Ecology and Biology of Desert Tortoises

Science Center Objects

WERC wildlife biologist Dr. Kristin Berry has more than forty years of experience studying Agassiz’s desert tortoise. As the lead on several USGS projects, Dr. Berry continues to lend her expertise to investigating the status, genetics, and behavior of Agassiz’s tortoise populations in the Mojave Desert.

Agassiz’s desert tortoise (Gopherus agassizii) was listed as “threatened” under California’s and the federal Endangered Species Acts in 1989 and 1990. Dr. Kristin Berry is leading scientific studies to better understand the tortoise’s ecology and offer her findings to the U.S. Bureau of Land Management, National Park Service, and other government agencies.

Demography of Agassiz’s Desert Tortoise Populations

This project is based on both short- and long-term studies (1 to 34 years) of status and trends in desert tortoise populations living in the Mojave and western Sonoran deserts. The project draws from several long-term plots and several other short-term study areas throughout the region. Research involves analysis of changes in densities, sex ratios, size-age class structure, death rates, causes of death, and condition of habitat. Dr. Berry has published papers and prepared reports on the negative associations between tortoises and recreational vehicle use and livestock grazing, among other human uses. Dr. Berry and her USGS team are currently analyzing data and preparing a manuscript for publication on a 34-year study at the Desert Tortoise Research Natural Area in the western Mojave Desert.

 

Genetics of Agassiz’s Desert Tortoises

Dr. Berry is involved in two projects on the genetics of Agassiz’s desert tortoise populations in the Mojave and western Sonoran deserts. Together, she and a team of scientists from the Royal Ontario Museum, Toronto and the University of Arizona identified a new species of desert tortoise, the Sonoran desert tortoise (Gopherus morafkai), and described the genetics of a hybrid zone in northwestern Arizona where both Agassiz’s desert tortoise and the Sonoran desert tortoise co-occur. This same team is examining regional genetics of Agassiz’s desert tortoise and testing the reliability of different analytical methods in drawing population boundaries. In the second project, Dr. Berry studies the genetics within an Agassiz’s desert tortoise population in the Central Mojave Desert. Specifically, she is looking at the genetic relationships between individuals and how behaviors such as dominance, aggression, and courtship might be affected.

 

Behavior of Agassiz’s Desert Tortoises

These projects examine the behavior and ecology of adult Agassiz’s desert tortoises:  (1) forage preferences; (2) use of burrows; and (3) social behavior and how dominance in adult males affects position in the male hierarchy, home range size and use, access to resources (food, cover sites), and access to female tortoises. With her former graduate student, Dr. Berry published a paper on the plant foods preferred by tortoises in the western Mojave Desert. Tortoises are very limited in what they can eat, preferring succulent green plants in the legume, evening primrose, mallow, and a few other families. They do not prefer non-native grasses. Dr. Berry and her USGS team reported on the types of shelters tortoises require to escape summer heat and the cold of winter. Burrows, dens, and rock shelters with long tunnels with substantial soil and rock cover are important for escaping the extremes of temperature and lack of moisture.