproduction, and movements

We translocated 120 Agassiz's desert tortoises to 5 sites in Nevada and Utah to evaluate the effects of translocation on tortoise survivorship, reproduction, and habitat use. Translocation sites included several elevations, and extended to sites with vegetation assemblages not typically associated with desert tortoises in order to explore the possibility of moving animals to upper elevation areas. We measured survivorship, reproduction, and movements of translocated and resident animals at each site. Survivorship was not significantly different between translocated and resident animals within and among sites, and survivorship was greater overall during non-drought years. The number of eggs produced by tortoises was similar for translocated and resident females, but differed among sites. Animals translocated to atypical habitat generally moved until they reached vegetation communities more typical of desert tortoise habitat. Even within typical tortoise habitat, tortoises tended to move greater distances in the first year after translocation than did residents, but their movements in the second or third year after translocation were indistinguishable from those of resident tortoises. Our data show that tortoises translocated into typical Mojave desert scrub habitats perform well; however, the large first-year movements of translocated tortoises have important management implications. Projects that employ translocations must consider how much area will be needed to contain translocated tortoises and whether roads need fencing to prevent the loss of animals.

Journal of Wildlife Management↗

USGS2012

Long-term growth of Desert Tortoises ( Gopherus agassizii ) in a southern Nevada population

Knowledge of growth rates, age at maturity, and longevity are important aspects of a species life history and are directly applicable to life table creation and population viability analyses. We measured the growth of a cohort of 17 semi-wild Desert Tortoises (Gopherus agassizii) located in Rock Valley, Nevada over a 47-yr period beginning in 1963. The tortoises were initially marked as hatchling and juvenile animals between the years 1963 and 1965 and ranged in size from 47 to 77 mm in plastron length. We assigned ages of 1-4 yr to the tortoises at initial capture based on their body size. These tortoises were recaptured, measured, and weighed approximately annually since their initial capture. Growth of male and female tortoises did not differ significantly until animals reached the age of 23-25 yr. Annual tortoise growth was correlated with the production of ephemeral vegetation, while accounting for size, sex, and repeated measurements of the animals as well as the interval between measurements. However, the production of ephemeral plants was likewise highly correlated (non-linearly) with winter rainfall. Stochastic predation events between 2003 and 2007 decimated this cohort of tortoises. The average age of the long-term surviving tortoises from this cohort was 43 yr with a range of 39-47 yr. Twelve of the tortoises survived to the age of 39 yr and 11 of the 12 reached 40 yr.

Journal of Herpetology↗

USGS2008

Assessing age in the desert tortoise Gopherus agassizii: Testing skeletochronology with individuals of known age

Eight desert tortoises Gopherus agassizii from a long-term mark-recapture study in the Mojave Desert, Nevada, USA, afforded an opportunity to examine the accuracy of skeletochronological age estimation on tortoises from a seasonal, yet environmentally erratic environment. These 8 tortoises were marked as hatchlings or within the first 2 yr of life, and their carcasses were salvaged from predator kills. Using d blind protocol, 2 skeletochronological protocols (correction-factor and ranking) provided age estimates for a set of 4 bony elements (humerus, scapula, femur, ilium) from these tortoises of known age. The age at death of the tortoises ranged from 15 to 50 yr. The most accurate protocol - ranking using the growth layers within each of the 4 elements - provided estimates from 21 to 47 yr, with the highest accuracy from the ilia. The results indicate that skeletochronological age estimation provides a reasonably accurate method for assessing the age at death of desert tortoises and, if used with a large sample of individuals, will provide a valuable tool for examining age-related mortality parameters in desert tortoise and likely in other gopher tortoises (Gopherus). ?? Inter-Research 2008.

Endangered Species Research↗

USGS2007

Return to the wild: Translocation as a tool in conservation of the desert tortoise ( Gopherus agassizii )

Translocation could be used as a tool in conservation of the threatened Mojave Desert Tortoise ( Gopherus agassizii ) by moving individuals from harm's way and into areas where they could contribute to conservation of the species. Numerous factors may affect the success of translocations, including the conditions experienced by tortoises in holding facilities while awaiting translocation. The tortoises available for our translocation study had been provided supplemental water during their years spent in a captive holding facility, potentially inducing carelessness in water conservation. In addition to generally investigating the efficacy of translocation, we compared the effects of continuing with the effects of ceasing the holding facility's water supplementation regimen. After exposure to one of the two water regimens, all tortoises were given the opportunity to hydrate immediately prior to release. We examined behavior, body mass, carapace length, movement, and mortality of tortoises for two activity seasons following release to the wild. Water supplementation was correlated with high rates of carapace growth and distant movements by males after release. Lengthy movements following translocation may be problematic for conservation planning, but this should be evaluated in light of the goals and circumstances of each translocation project. Although the mortality rate was 21.4% in 1997, data suggest that drought conditions at the site rather than the translocation itself negatively affected the tortoises. None of the tortoises died during their second season at the site. Our results indicate that translocation should be considered a useful tool in conservation of the Desert Tortoise.

Nevada↗

USGS2003