Detailed depictions of predator and prey interactions have been chronicled in artwork found in tombs of the likes of Tutankhamun and Khnumhotep II. Now, researchers examining Egyptian mammal records over the last several millennia found that local species extinctions were often linked with abrupt climate drying and the growth of human populations. The study, published in Proceedings of the National Academy of Sciences this week, traced the collapse of an ecological network over 6,000 years ago.
In that time, there were five episodes of dramatic changes in the mammal community. Of those, "there were three large pulses of aridification as Egypt went from a wetter to a drier climate, starting with the end of the African Humid Period 5,500 years ago when the monsoons shifted to the south," says Justin Yeakel of the Santa Fe Institute. "At the same time, human population densities were increasing, and competition for space along the Nile Valley would have had a large impact on animal populations."
The once rich and diverse community included 37 species of large mammals, which has since dwindled down to the eight that remain today. Among those recorded in artwork from the late Predynastic Period before 3100 BC -- but are no longer found in Egypt -- are lions, wild dogs, elephants, oryx, hartebeest, and giraffes.
To reconstruct how animal communities changed, Yeakel and colleagues examined paleontological, archaeological, and historical records of mammals living in the Nile Valley during the Holocene. Then they used ecological modeling and analyses of predatory-prey networks to look at the effects of these changes over time. Pictured to the right are carvings from the ivory handle of a Predynastic ritual knife.
The team found that local extinctions of mammals led to the steady decline of community stability. When there were lots of species, the loss of any one had relatively little impact on how the ecosystem functioned -- but in more recent times, they’ve become much more sensitive to disruptions.
"As the number of species declined, one of the primary things that was lost was the ecological redundancy of the system,” Yeakel explains in a news release. Having multiple herbivores like gazelles, ibex, and sheep meant many different predators could prey on them. With fewer 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 team also used their model to predict extinction risk, using artwork to test their predictions. Sure enough, species that were more sensitive in theory did in fact disappear sooner, Science reports.
Dry periods also coincided with upheaval in human societies, including the collapse of the Old Kingdom 4,000 years ago and the fall of the New Kingdom 3,000 years ago. The most recent major shift in mammalian communities occurred about 100 years ago: The loss of wild boar, white antelope, and the leopard had a disproportionately large impact on ecosystem stability. “As you lose diversity, you lose redundancy in the system,” Yeakel tells Science, “and the importance of each organism becomes magnified.”
Images: Ashmolean Museum from J.D. Yeakel et al., PNAS 2014 (top), Charles Edwin Wilbour Fund, Brooklyn Museum (middle)