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These data were compiled to evaluate the reproductive ecology of Agassiz's desert tortoises (Gopherus agassizzi) in the Sonoran Desert of California using two populations within Joshua Tree National Park, including five reproductive seasons that spanned 20 years (1997-1999, 2015-2016). Compared to their conspecifics inhabiting the Mojave Desert, the reproductive ecology of G. agassizii in the Sonoran Desert is understudied. Climatic variation between the two deserts can affect reproductive ecology, including fecundity and clutch phenology. Mature female tortoises (straight-line carapace length ≥ 20 cm) outfitted with radiotransmitters were located and X-radiographed approximately every 10-14 days during the reproductive...
Categories: Data;
Tags: Agassiz's desert tortoise,
California,
Ecology,
Geography,
Gopherus agassizii,
To elucidate ecological effects of variation in the temporal distribution of a limiting resource (water in the Mojave Desert), energetics of two free-living populations of desert tortoises (Gopherus [=Xerobates] agassizii) were studied concurrently over 18 mo with use of doubly-labeled water. Field metabolic rates (FMR) and feeding rates (estimated from rates of water influx and rates of change in dry mass) were highly variable. This variability was manifested at several levels, including seasonal changes within populations, year-to-year differences within populations, and differences between populations. Underlying observed patterns and contrasts was considerable variation among individuals. Much of the variation...
Categories: Publication;
Types: Citation,
Journal Citation;
Tags: Ecology,
Gopherus agassizii,
desert herbivore,
differences,
field energetics,
Percent perennial plant cover. The total perennial plant cover data were modeled using Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) collected by the MODIS satellite and composited over 16-day intervals (Wallace and others, 2008), combined with field measurements of total perennial cover, estimated from line intercept transects at locations across the Mojave Desert (Webb and others, 2003, 2009; Thomas and others, unpublished data; Wallace and others, 2008). Total perennial cover was related to elevation and 2001 through 2004 MODIS-EVI data at the transect locations (R2 = 0.82), and the resulting model was used to extrapolate cover estimates for the remaining study area. The...
Average surface roughness. Surface roughness was calculated at a 30-m cell size using the method specified by Hobson (1972). Average surface roughness was then calculated as the average value of surface roughness in each 1-km2 grid cell. Derived from 30m NED DEM.
Mean wet season precipitation for 30-year normal period.
Annual growth potential is an environmental data layer that is a proxy for annual plant biomass, which reflects potential forage for tortoises. This data layer was derived by calculating the difference in greenness (a measure of plant growth) between two highly contrasting years of annual plant production (Wallace and Thomas, 2008). The difference between MODIS-EVI images for 2002 (a very dry year) and 2005 (a very wet year) had high correlation with field measurements of annual plant cover collected on 36 plots in the Mojave National Preserve in 2005 (R2 = 0.63, p=0.01). The resulting values represent the potential for site specific food availability for desert tortoise.
Percent smoothness. First, Surface roughness was calculated at a 30-m cell size using the method specified by Hobson (1972). Average surface roughness was calculated as the average value of surface roughness in each 1-km2 grid cell. Percent smooth. Percentage of each 1-km2 cell that was "smooth" and "rough" was assessed by measuring the proportion of 30-m average roughness grid cells that were < 1.01 (threshold for smooth). Derived from 30m NED DEM.
Percent rocks. Obtained from STATSGO soils database.
Average soil depth to bedrock. Obtained from STATSGO soils database.
Habitat modeling is an important tool used to simulate the potential distribution of a species for a variety of basic and applied questions. The desert tortoise (Gopherus agassizii) is a federally listed threatened species in the Mojave Desert and parts of the Sonoran Desert of California, Nevada, Utah, and Arizona. Land managers in this region require reliable information about the potential distribution of desert tortoise habitat to plan conservation efforts, guide monitoring activities, monitor changes in the amount and quality of habitat available, minimize and mitigate disturbances, and ultimately to assess the status of the tortoise and its habitat toward recovery of the species. By applying information from...
Percent surface roughness. Surface roughness was calculated at a 30-m cell size using the method specified by Hobson (1972). First average surface roughness was calculated as the average value of surface roughness in each 1-km2 grid cell. Percentage of each 1-km2 cell that was "rough" was assessed by measuring the proportion of 30-m average roughness grid cells that were > 1.11 (threshold for rough). Derived from 30m NED DEM.
Wet season precipitation, spatially distributed coefficient of variation. Blainey and others (2007).
The aspect of each 1-km2 grid cell was represented by eastness and northness (Zar, 1999), which are variables that represent aspect by converting the 1 to 360º range of possible azimuths into a range of -1 to 1, where -1 = south or west and 1 = north or east for northness and eastness, respectively.
Digitized vector data of Desert Tortoise (gopherus agassizii suitable habitat in Arizona. Digitized from 1:100,000 scale manuscripts prepared by Field Office Wildlife Specialists or digitized on-screen and edited at 1:100,000 scale or larger by GIS specialists. The criteria used included elevation, terrain, climate and vegetation. Category definitions: Category 1: 1. Habitat Area essential to maintenance of large, viable populations. 2. Conflicts are resolvable. 3. Medium to high density or low density contiguous with medium or high density. 4. Increasing, stable, or decreasing population.Category 2: 1. Habitat Area may be essential to maintenance of viable population. 2. Most conflicts are resolvable. 3. Medium...
Mean dry season precipitation for 30-year normal period.
Dry season precipitation, spatially distributed coefficient of variation. Blainey and others (2007).
The aspect of each 1-km2 grid cell was represented by eastness and northness (Zar, 1999), which are variables that represent aspect by converting the 1 to 360º range of possible azimuths into a range of -1 to 1, where -1 = south or west and 1 = north or east for northness and eastness, respectively.
Maxent model output for desert tortoise (Gopherus agassizii) in the Mojave and parts of the Sonoran Deserts of California, Nevada, Utah, and Arizona, USA.
Slope. Derived from 30m NED DEM.
Elevation. From 30m NED DEM.
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