Spring Valley, CA, United States
Spring Valley, CA, United States

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Esque T.C.,U.S. Geological Survey | Nussear K.E.,U.S. Geological Survey | Drake K.K.,U.S. Geological Survey | Walde A.D.,QinetiQ | And 8 more authors.
Endangered Species Research | Year: 2010

Understanding predator-prey relationships can be pivotal in the conservation of species. For 2 decades, desert tortoise Gopherus agassizii populations have declined, yet quantitative evidence regarding the causes of declines is scarce. In 2005, Ft. Irwin National Training Center, California, USA, implemented a translocation project including 2 yr of baseline monitoring of desert tortoises. Unusually high predation on tortoises was observed after translocation occurred. We conducted a retrospective analysis of predation and found that translocation did not affect the probability of predation: translocated, resident, and control tortoises all had similar levels of predation. However, predation rates were higher near human population concentrations, at lower elevation sites, and for smaller tortoises and females. Furthermore, high mortality rates were not limited to the National Training Center. In 2008, elevated mortality (as high as 43%) occurred throughout the listed range of the desert tortoise. Although no temporal prey base data are available for analysis from any of the study sites, we hypothesize that low population levels of typical coyote Canis latrans prey (i.e. jackrabbits Lepus californicus and other small animals) due to drought conditions influenced high predation rates in previous years. Predation may have been exacerbated in areas with high levels of subsidized predators. Many historical reports of increased predation, and our observation of a range-wide pattern, may indicate that high predation rates are more common than generally considered and may impact recovery of the desert tortoise throughout its range. © Inter-Research 2010.


Hinderle D.,San Diego State University | Lewison R.L.,San Diego State University | Walde A.D.,Walde Research and Environmental Consulting | Deutschman D.,San Diego State University | And 2 more authors.
Journal of Wildlife Management | Year: 2015

Translocation of threatened or vulnerable species is a tool increasingly used for conservation and management. However, in some species, homing and movement behaviors may undermine the success of translocation efforts. For the federally protected Agassiz's desert tortoise (Gopherus agassizii), translocation is a strategy used to manage declining populations, yet homing behavior in this species is poorly understood. To explore homing behavior and movement patterns after translocation, we radio tracked 80 tortoises during a 2-phase experimental translocation. Phase 1 included 40 tortoises that were translocated, then monitored for a period of 37 days (21 Sep-28 Oct 2009), and phase 2 included a different group of 40 tortoises that were translocated and then monitored for 186 days (13 Apr-20 Oct 2010). In both phases, we assigned tortoises randomly to 1 of 3 treatment groups: translocated (displaced 2, 5, or 8 km from their source location), handling control, or control. After translocation, 20% of the translocated tortoises were able to navigate to their source location, and translocation distance had an effect on their ability to navigate home. We found 44% of tortoises in the 2-km translocated group returned home; 1 tortoise in the 5-km group, and no tortoises in the 8-km translocated group returned. The time required to reach home ranged from 5 to 37 days for the 2-km group, and 34 days for the 5-km group. We deemed tortoises to have homed successfully if they returned to their source location within 37 days of translocation as this reflected the duration of phase 1 and allowed for a balanced comparison between the 2 phases. We found that translocated tortoises moved at least 1.5 times more overall than the control groups, with some individuals moving >10 km from the translocation site. These patterns persisted even after accounting for seasonal and sex differences in distance traveled. By identifying homing behaviors and quantifying post-translocation movement patterns, this experiment addressed a key data gap in tortoise behavior that may limit the efficacy of tortoise translocation efforts. Our results point to the need to account for behavioral responses of tortoises to minimize risk to translocated individuals and maximize the success of translocation projects. © 2014 The Wildlife Society.


Latch E.K.,University of Wisconsin - Milwaukee | Latch E.K.,Center for Conservation and Evolutionary Genetics | Boarman W.I.,Conservation Science Research and Consulting | Walde A.,Walde Research and Environmental Consulting | Fleischer R.C.,Center for Conservation and Evolutionary Genetics
PLoS ONE | Year: 2011

Characterizing the effects of landscape features on genetic variation is essential for understanding how landscapes shape patterns of gene flow and spatial genetic structure of populations. Most landscape genetics studies have focused on patterns of gene flow at a regional scale. However, the genetic structure of populations at a local scale may be influenced by a unique suite of landscape variables that have little bearing on connectivity patterns observed at broader spatial scales. We investigated fine-scale spatial patterns of genetic variation and gene flow in relation to features of the landscape in desert tortoise (Gopherus agassizii), using 859 tortoises genotyped at 16 microsatellite loci with associated data on geographic location, sex, elevation, slope, and soil type, and spatial relationship to putative barriers (power lines, roads). We used spatially explicit and non-explicit Bayesian clustering algorithms to partition the sample into discrete clusters, and characterize the relationships between genetic distance and ecological variables to identify factors with the greatest influence on gene flow at a local scale. Desert tortoises exhibit weak genetic structure at a local scale, and we identified two subpopulations across the study area. Although genetic differentiation between the subpopulations was low, our landscape genetic analysis identified both natural (slope) and anthropogenic (roads) landscape variables that have significantly influenced gene flow within this local population. We show that desert tortoise movements at a local scale are influenced by features of the landscape, and that these features are different than those that influence gene flow at larger scales. Our findings are important for desert tortoise conservation and management, particularly in light of recent translocation efforts in the region. More generally, our results indicate that recent landscape changes can affect gene flow at a local scale and that their effects can be detected almost immediately.


Camp R.J.,Colorado State University | Camp R.J.,University of Hawaii at Hilo | Hagan M.,U.S. Air force | Boarman W.I.,U.S. Geological Survey | And 4 more authors.
Western North American Naturalist | Year: 2013

Capturing Common Ravens (Corvus corax) is very difficult. Several methods are currently used, but none effectively catch large numbers (>25 birds) of ravens at one time. Efficient capture of large numbers of ravens is needed for some autecology studies. We describe and evaluate the effectiveness of using a prebaited rocket net trapping method for simultaneously capturing large numbers of ravens. The study sites were within 2 landfills in California's Mojave Desert, one at Edwards Air Force Base and the other at Fort Irwin National Training Center. We captured 283 ravens on 5 trapping occasions between 1995 and 1997, with an average of 57 birds per trapping occasion. We observed greater numbers of ravens at the bait sites with increasing bait duration, and these numbers appeared to level off after 25 to 30 days of baiting. Longer bait durations may habituate ravens to the resource and compensate for their wariness, which could increase capture success. More than half of the ravens captured (55%) were adults, and subadults composed the remaining age class (42%). Only 3% of the ravens captured were hatch-year birds, a result of trapping early in the breeding season. Using rocket nets is a safe and effective method to capture large numbers of ravens.

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