East Africa’s Mara River Relies on Hippo Poop to Transport a Key Nutrient

Hippopotamuses play a crucial role in East Africa’s ecosystem, ingesting copious amounts of silica-laden grass and excreting 880 pounds of the nutrient into Kenya’s Mara River every day. In turn, this hippo poop-produced silica helps single-celled algae, or diatoms, build their cell walls, ensuring both the aquatic plant’s survival and that of the many organisms reliant on algae as a key food source, according to a new Science Advances study.

“Hippos act as a kind of conveyor belt, transporting silica from land to water,” Jonas Schoelynck, a biologist at Belgium’s University of Antwerp and the study's lead author, tells Ruby Prosser Scully at New Scientist.

Overall, Brian Kahn notes for Earther, more than three-quarters of the Mara River’s silica stems from droppings left by hippos, which spend half of their day eating upward of 55 pounds of grass and the other half lounging—and pooping—in communal pools. Although many of Africa’s grazing animals consume roughly the same amount of grass as hippos, they spend less time in the water, instead leaving their excrement scattered across the savannah. From here, Schoelynck explains to the Independent’s Alex Matthews-King, the feces decomposes, releasing nutrients back into the earth.

The researchers’ findings reveal how hippos support the region’s diverse wildlife, but as Matthews-King writes, poaching, habitat loss and human conflict pose increasingly devastating threats to the animal’s dwindling African population. If hippos drop significantly in number, they stop carrying silica from the savannahs to rivers and lakes, triggering a similar decline in diatoms that could give rise to toxic species such as “life-suffocating” cyanobacteria blooms. Eventually, these silica-deprived marine ecosystems could even become what Matthews-King terms “environmental dead zones.”

Unfortunately, it’s already too late to save a significant percentage of Africa’s hippo populations, particularly along rivers draining into the continent’s largest lake, Victoria. As Cosmos’ Mark Bruer reports, Africa’s hippos dropped in number by up to 20 percent between 1996 and 2004. And over the next three generations, this figure is expected to decline by an additional 30 percent.

There’s at least one potential upside to this downward trend: According to a 2018 study published in Nature Communications, excessive amounts of hippo poop deposited into the Mara cut off oxygen flow and suffocated fish. With fewer hippos around, these fish may have a better chance of survival. But at the same time, as Earther’s Kahn writes: “What taketh away the fish also giveth life to Lake Victoria’s diatoms.”

For now, Lake Victoria appears to have access to enough silica to last “several decades,” as Schoelynck notes in a press release.

“But in the long run there is probably going to be a problem,” he concludes. “If the diatoms do not get enough silicon, they are replaced by pest algae, which have all sorts of unpleasant consequences, such as a lack of oxygen and the associated death of fish. And fishing is an important source of food for the people of Lake Victoria.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's associate digital editor, history.