Depictions of animals in ancient Egyptian artifacts have helped scientists assemble a detailed record of the large mammals that lived in the Nile Valley over the past 6,000 years. A new analysis of this record shows that species extinctions, probably caused by a drying climate and growing human population in the region, have made the ecosystem progressively less stable. The study, published September 8 in Proceedings of the National Academy of Sciences (PNAS), found that local extinctions of mammal species led to a steady decline in the stability of the animal communities in the Nile Valley. When there were many species in the community, the loss of any one species had relatively little impact on the functioning of the ecosystem, whereas it is now much more sensitive to perturbations, according to first author Justin Yeakel, who worked on the study as a graduate student at the University of California, Santa Cruz, and is now a postdoctoral fellow at the Santa Fe Institute.

Around six millennia ago, there were 37 species of large-bodied mammals in Egypt, but only eight species remain today. Among the species recorded in artwork from the late Predynastic Period (before 3100 BC) but no longer found in Egypt are lions, wild dogs, elephants, oryx, hartebeest, and giraffe.

"What was once a rich and diverse mammalian community is very different now," Yeakel said. "As the number of species declined, one of the primary things that was lost was the ecological redundancy of the system. There were multiple species of gazelles and other small herbivores, which are important because so many different predators prey on them. When there are fewer of those small herbivores, the loss of any one species has a much greater effect on the stability of the system and can lead to additional extinctions."

The new study is based on records compiled by zoologist Dale Osborne, whose 1998 book The Mammals of Ancient Egypt provides a detailed picture of the region's historical animal communities based on archaeological and paleontological evidence as well as historical records. "Dale Osborne compiled an incredible database of when species were represented in artwork and how that changed over time. His work allowed us to use ecological modeling techniques to look at the ramifications of those changes," Yeakel said.

The study had its origins in 2010, when Yeakel visited a Tutankhamun exhibition in San Francisco with coauthor Nathaniel Dominy, then an anthropology professor at UC Santa Cruz and now at Dartmouth. "We were amazed at the artwork and the depictions of animals, and we realized they were recording observations of the natural world. Nate was aware of Dale Osborne's book, and we started thinking about how we could take advantage of those records," Yeakel said.