Scientists have used chemical markers to prove that a wide variety of fish and aquatic insects in Africa get their nutrients from an unexpected source: hippo dung.
The common hippopotamus can spend up to 16 hours a day immersed in rivers and lakes. Lumbering out of the water at night, they then graze on tropical grasses, consuming 80 to 100 pounds in one meal.
By daybreak, having eaten their fill, they return to their daytime hangout to rest, digest, and poop. Millions of tons of dung enters Africa’s aquatic ecosystems every year.
Aquatic vs. terrestrial
“[W]e use tools from chemistry to directly demonstrate that these hippo nutrients are being directly picked up and used by aquatic animals,” says Douglas McCauley, an assistant professor in the ecology, evolution and marine biology department at University of California, Santa Barbara.
“Ecologists are really interested in how materials and energy flow across ecosystems, and here is a very clear boundary—aquatic versus terrestrial,” he says.
“These two worlds are clearly distinct, but our research shows that wildlife such as hippos build important connections across these ecosystem gaps. Our study confirms that hippos are bringing a part of terrestrial ecology—nutrients and energy—into this other domain of rivers.”
Back end of the hippo
The research, published in the journal Ecosphere, shows that some species of river fish—both in their native habitat of Kenya’s Ewaso Ng’iro River and in the laboratory—feed on the nutrients from hippo dung.
The scientists were able to use stable isotopes, a class of natural chemical markers, to trace the flow of organic matter through the food pipeline, from the back end of the hippo to the tissue of river fish and insects. The results demonstrate that these aquatic consumers absorb nutrients from hippo dung as part of their diet.
Further, the importance of the hippo as a food source is contingent on the conditions of the river. For example, the uptake of nutrients from hippo dung was most pronounced during periods of low river flow, which were caused by seasonal changes in rainfall.
River food web
“When the river is high, it seems to be diluting a lot of the material that the hippos are bringing in and the animals in the river just can’t get to it quickly enough,” McCauley says. “And when it’s dry, these materials concentrate in these pools and the animals are able to make better use of them.”
The findings are important not only because they characterize the importance of hippos in the river food web but also because rivers in East Africa are changing rapidly, McCauley says. “Climate change and regional development are certainly changing river flow.
“With hippo populations declining in Africa and water regimes changing rapidly, it is critically important that we understand more about the ecological role of hippos. The linkages that we highlight in our research illustrate that the fate of the hippo is intimately linked to the fate of whole food webs and to the functioning of entire ecosystems.”
The hippos can be viewed live via a webcam focused on a pool McCauley used in his research.
The National Science Foundation supported the work.
Source: UC Santa Barbara