Tourists on African wildlife safaris might be surprised to learn the locations they are visiting are bullshit, literally. That's not to say the sites aren’t worth the journey, just that the reason they have so many wild animals is that many years ago, vast numbers of cattle defecated there.
Africans have been extensively cattle herding for some 3,500 years. Although the animals ranged widely during the day in search of food, their nights were spent in settlement corrals, safe from predators and thieves. There, cows, sheep, and goats did their business, concentrating nutrients in small locations.
"Ecologists have suggested that wildlife movements, including the Serengeti's famous wildebeest migrations, may be influenced by the location of nutrient-rich soil patches that green rapidly during the rains," said Professor Fiona Marshall of Washington University in St Louis in a statement. "Our research suggests that some of these patches may be the result of prehistoric pastoral settlement in African savannahs."
The migrations of herd animals like zebra and wildebeest are central to the ecology of Africa's grasslands. Marshall was part of a team that mapped these movements and used satellite imaging and soil analysis to study how they were influenced by particularly nutrient-rich soils.
In Nature, Marshall and co-authors show these routes favor oval-shaped hotspots 60-140 meters (200-450 feet) across where grass sprouts abundantly after rains. The herbivores cluster at these sites, particularly in the mornings, attracting predators and tourists alike. Volcanic rocks, fire and termite mounds all influence local soil fertility, but the authors argue these don’t explain the size and pattern of these hotspots.
Instead, they argue, the sites are where herders once built corrals.
By studying five locations known to have been used for this purpose between 1,500 and 3,700 years ago, the authors confirmed that, even after all this time, soils there are richer in essential nutrients, including phosphorous, magnesium and calcium. Nitrogen isotope ratios matched those in herbivore manure.
Such sites favor the growth of grasses over woody shrubs – not only providing most grazers with their preferred food, but offering less cover for ambush predators. Even birds congregate here, drawn by the invertebrates that populate richer soils.
Of course, the original dung is long gone. However, nutrient boosted locations attract migratory animals, who recycle much of what they have consumed on location, either in dung of their own, or through their bones if they die on-site.
This concentration of nutrients into hotspots further depletes surrounding areas, but senior author Professor Stanley Ambrose of the University of Illinois said the concentrations makes the entire ecosystem more resilient.
Marshall argued: “When present-day herders are mobile and live at relatively low densities, they have few long-term negative impacts on the environment and some significant positive ones." However, the often fierce debate as to the ecological effects of higher density but mobile livestock populations still has a long way to run.