Article by Shreya Dasgupta.
In the East African savanna landscape, hippos and their poop play a critical role. By chomping on large amounts of silica-rich grass at night, then defecating into the Mara River during the day, the giant grazers help move silicon from the land to the water — something that’s vital for the health of the river and lakes further downstream, a new study has found.
Jonas Schoelynck, a postdoctoral researcher at the University of Antwerp, Belgium, and lead author of the study, was attending a presentation on the role of hippos (Hippopotamus amphibius) in the Mara ecosystem when he first got interested in how hippos might be moving silicon (Si). “It was there that the idea sparked and that I connected the behavior of hippos with Si cycling,” Schoelynck told Mongabay. “I had seen the hippos before in the wild and I knew that they spread tons of dung in the water while twisting their tail around. After that, it was only a matter of finding the right team and resources to conduct an expedition to the Masai Mara.”
Upon reaching Kenya, Schoelynck and his colleagues collected samples of soil, water, grass and hippo feces from various points along the Mara River. In some stretches of the river, the team spotted up to 80 hippos, while in others there were few to none of the pachyderms.
The researchers analyzed these samples and found that the hippos within their study area were consuming about 800 kilograms (1,760 pounds) of silicon daily through the plants they ate. About half of this silicon ended up in the water when the grazers excreted. Further calculations showed that the hippos alone contributed to more than 76 percent of the silicon transported along the Mara River. In short, hippos act as silicon pumps.
The researchers did not look at other grazers in the landscape. But hippos split their 24 hours between grazing on land, then spending a considerable amount of time in rivers to cool off their bodies. It’s in the water that they tend to defecate, their tails violently spraying their dung around. “It is the hippos’ diel [24-hour] behavior that causes the massive transport of Si to the river, contrary to all other animals, even though all other grazers together outnumber the resident hippo population,” Schoelynck said. “The further away from the river, the less likely it becomes that the Si in animal droppings reaches the river, and chances are bigger that it will be recycled by the Savannah grass.”
Silicon is vital for the Mara River ecosystem. This is because East African lakes like Lake Victoria, the world’s largest tropical lake, depend on silicon flowing into them from rivers like the Mara to support the food chains that they host. Diatoms, a type of single-celled photosynthesizing algae that live in the water, produce oxygen and take up carbon, and form the base of these food webs. Silicon forms a key part of the diatoms’ cells.
If the Mara River’s hippo population declined, it could lead to a reduction in the amount of silicon that makes its way to the lakes. This in turn could result in “pest algae” replacing the diatoms, which need an optimal carbon-silicon-nitrogen-phosphorus nutrient ratio of 106:15:16:1, according to Schoelynck. These algae could then cause a lack of oxygen in the water, killing the lake’s fish and affecting people who depend on fishing for their livelihoods.
Industrial pollution and fertilizer runoff from farms have already led to toxic algal blooms in parts of Lake Victoria and other East African lakes by increasing the input of nutrients like nitrogen and phosphorus. A decline in silicon reaching the lakes could worsen this imbalance.
It “could be sufficient to further limit diatom growth in Lake Victoria, where increased nitrogen:silicon and phosphorus:silicon ratios have already caused a phytoplankton transition to year-round dominance by cyanobacteria since the late 1980s, and the diversity of planktonic diatom communities has declined dramatically,” Schoelynck said.
Banner image of hippos in Kenya by Rhett A. Butler/Mongabay.
Schoelynck, J., Subalusky, A. L., Struyf, E., Dutton, C. L., Unzué-Belmonte, D., Vijver, B. V., . . . Frings, P. (2019). Hippos (Hippopotamus amphibius): The animal silicon pump. Science Advances,5(5). doi: 10.1126/sciadv.aav0395