Spy satellite photographs have documented a desert invasion. But don't stockpile food and water in some survivalist's bomb shelter just yet. This invasion consists of desert plants that have converted parts of the original short-grass prairie in the American Southwest into a more barren land that is full of thorny, spindly flora that no cow wants to eat. Chief among these invaders is the 8-foot-tall woody desert shrub known as honey mesquite -- the same species that is popular among suburbanites as charcoal briquettes for their backyard barbecues. Botany professor William Schlesinger of Duke University in Durham, N.C., and his colleagues suspected that these desert plants were spreading but wanted more hard evidence. They needed to see how quickly this desertification was happening at their study site in the Jornada Basin. If it was a short-term process that began quickly, that would indicate that a naturally occurring fluctuation such as sudden drought was the probable cause. On the other hand, if plants such as honey mesquite gradually took over grassland species in a prolonged process, that would suggest that human causes such as overgrazing and excessive fire suppression were to blame. To get an idea of the speed and extent of the spread of honey mesquite, they turned to an archive of old spy photographs that was recently declassified for scientific use by a government committee headed by Vice President Al Gore (Science, Oct. 1, 1999). The material, located at the Earth Research Observation System data center in Sioux Falls, S.D., contains photos taken from a variety of satellite reconnaissance systems operating at different wavelengths over decades. With the aid of a National Science Foundation grant, the researchers combed through the archive for images of their study site. By adding old photos taken by airplanes to the mix, they were able to document the spread of honey mesquite going back about 60 years, albeit with some small gaps in the historical record. "We've pretty much ruled out Mother Nature," said Schlesinger. "The loss of grassland is largely due to man-made factors." The satellite imagery was invaluable, he added. "We couldn't have done it anywhere near as well. We would not have had the frequency of coverage necessary." He hopes to locate more old photographs of the area and fill in the missing years.