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Hippo excrement is vital for life in rivers

  • Written by  Katie Burton
  • Published in Wildlife
Hippo excrement is vital for life in rivers
18 Jul
A look at the contribution of hippos to the savannah ecosystem has revealed that their preferred lifestyle is crucial for maintaining life in the rivers they laze in

To some, it might sound like an enviable lifestyle. Hippos in the wild spend their night grazing grass on the savannah and their days digesting lazily in the water, protected from the African sun. But enviable or not, this routine appears to be more important than previously noted, and it all comes down to excrement.

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An important consequence of the hippo lifestyle is that vast quantities of their excrement, made up of the savannah grasses, finds its way into rivers and lakes. This deposit system is unusual as most animals stick to the grasslands for the majority of their business. By studying the hippos of the Mara river, situated within Kenya’s Masai Mara reserve, a group of international scientists have now demonstrated that this process transfers nutrients and chemicals from the land to the water in a way that is crucial for the health of the marine ecosystem. 

A bloat of hippos swimmingHippos are unusual becasue they spend their days digesting lazily in the water Of particular interest to the researchers, whose results are published in the journal Science Advances, is the chemical silicon, which the grasses absorb from the soil. By measuring the amount of silicon entering the river and the amount leaving it they identified an increase that had to come from within the Massai Mara itself. Through further testing they were then able to demonstrate that 73% of this increase was due to hippo excrement. 

The finding is significant due to the crucial role silicon plays in maintaining marine life. Silicon is vital for diatoms, a type of phytoplankton that produce oxygen and are the foundation of the food chain in most aquatic systems. In addition, silicon offers a balancing effect to excess nitrogen and phosphorus – an increasing problem in African river systems due to run-off from agricultural processes. Because of these crucial attributes the decline of hippos, which are considered a vulnerable species by the IUCN due to hunting and habitat loss, is concerning. Further loses could have consequences for the Mara River and for Lake Victoria, one of African Great Lakes which the river feeds. 

‘While Lake Victoria’s silicon supply will last for the next few decades, there will likely be a problem in the long term,’ wrote Jonas Schoelynck, lead author of the study from the University of Antwerp. ‘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.’ This effect could be exacerbated in other regions of Africa, such as the Congo Basin, where the soils are already lower in silicon and where wildlife may not be as protected. 

A lonely hippo almost submerged in the riverHippos are considered a vulnerable species due to hunting and habitat loss Schoelynck adds that in other countries where silicon has already been depleted from river catchment areas due to replacement of forest with agricultural land, engineers are considering adding silicon to the soil artificially. But he stresses that the long-term effects of this practice aren’t known, and that the only sustainable mitigation measure is to preserve those natural ecosystems which naturally transfer silicon to water-tidal marshes are one example. 

‘What I hope, and I think I have succeeded already, is to get silica research attention, because it is an understudied element and it is really crucial for our planet, for food webs and the climate,’ says Schoelynck. When it comes to the role of hippos this means understanding that ‘they are much more than a fun tourist attraction, they really are a key species in the savannah ecosystem, and we need to start realising this’.

This was published in the July 2019 edition of Geographical magazine

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