Hippos help African lakes breathe

UAntwerp biologists find evidence that dwindling hippo numbers could lead to food crisis​.

Hippopotamus droppings play an important role in the ecosystems of African lakes and rivers. Because of a sharp decline in the number of hippos, these ecosystems are now being jeopardised. “In the long run, this could lead to food shortages around Lake Victoria,” say biologists at UAntwerp.

Hippo  

Hippos living in the wild have a unique lifestyle. At night they graze the savannas, consuming dozens of kilos of fresh grass. During the day, they mostly just laze about together in the water, far from enemies and protected from the scorching African sun. As they laze away the day, they digest the grass, resulting in enormous amounts of excrement.

Nijlpaardenmest  Hippo

“Hippos are unique compared to other large grazing animals in the savanna,” explains biologist Jonas Schoelynck (UAntwerp). “The nutrients contained in the droppings of most grazers usually end up back in the savanna, where they are reabsorbed by the plants. This is not the case with hippos: they act as a kind of nutrient conveyor belt to the water in rivers and lakes.”

Problems for the food web
In a recent study published in the journal Science Advances, Schoelynck and his colleague Eric Struyf show the vital importance of this conveyor belt function to aquatic life. Schoelynck: “The grass contains silicon. The plants absorb this silicon from water in the soil, which makes them more robust and protects them from disease, as well as from being eaten by small animals to a certain extent.”

The researchers have now shown that it is largely thanks to hippos that this silicon ends up in subtropical rivers. “Silicon is vital for diatoms, which are single-celled algae living in water that produce oxygen and are the foundation of the food chain in most aquatic systems,” says Struyf. “In the event of a shortage of silicon, the diatom population could collapse, with dire consequences for the entire food web in a lake or river.”

Important fishing
The research was part of an expedition to the Mara River in the Masaai Mara Nature Reserve. Hippos are major contributors to the total amount of silicon in the river which runs through the park and eventually flows into Lake Victoria. This means their contribution to the silicon concentration in Lake Victoria is also significant.

However, the number of hippos in Africa has dwindled in recent years due to hunting and loss of habitat, which has led to a drastic reduction in this conveyor belt function. Schoelynck: “While Lake Victoria’s silicon supply will last for the next few decades, there will likely be a problem in the long term. When the diatoms run out of silicon they will be replaced by pest algae, and this will have all kinds of unpleasant consequences, including oxygen depletion and associated fish mortality. The latter could be a real problem, as fishing is an important means of putting food on the table for the people living around Lake Victoria.”

Hippo
 

Unexpected consequences
The research shows how seemingly unconnected processes in land ecosystems can have unexpectedly large consequences for the vital and very biodiverse aquatic systems in Africa. On the entire continent of Africa, up to 90% of hippos have been killed off in recent decades. The Antwerp scientists have now proved that this is likely to change the nutrient balance in African rivers and lakes.