Agassiz’s desert tortoises (Gopherus agassizii) approach the southern edge of their mostly Mojave Desert range near Joshua Tree National Park. Modern desert tortoise research started in the Park in 1978 when the first tortoise population census was conducted on a one square mile area in the Pinto Basin known as the “Barrow Plot.” U.S. Geological Survey research began at the plot in 1997 and continued intermittently until 2012, providing important long-term data on variation in population size and survivorship of long-lived tortoises during a period of widely variable environmental conditions. As a threatened species under the Endangered Species Act, resource managers need information on how best to manage for healthy tortoise populations. Other long-term research is focused on tortoise populations living near the mouth of Cottonwood Canyon in the Sonoran Desert portion of the Park. Research at Cottonwood examined the behavior, movements and reproductive ecology of tortoises using radio-telemetry and X-radiography. Scientists from the U.S. Geological Survey and the National Park Service have a long history of collaboration on tortoise research at Joshua Tree National Park.
Background & Importance
Agassiz’s desert tortoise populations are declining across their vast range in the Mojave and Sonoran deserts of Arizona, California, Nevada and Utah. Research is needed by resource management agencies to help recover populations and ultimately delist them from their status as threatened species under the Endangered Species Act.
Research is also needed to understand how populations will respond to utility-scale solar and wind energy development in desert tortoise habitat and to understand how they will respond to climate change. Large areas of the Southwestern Desert are developed or slated for development of large renewable energy projects, often requiring relocation of tortoises with results that are not always predictable. This research also supports the monitoring requirements for desert tortoises under the Coachella Valley Multiple Species Habitat Conservation Plan.
General Methods
Population censuses for tortoises often used plot-based surveys of one square mile study plots and this is how work started at the Barrow Plot in Joshua Tree National Park. Teams of scientists and technicians walk closely-spaced transects across the plot, noting all tortoises and tortoise sign (tortoise burrows, shells, and scat). Live tortoises are marked with unique and permanent notches on their shell that allow researchers to identify individuals at subsequent captures to measure growth, movements, etc. An advantage of plot-based surveys is that detailed recapture data can be analyzed over time for a cohort of marked tortoises since they generally don’t move great distances but they can live a long time. A disadvantage is that extrapolation of density results to surrounding areas can be flawed due to the patchy distribution of habitats and tortoises in the larger desert landscape.
Individual tortoises are measured and weighed, and sex is determined by examining shell shape and tail size. A small sample of tortoises are outfitted with radio transmitters that allow scientists to find them on subsequent visits to the field site. Adult female tortoises are X-rayed to determine the clutch size, clutch frequency (they produce up to two clutches per female per year in Joshua Tree National Park), and egg width. Previously published research by the Principal Investigator determined that this technique poses minimal risk to females and embryos. Small blood samples are occasionally taken to obtain DNA used to compare genetic differences between populations. In addition, wildlife “trail cameras” were used in 2015 to detect tortoise predators.
Important Results
To date, research has demonstrated that National Parks provide good protection to desert tortoise populations but they cannot protect them from all the negative impacts of drought, fire, disease, climate change, and road mortality. For example, analysis of long-term data from the Barrow Plot found a strong relationship between average rainfall amounts and survival of tortoises. Simply put, tortoise populations decreased in size during droughts. What was once a robust and large population of tortoises in the early 1990s declined precipitously by 2012. The effects of the drought also caused predators like coyotes to eat more tortoises instead of their normal prey: rabbits and rodents. Research at Cottonwood demonstrates that tortoises often spend most of the year in steep boulder piles and small mountains, even nesting there. Elsewhere in California, tortoises appear to prefer flatter ground with more sandy soils for digging burrows. Steep terrain may be underappreciated by resource managers as habitat for desert tortoises in some areas.
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Desert Tortoise Ecology and Renewable Energy Development
- Overview
Agassiz’s desert tortoises (Gopherus agassizii) approach the southern edge of their mostly Mojave Desert range near Joshua Tree National Park. Modern desert tortoise research started in the Park in 1978 when the first tortoise population census was conducted on a one square mile area in the Pinto Basin known as the “Barrow Plot.” U.S. Geological Survey research began at the plot in 1997 and continued intermittently until 2012, providing important long-term data on variation in population size and survivorship of long-lived tortoises during a period of widely variable environmental conditions. As a threatened species under the Endangered Species Act, resource managers need information on how best to manage for healthy tortoise populations. Other long-term research is focused on tortoise populations living near the mouth of Cottonwood Canyon in the Sonoran Desert portion of the Park. Research at Cottonwood examined the behavior, movements and reproductive ecology of tortoises using radio-telemetry and X-radiography. Scientists from the U.S. Geological Survey and the National Park Service have a long history of collaboration on tortoise research at Joshua Tree National Park.
Sources/Usage: Public Domain.USGS Research Ecologist Jeff Lovich gets ready to release a male desert tortoise into its cave-like shelter in southern Joshua Tree National Park after collecting data. Photo by Shellie Puffer, USGS, SBSC. Background & Importance
Agassiz’s desert tortoise populations are declining across their vast range in the Mojave and Sonoran deserts of Arizona, California, Nevada and Utah. Research is needed by resource management agencies to help recover populations and ultimately delist them from their status as threatened species under the Endangered Species Act.
