Why water ecosystems in Africa are dependent on hippos

A new study from the German Research Centre for Geosciences GFZ is describing the important role that hippos play in the ecosystem of African lakes and rivers. It turns out that, through the large quantities of fecal matter that they leave behind in the water, hippos are delivering much-needed silicon.

“Hippos differ from other large grazing animals in the savannah,” said study first author Jonas Schoelynck. “The nutrients in the excrements of most grazers largely end up back in the savannah again, where they are reabsorbed by the plants. This is not the case with hippos: they act as a kind of nutrient pump from the land to rivers and lakes.”

For the investigation, the researchers travelled to the Mara River in the Masaai Mara Nature Reserve in Kenya where they collected samples.

“The grass that hippos eat contains silicon,” said Schoelynck. “The grass absorbs this silicon from the groundwater. It gives it the strength it needs, protects it from disease and, to a limited extent, from grazing by small animals.”

Back in the lab, Patrick Frings analyzed the isotopic composition of silicon in samples of plants, water, and hippo excrements from the Mara River. “The isotope analysis enabled us to reconstruct the transport path of the silicon,” explained Frings.

The researchers found that a large part of the silicon in the Mara River was transported there by hippos. The grazing animals absorbed a total of 800 kilograms of silicon per day through the plants they ate, half of which ended up in the water via excretion of hippo faeces.

Calculations made by the researchers revealed that the hippos’ silicon contribution influences over 76 percent of the total silicon transported along the Mara River through various ecological mechanisms.

“Our results are completely new,” said Frings. “So far, it has not been assumed that grazing wild animals could have such an influence on the transport of silicon from land to lakes. This process is crucial for the entire land-water ecosystem. In the past, however, it has simply been overlooked.”

Silicon is critical to the survival of diatoms, which are unicellular algae that produce oxygen and form the basis of the food chain in many water ecosystems. This algae population can collapse if there is not enough silicon, resulting in harmful consequences for the entire food web in the lake or river, according to the researchers.

In Africa, hippos are vanishing due to hunting and habitat loss. The experts said that their function as animal silicon pumps has thus been partially lost.

“Lake Victoria, into which the Mara River flows, can survive for several decades with its current silicon supply,” said Schoelynck. “But in the long run there is probably going to be a problem.”

“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.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Jonas Schoelynck