Desert Tortoise Ecology in Joshua Tree National Park

Science Center Objects

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.

Yellow poppies flowering in Joshua National Park
If winter rains are adequate, the Sonoran Desert in Joshua Tree National Park springs to life with wildflowers like these poppies. 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.)

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, 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 clutch size, clutch frequency (they produce up to two clutches per female per year in Joshua Tree National Park) and egg width. Previous 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.

U.S. Geological Survey Research Ecologist Jeff Lovich gets ready to release a male desert tortoise into its cave-like shelter in
U.S. Geological Survey 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.(Credit: Shellie Puffer, USGS. Public domain.)
Landscape consisting of arid shrubland.
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.)