Six thousand years ago, Egyptian lions hunted wildebeests and zebras in a landscape that resembled the Serengeti more than the Sahara. Since then, the number of large mammal species has decreased from 37 to eight, says quantitative ecologist Justin Yeakel of the Santa Fe Institute. New research using ancient animal depictions tracks the collapse of Egypt’s ecological networks one extinction at a time, offering a glimpse into how climate change and human impacts have altered the structure and stability of ecosystems over millennia.
People in Egypt have been observing the natural world since long before they built the pyramids. Prehistoric rock drawings depict hippopotamuses, giraffes, elephants, hartebeests, and foxes. Ostrich and ibex are carved into a 5000-year-old ceremonial palette. Later, hunting scenes on ancient Egyptian tombs teemed with wildlife. Yeakel created a timeline based on existing records from paleontology, archaeology, and art, which picks up about where the fossils leave off and zooms in on a much shorter time scale. He used it to find out which species died out when and how their loss affected the rest of the ecological network.
As Yeakel and co-authors report online today in the Proceedings of the National Academy of Sciences, the most dramatic shifts in climate and land use accompanied the most dramatic shifts in the number of predators relative to the number of prey species. Three of the five shifts happened at the same time as sudden dryings of the Nile Valley that may also have catalyzed the rise and fall of dynasties. A fourth shift occurred with population growth and industrialization in modern Egypt.
The researchers explored whether some of the ecological networks were more vulnerable than others. For each mammal community of the last 6000 years, they assembled possible predator-prey networks based on the body size of the animals (a cheetah is more likely to hunt a hedgehog than vice versa)—a system that correctly predicts who eats whom up to 74% of the time in modern African systems. Then they modeled the stability of each ecological network: How likely is a small change to cause a complete collapse?
The most ancient and species-rich ecosystems were resilient. But the networks became less and less stable through time. With each extinction, the mammals that depended on that species become more vulnerable to collapse themselves. The loss of the wild boar, the white antelope, and the leopard in the last 150 years caused the most precipitous drop in stability yet. “As you lose diversity, you lose redundancy in the system, and the importance of each organism becomes magnified,” Yeakel says.
As a result, the remaining eight large mammal species in Egypt—including striped hyenas, golden jackals, and the Egyptian fox—are now more vulnerable than they’ve been in more than 12,000 years, Yeakel says. Some of the most important of those eight are already in trouble. By calculating the stability of the modern Egyptian predator-prey network with and without each species, the team found a few whose presence stabilizes the whole system—primarily small herbivores eaten by many predators, including gazelles, ibex, and Barbary sheep. One gazelle species is now critically endangered, and Barbary sheep are less common in the Western Desert than they were even 30 years ago, says Egyptologist Salima Ikram of American University in Cairo, who was not involved in the work.
The researchers also used their model to predict extinction risk, a measure that’s important for conservation planning but hard to observe. The artistic record offers an unusual chance to test these predictions on extant species at shorter timescales. Looking back in history, the researchers found that the theoretically more sensitive species did in fact disappear from Egypt sooner.
Still, this window on the past is less than perfect, warns Linda Evans, an environmental historian at Macquarie University in Sydney, Australia, who studies representations of animals in ancient Egyptian art. Just because an animal appears on a tomb doesn’t necessarily mean it existed at the time, as artists in later periods copied older tombs. Ancient Egyptians “didn’t just depict what they saw,” Evans says. “Egyptian art has a grammar to it. You have to be really careful and guarded about the conclusions you draw.”
Ikram agrees that ancient Egyptian artists had more on their minds than what they saw in the real world. But while Yeakel’s timeline may not be perfect, it is probably “a good mapping of what was present when and where, and how the different species affected one another.”