Sources/Usage: Public Domain.A female Agassiz's desert tortoise at Joshua Tree National Park lounges in the entrance of her burrow, wearing a USGS radio. Research is also needed to understand how populations will respond to utility-scale solar and wind energy development in desert tortoise habitat and to understand how they will respond to climate change. Large areas of the Southwestern Desert are developed or slated for development of large renewable energy projects, often requiring relocation of tortoises with results that are not always predictable. This research also supports the monitoring requirements for desert tortoises under the Coachella Valley Multiple Species Habitat Conservation Plan.
General Methods
Population censuses for tortoises often used plot-based surveys of one square mile study plots and this is how work started at the Barrow Plot in Joshua Tree National Park. Teams of scientists and technicians walk closely-spaced transects across the plot, noting all tortoises and tortoise sign (tortoise burrows, shells, and scat). Live tortoises are marked with unique and permanent notches on their shell that allow researchers to identify individuals at subsequent captures to measure growth, movements, etc. An advantage of plot-based surveys is that detailed recapture data can be analyzed over time for a cohort of marked tortoises since they generally don’t move great distances but they can live a long time. A disadvantage is that extrapolation of density results to surrounding areas can be flawed due to the patchy distribution of habitats and tortoises in the larger desert landscape.
If winter rains are adequate, the Sonoran Desert in Joshua Tree National Park springs to life with wildflowers like these desert dandelions. Wildflowers are an important source of food and moisture for desert tortoises and may only be available for a few weeks of the year before they dry out and lose much of their nutritional value. (Credit: Shellie Puffer, USGS. Public domain.) Individual tortoises are measured and weighed, and sex is determined by examining shell shape and tail size. A small sample of tortoises are outfitted with radio transmitters that allow scientists to find them on subsequent visits to the field site. Adult female tortoises are X-rayed to determine the clutch size, clutch frequency (they produce up to two clutches per female per year in Joshua Tree National Park), and egg width. Previously published research by the Principal Investigator determined that this technique poses minimal risk to females and embryos. Small blood samples are occasionally taken to obtain DNA used to compare genetic differences between populations. In addition, wildlife “trail cameras” were used in 2015 to detect tortoise predators.
Sunset on the Barrow Plot in eastern Joshua Tree National Park. This area used to be inhabited by a healthy population of desert tortoises, but by 2012 many were dead due to the effects of protracted drought and predation by coyotes. Photo taken May 4, 2012.(Credit: Jeff Lovich, USGS. Public domain.) Important Results
To date, research has demonstrated that National Parks provide good protection to desert tortoise populations but they cannot protect them from all the negative impacts of drought, fire, disease, climate change, and road mortality. For example, analysis of long-term data from the Barrow Plot found a strong relationship between average rainfall amounts and survival of tortoises. Simply put, tortoise populations decreased in size during droughts. What was once a robust and large population of tortoises in the early 1990s declined precipitously by 2012. The effects of the drought also caused predators like coyotes to eat more tortoises instead of their normal prey: rabbits and rodents. Research at Cottonwood demonstrates that tortoises often spend most of the year in steep boulder piles and small mountains, even nesting there. Elsewhere in California, tortoises appear to prefer flatter ground with more sandy soils for digging burrows. Steep terrain may be underappreciated by resource managers as habitat for desert tortoises in some areas.
Sources/Usage: Public Domain. Visit Media to see details.A radioed male tortoise in the Santa Rosa Mountains, CA eating a beavertail prickly pear cactus. During the extreme drought conditions we are experiencing, that is the only food available for them with any moisture. They have to cope with all the little spiny gloccids (hair-like spines) that cover the pads of the cactus. Video provided by Dr. Bill Hoese at California State University, Fullerton provided to Jeff Lovich, SBSC, USGS with permission to use.
Sources/Usage: Public Domain.Scattered mature Joshua trees in this northernmost stand of Joshua trees are surrounded by abundant seedlings and saplings. USGS, public domain.
Sources/Usage: Public Domain.A desert tortoise eating at a study site. The video was taken in March 2022 by Jeff Lovich, SBSC. - Science
Below are other science projects related to this project.
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The United States is developing renewable energy resources, especially solar, at a rapid rate. Although renewable energy development is widely perceived by the public as “green technology,” construction, operation, maintenance, and eventual decommissioning of facilities all have known and potential negative impacts to natural resources, including plant communities and wildlife. This is especially...Turtle Ecology
Turtles are among the most recognizable and iconic of animals. Any animal with a shell and a backbone is a turtle whether they are called turtles, tortoises, or terrapins. In fact, terrapin is an Algonquian Native American name for turtle. Worldwide there are 356 turtle species on all continents except for Antarctica. The United States has more species than any other country with about 62...Desert Tortoise Ecology and Renewable Energy Development
The desert Southwest is experiencing rapid development of utility-scale solar and wind energy facilities. Although clean renewable energy has environmental benefits, it can also have negative impacts on wildlife and their habitats. Understanding those impacts and effectively mitigating them is a major goal of industry and resource managers. One species of particular concern is Agassiz’s desert... - Publications
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