Hippos crucial to silicon cycle in African river food webs

Africa’s rivers and lakes need hippo poo to thrive, and declining hippo numbers threaten food chains and human health, research has revealed.

Published in the journal Science Advances, the study highlights the vital role of hippopotamus waste in savannah ecosystems, including rivers flowing into Lake Victoria. 

A team headed by Jonas Schoelynck from the University of Antwerp in Belgium studied hippopotamuses (Hippopotamus amphibious) living along Kenya’s Mara River to find out how silicon extracted from plants gets into the rivers and lakes. 

Silicon is an essential building block used by single-cell algae called diatoms, which provide the base for the food chain, and are also responsible for much of the carbon uptake in the region. Globally, diatoms account for about 20% of all photosynthesis.

The researchers say that although the role of plants in capturing silicon from the soil and releasing it as they decompose is well understood, the function of large grazing fauna in the process had not been explored. {%recommended 8278%}

The role of hippos turns out to be critical. The team found that they contribute a massive 76% of the flow of silicon in the Mara River, leading researchers to dub the animals “land-to-river silicon pumps”. 

It all comes down to the animals’ unique habits. 

Each night, the semi-aquatic giants eat large quantities of silicon-rich savannah grasses, then typically rest communally in “hippo pools” in the river during the day. While resting, they excrete into the water – producing 11.1 tonnes a day in the Maasai Mara National Reserve, where much of the sampling was done. 

Hippo faeces contain large amounts of silicon, which is released as the dung dissolves.

This silicon then washes downstream into lakes and is used by diatoms to build their intricate walls. Without it, they cannot grow.

Changes in the population of hippo “silicon pumps” could have widespread ecological and climate impacts, Schoelynck and colleagues note.

“Unintended downstream repercussions to Lake Victoria and other Rift Valley lake ecology are possible if hippos are removed from the savannah ecosystem,” they write.

“If riverine [silicone] delivery to these lakes is reduced, then this could induce

algal community shifts with knock-on effects on the food web structure

and human well-being in that region.”

Unfortunately, this is already happening. Most rivers draining into Lake Victoria have been cleared of hippo populations. Throughout Africa, species numbers decreased by up to 20% from 1996 to 2004 and are expected to decrease by a further 30% over the next three generations.

In Lake Victoria, changes in the balance of silicon and other elements have resulted in a decline of diatom populations and diversity, and an increase in cyanobacteria, which can be toxic to humans and animals.

“Our work suggests that decimation of the hippo population over the past decades could have contributed,” the researchers write.