Desert Tortoise Ecology, Health, Habitat, and Conservation Biology

Science Center Objects

The desert tortoise is listed as “threatened” under the Endangered Species Act. USGS WERC scientists, along with project partners have been conducting long-term analyses on how changes in the southwestern deserts of the United States can affect desert tortoise populations. Dr. Todd Esque and his team are investigating how habitat disturbances and restoration projects influence tortoise populations through research in nutrition, physiology, epidemiology, genetics, and demographics.

Tiny desert tortoise found during a population census

This tiny tortoise, about the size of a half dollar, was found during a population census of desert tortoises in the extremely rugged McCollough Mountains of southern Nevada. The tortoise was assigned an individual number (on top of the shell) in anticipation of measuring growth rates in future years, and monitoring survivorship among several different populations. A minute blood sample was taken from this and the other tortoises that were found to understand relatedness among the tortoises at this site, understand which tortoises make contributions to future generations, learn about connectivity among tortoise populations on either side of the mountain range and how solar development and highways influence natural connectivity among populations.(Credit: Todd C. Esque, USGS WERC. Public domain.)

Desert Tortoise Ecology

Desert tortoise diet, foraging ecology, and nutrition are key factors to understand the health and growth of desert tortoise populations. Plant communities are a food source and provide cover from predators and environmental extremes. In addition, diet can influence egg production. Dr. Todd Esque and his collaborators are investigating how annual grasses and herbaceous annuals influence the physiology and ecology of desert tortoises. Information about production, cover, physical structure and species composition of plant communities provide data that can be used to analyze relationships between tortoises and their habitats. Dr. Todd Esque and WERC researchers with federal management agencies developed a sophisticated database to integrate data such as (but not limited to): locality information, dates, participants, habitat parameters, tortoise presence, prevalence, and demographics in relation to the mitigation of the factors that threaten this species. This project is a collaborative effort between USGS, USDI- Bureau of Land Management, U.S. Fish and Wildlife Service, and private industry.

Dr. Todd Esque, Dr. Kristina Drake, and collaborators are conducting experiments to identify how changes in tortoise diets influence their health and growth. Recent work indicated how just one growth season on a diet including the invasive annual grass, red brome (Bromus rubens) severely reduced growth and survival of juvenile desert tortoises. In comparison to four species of native wildflowers, the red brome diet has poor nutrition, and can cause internal and external injuries to small tortoises. Because red brome can occur throughout the range of the tortoise, it may work as a demographic filter that reduces the number of individual tortoises being recruited into populations, thus partially explaining population declines throughout most of its range and responsible for listing the tortoise under the Endangered Species Act. New research being implemented tests the effectiveness of desert restoration programs in providing diet and cover plants that are beneficial for juvenile desert tortoise by releasing them in combination with restoration experiments. 

 

Effects of fire on desert tortoise habitat

This old male tortoise was experimentally translocated to a site that had been previously burned in wildfire to understand how tortoise grow and survive in habitat that is disturbed by fires. While tortoises will eat the bright green brome grass, commonly named red brome (Bromus rubens), it is nutritionally less valuable than native wildflowers that are the preferred and healthier food for desert tortoises. The stiff awns at the top of the grass can injure desert tortoises and other wildlife. (Credit: Todd C. Esque, USGS WERC. Public domain.)

Effects of Wildfire on Desert Tortoise Habitats

Fire is widely regarded as a key evolutionary force in fire-prone ecosystems. Ecosystems subjected to novel fire regimes may experience profound changes that are difficult to predict, including losses of vegetation cover and diversity, losses to seed banks, changes in demographic processes, increased erosion, changes in nutrient availability, increased dominance of invasive species, and transitions to alternative ecological states. 

In the deserts of the southwestern United States, fire size and frequency have increased substantially over the last several decades because of an invasive grass/fire feedback cycle. Invasive annual species are able to establish fuel loads capable of sustaining large-scale wildfires following years of high rainfall. Native perennial vegetation is not well-adapted to fire in these environments and may be reduced or eliminated, potentially affecting wildlife species such as the desert tortoise.

USGS WERC has been conducting analyses on the long-term impacts of burned areas on desert habitat, assessing potential implications for land management and desert tortoise conservation.

 

Desert Tortoise Population Connectivity – Gene Flow and Demographic Processes

The expansive valleys and mountain passes in the Mojave Desert comprise of the habitat for the Mojave Desert tortoise. Increasing demand for alternative energy, is creating greater pressure on ecosystems in the desert southwest. As urbanized areas, energy development, recreation areas, military training sites, and their associated infrastructure are established across southwestern landscapes, conservation corridors are invoked with increasing frequency as mitigation tools for large scale man-made disturbances.

Conservation corridors theoretically allow gene flow continuity among populations of species. The desert tortoise is a slowly reproducing animal, which can make it difficult to monitor gene flow. WERC researchers are using modern technology to identify existing corridors that have the potential to promote gene flow. Using network analysis to document tortoise-to-tortoise contacts, data can illustrate tortoise sociality and assisting in the analysis genetic connectivity or disease transmission dynamics.

In collaboration with the University of Nevada – Reno, the US Bureau of Land Management, the US Fish and Wildlife Service, and private industry, Dr. Todd Esque and Dr. Amy Vandergast have established a network of 10 replicated research study sites to determine the extent of population connectivity and demographic processes in proximity to man-made landscape pinch points created by solar development sites, and natural topographic features (mountain passes), in comparison to open desert sites.