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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 tortoise (Gopherus agassizii), a threatened species under the Endangered Species Act. Tortoises are declining throughout their range in the Mojave and Sonoran Deserts and resource managers need information on how best to manage for healthy tortoise populations. Threats to their recovery include road mortality; habitat destruction, including from energy development; disease and predators; and climate change. I have been studying the ecology and behavior of a desert tortoise population in an operating wind farm since 1995. The wind farm was permitted and developed after 1983. The overarching focus of this research was to determine how desert tortoise ecology and behavior at the site differ from tortoise populations living in more natural environments.
Background & Importance
Resource managers in the desert Southwest are challenged with trying to recover declining populations of the threatened Agassiz’s desert tortoise at the same time that large renewable energy projects are being permitted, developed and operated in desert tortoise habitat. To date, research conducted by the U.S. Geological Survey scientists represents the only body of published scientific literature on the effects of wind energy operation on desert tortoise ecology, behavior and survival. The information generated from this research allows resource managers to make informed decisions on project siting, design and mitigation strategies.
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. Individual tortoises are measured, weighed and sex is determined by examining shell shape and tail size. A small sample of tortoise 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 three clutches per female per year at the Mesa wind farm) 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 2013 to observe tortoise behavior and document use of tortoise burrows by other species.
Research in the 1990s suggested that tortoises occupied virtually all areas of the wind farm including those in proximity to turbines and roads. In fact, tortoise burrows were often constructed in proximity to some of these features. Over time, accumulation of thousands of capture points and tortoise burrow locations now suggests that tortoises generally avoid the areas of greatest road and turbine concentration. It is important to note that the effects of site operation and maintenance do not affect all species the same. The deaths of two golden eagles were documented due to collisions with turbines at that time. Tortoise mortality has resulted from site effects including a tortoise that was entombed in a culvert under a road during a rain event, a road-killed female tortoise with eggs, and a tortoise killed by a fire. While that particular fire was unrelated to site activities, turbine-induced fires were a frequent occurrence at the site and remain a threat to tortoises as shown by burn injuries on adult tortoises.
Tortoises at the wind farm have high reproductive output due to the location of the site. It is situated on the edge of the desert where winter rain that affects the abundance of tortoise food plants is more reliable. Females at the site produce more eggs per year (up to 15) than any other desert tortoise population known. They can produce up to three clutches per year with an average of about four eggs per clutch. Some females exhibit nest-guarding behavior at the site. Hatching success of eggs was over 65% after accounting for losses due to infertility, predators and other effects. Overall, growth, population structure and survivorship were not different when comparing Mesa tortoises to populations living in more natural environments.
Unfortunately, it is still unclear if wind farm development and operation is compatible with recovery of threatened populations of desert tortoises. The protected nature of the site does convey an advantage to the tortoises living there, but mortality has occurred due to site operation.
Although this research project spans almost 20 years as of the last year of study (2013), results have to be placed in the context of the population dynamics of a species that takes 14+ years to reach sexual maturity and can live over 50 years. Research has just covered a single generation of this population of tortoises. Also, evidence to date suggests that many of the tortoises at the site are old individuals and evidence of younger tortoises growing into the adult population is extremely limited. The long-term fate of this tortoise population will require additional monitoring and research.
Below are other science projects associated with this project.
Below are publications associated with this project.
Below are partners associated with this project.