We owe some fertile landscapes to lots of ancient poop

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Some of the African savanna’s most fertile and biologically diverse wildlife hotspots owe their vitality to heaps of dung deposited there over thousands of years.

“Many of the iconic wild African landscapes, like the Mara Serengeti, have been shaped by the activities of prehistoric herders over the last 3,000 years,” says anthropologist Fiona Marshall, professor at Washington University in St. Louis and a senior author of the study in Nature.

“Our research shows that the positive impacts of increased soil fertility in herder settlement corrals can last for thousands of years,” she says. “The longevity of these nutrient hotspots demonstrates the surprising long-term legacy of ancient herders whose cattle, goats, and sheep helped enrich and diversify the vast savanna landscapes of Africa over three millennia.”

Kenya’s hotspots

The study, which focuses on wildlife hotspots in Kenya, documents how the cultural practices and movement patterns of ancient herders and their livestock continue to influence an array of seemingly wild and natural phenomena.

“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,” Marshall says.

“Our research suggests that some of these patches may be the result of prehistoric pastoral settlement in African savannas.”

Based on satellite imaging and detailed analyses of soil nutrients, isotopes, and spatial characteristics at ancient Neolithic herder sites in East Africa, the study offers a surprisingly simple explanation for how oval-shaped wildlife hotspots measuring about 100 meters in diameter evolved in a region where grasslands are naturally low in soil nutrients—manure happens.

Soil enrichment

For millions of wildebeest, zebras, gazelles, and the carnivores who hunt them, migration patterns revolve around an age-old quest for the lush grasses that spring up on fertile soils after seasonal rains.

While other research shows that fire, termite mounds, and volcanic sediments may contribute to the varying fertility of savanna soils, the new study confirms that ancient livestock dung has long been an important catalyst in an ongoing cycle of soil enrichment—one that continues to attract diverse wildlife to the sites of abandoned livestock corrals.

For 2,000-3,000 years, savanna grasslands of southwestern Kenya were home to groups of nomadic herders who moved their camps often in search of greener pastures. They herded the livestock that grazed the open savanna by day into small, oval-shaped corrals within settlements at night for protection from predators and rustlers.

As manure piled up in these temporary corrals, scarce nutrients from surrounding grasslands also began to accumulate, creating fertility hotspots that attracted herds of both wild and domesticated grazers for years to come.

Thus, over the millennia, the cultural practices of mobile herders had the unintended consequence of creating spatially stable fertile environmental niches for an array of wildlife, the study reports.

3,000-year-old dung

While the herding activities of mobile communities of modern and historic Maasai and Turkana have been shown to enrich savanna soils, little has been known about the lasting impact of Africa’s earliest food producers, herders who moved south from the Sahara 2,000-5,000 years ago.

Researchers examined five Neolithic pastoral sites in southern Kenya, ranging in age from 1,550-3,700 years, and found that the sites still contain nutrient-rich sediments resulting from livestock dung deposited as far back as 3,000 years.

As compared to the surrounding savanna, ancient pastoral sites were found to have significantly higher levels of phosphorous, magnesium, calcium, and other nutrients essential for plant growth and animal health and reproduction.

Observed from the ground and via satellite, these ancient pastoral sites appear as treeless, open grassy patches within larger expanses of wooded savanna grasslands.

Excavations show that the abandoned settlement footprints are loosely defined by a visually distinct, fine-grained layer of grey sediment, now located about a half-meter below the surface and up to a foot thick in places.

Over the millennia, the increasing fertility of these ancient settlement sites have increased the spatial and biological diversity of savannas.

Ecological research shows that in fertile corral soils grasses tend to outcompete woody vegetation, creating open glades of nutritious grasses where grazing animals are also less vulnerable to predators.

Wild herbivores that forage there, such as gazelle, wildebeest, zebra, and warthogs, exert a positive influence on plant productivity and nutrient turnover. Hotspot fertility generates rapid plant regrowth after rains and provides essential nutrients for pregnant and lactating herbivores.

High fertility soils also means more soil invertebrates—worms, dung beetles, and other insects, which, in turn, attract more of the birds, reptiles, and elephant shrews that feed on them. As reported in a 2013 study, a species of gecko that thrives in these nutrient hotspots is virtually non-existent in the open savanna between them.

Ecosystems around the world

By establishing the role that the early herders played in enriching Africa’s savanna soils, the new study offers more evidence for the entwined nature of human activities and other ecological influences on the landscapes in which we live.

In the United States, recent studies have shown that early Native Americans used fire to manipulate the migration patterns of buffalo herds, changing the climate and ecological balance of the Great Plains in the process. Researchers suspect that ancient pastoralists also may also have influenced grassland ecology in pastoral regions of Central Asia and South America.

In the Serengeti, research shows that the modern wildebeest migrations also contribute to the redistribution of scarce savanna nutrients as they drown or fall prey to crocodiles while crossing the Mara River. Their dead bodies add large quantities of phosphorus, nitrogen and carbon to the river ecosystem each year.

In showing that ancient herding sites may still be influencing modern wildebeest migrations, the finding are relevant to ongoing debates over the effect of livestock grazing on biological diversity and wildlife survival in grasslands, both in the Serengeti and around the globe.

“Ecologists have shown that 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,” Marshall says.

“Our findings provide a new perspective on how human herding activities have influenced, and sometimes enriched, the ecology of African grasslands. From a policy perspective, it suggests that there are ecological costs to increasing sedentization of herders.”

Additional coauthors are from Washington University in St. Louis, the University of Illinois, the University of Haifa, and the National Museums of Kenya.

The International Center for Energy, Environment and Sustainability, the Nano Research Facility, the Liu and the Kidder laboratories at Washington University in St Louis, the Environmental Isotope Paleobiogeochemistry Laboratory at the University of Illinois, and the National Science Foundation funded the work.

Source: Washington University in St. Louis