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Timothy A. Shields

Publications and source records attributed to Timothy A. Shields.

4 recordsLinked to original sources
USGS2021

Efficacy of manipulating reproduction of common ravens to conserve sensitive prey species: Three case studies

Expansion of human enterprise across western North America has resulted in an increase in availability of anthropogenic resource subsidies for generalist species. This has led to increases in generalists’ population numbers across landscapes that were previously less suitable for their current demographic rates. Of particular concern are growing populations of common ravens ( Corvus corax ; ravens), because predation by ravens is linked to population declines of sensitive species. Ecosystem managers seek management options for mitigating the adverse effects of raven predation where unsustainable predator–prey conflicts exist. We present 3 case studies examining how manipulating reproductive success of ravens influences demographic rates of 2 sensitive prey species. Two case studies examine impacts of removing raven nests or oiling raven eggs on nest survival of greater sage-grouse ( Centrocercus urophasianus ; sage-grouse) within Wyoming and the Great Basin of California and Nevada, USA, respectively. The third case study uses Mojave desert tortoise ( Gopherus agassizii ; tortoise) decoys to examine effects of oiling raven eggs on depredation rates of juvenile tortoises in the Mojave Desert in California. Initial trial years from all 3 case studies were consistent in finding improved vital rates associated with the application of strategies for reducing reproductive success of ravens. Specifically, removal of raven nests resulted in increased nest survival of sage-grouse within treatment areas where predation by ravens was the primary cause of nest failure. In addition, nest survival of sage-grouse and survival of juvenile tortoise decoys was higher following a treatment of oiling the eggs of ravens in their nests at 2 sites within the Great Basin and 4 tortoise conservation areas in the Mojave Desert in California. Along with specialized technologies that can make techniques such as egg-oiling more feasible, these findings support these management practices as important tools for managing ravens, especially in areas where breeding ravens have negative impacts on sensitive prey species.

California, Nevada, Wyoming↗
USGS2020

The catastrophic decline of tortoises at a fenced natural area

Agassiz’s desert tortoise ( Gopherus agassizii ), a threatened species of the southwestern United States, has severely declined to the point where 76% of populations in critical habitat (Tortoise Conservation Areas) are below viability. The potential for rapid recovery of wild populations is low because females require 12–20 years to reach reproductive maturity and produce few eggs annually. We report on a 34‐year mark‐recapture study of tortoises initiated in 1979 at the Desert Tortoise Research Natural Area in the western Mojave Desert, California, USA, and provide substantive data on challenges faced by the species. In 1980, the United States Congress designated the Research Natural Area and protected the land from recreational vehicles, livestock grazing, and mining with a wildlife‐permeable fence. The 7.77‐km 2 study area, centered on interpretive facilities, included land both within the Natural Area and outside the fence. We expected greater benefits to accrue to the tortoises and habitat inside compared to outside. Our objectives were to conduct a demographic study, analyze and model changes in the tortoise population and habitat, and compare the effectiveness of fencing to protect populations and habitat inside the fence versus outside, where populations and habitat were unprotected. We conducted surveys in spring in each of 7 survey years from 1979, when the fence was under construction, through 2012. We compared populations inside to those outside the fence by survey year for changes in distribution, structure by size and relative age, sex ratios, death rates of adults, and causes of death for all sizes of tortoises. We used a Bayesian implementation of a Jolly Seber model for mark‐recapture data. We modeled detection, density, growth and transition of tortoises to larger size‐age classes, movements from inside the protective fence to outside and vice versa, and survival. After the second and subsequent survey years, we added surveys to monitor vegetation and habitat changes, conduct health assessments, and collect data on counts of predators and predator sign. At the beginning of the study, counts and densities for all sizes of tortoises were high, but densities were approximately 24% higher inside the fence than outside. By 2002, the low point in densities, densities had declined 90% inside the fence and 95% outside. Between 2002 and 2012, the population inside the fence showed signs of improving with a 54% increase in density. Outside the fence, densities remained low. At the end of the study, when we considered the initial differences in location, densities inside the fence were roughly 2.5 times higher than outside. The pattern of densities was similar for male and female adults. When evaluating survival by blocks of years, survivorship was higher in 1979–1989 than in 1989–2002 (the low point) and highest from 2002 to 2012. Recruitment and survival of adult females into the population was important for growing the population, but survival of all sizes, including juveniles, was also critical.

California↗
USGS2013

Multiple factors affect a population of Agassiz's desert tortoise ( Gopherus agassizii ) in the Northwestern Mojave Desert

Numerous factors have contributed to declines in populations of the federally threatened Agassiz's Desert Tortoise ( Gopherus agassizii ) and continue to limit recovery. In 2010, we surveyed a low-density population on a military test facility in the northwestern Mojave Desert of California, USA, to evaluate population status and identify potential factors contributing to distribution and low densities. Estimated densities of live tortoises ranged spatially from 1.2/km 2 to 15.1/km 2 . Although only one death of a breeding-age tortoise was recorded for the 4-yr period prior to the survey, remains of 16 juvenile and immature tortoises were found, and most showed signs of predation by Common Ravens ( Corvus corax ) and mammals. Predation may have limited recruitment of young tortoises into the adult size classes. To evaluate the relative importance of different types of impacts to tortoises, we developed predictive models for spatially explicit densities of tortoise sign and live tortoises using topography (i.e., slope), predators (Common Raven, signs of mammalian predators), and anthropogenic impacts (distances from paved road and denuded areas, density of ordnance fragments) as covariates. Models suggest that densities of tortoise sign increased with slope and signs of mammalian predators and decreased with Common Ravens, while also varying based on interaction effects involving these predictors as well as distances from paved roads, denuded areas, and ordnance. Similarly, densities of live tortoises varied by interaction effects among distances to denuded areas and paved roads, density of ordnance fragments, and slope. Thus multiple factors predict the densities and distribution of this population.

California;Nevada↗