Monday, March 19 , 2018, 10:40 am | Fair 58º


UCSB’s Douglas McCauley Studies the Life Force of African Rivers

Red-billed oxopeckers sit on the back of a hippo taking a nap in the river.
Red-billed oxopeckers sit on the back of a hippo taking a nap in the river. (Douglas McCauley photo)

The common hippopotamus can spend up to 16 hours a day immersed in rivers and lakes. Lumbering out of the water at night, these herbivores graze on tropical grasses and consume 80 to 100 pounds in one meal.

By daybreak, having eaten their fill, they return to their daytime resting area to rest, digest and, eventually, eliminate. This natural process results in millions of tons of hippo dung entering Africa’s aquatic ecosystems every year.

However, as distasteful as that might seem, the hippos’ deposits actually serve an important ecological function. A new study by UC Santa Barbara’s Douglas McCauley and colleagues reveals that the organic matter produced by hippos is a source of nutrition for a variety of river fish and aquatic insects. The researchers’ findings appear today in the journal Ecosphere.

“The ecological importance of hippopotamus-vectored subsidies has been widely speculated, but we use tools from chemistry to directly demonstrate that these hippo nutrients are being directly picked up and used by aquatic animals,” said McCauley, an assistant professor in UCSB’s Department of Ecology, Evolution and Marine Biology.

“Ecologists are really interested in how materials and energy flow across ecosystems, and here is a very clear boundary — aquatic versus terrestrial,” he added. “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.”

McCauley’s team found that some species of river fish — both in their native habitat of Kenya’s Ewaso Ng’iro River and in the laboratory — fed 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.

An additional discovery showed that the importance of the hippo as a food source is contingent on the conditions of the river. For example, the researchers found that the uptake of nutrients from hippo-vectored organic matter was most pronounced during periods of low river flow, which were caused by seasonal changes in rainfall.

“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 explained. “And when it’s dry, these materials concentrate in these pools and the animals are able to make better use of them.”

These 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. “Climate change and regional development are certainly changing river flow,” McCauley said.

“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,” he added. “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 stars of this research can be viewed live via a webcam focused on a hippo pool that served as one of McCauley’s main research sites.

Funding for this work was provided by the National Science Foundation.

— Julie Cohen represents the UCSB Office of Public Affairs and Communications.

  • Ask
  • Vote
  • Investigate
  • Answer

Noozhawk Asks: What’s Your Question?

Welcome to Noozhawk Asks, a new feature in which you ask the questions, you help decide what Noozhawk investigates, and you work with us to find the answers.

Here’s how it works: You share your questions with us in the nearby box. In some cases, we may work with you to find the answers. In others, we may ask you to vote on your top choices to help us narrow the scope. And we’ll be regularly asking you for your feedback on a specific issue or topic.

We also expect to work together with the reader who asked the winning questions to find the answer together. Noozhawk’s objective is to come at questions from a place of curiosity and openness, and we believe a transparent collaboration is the key to achieve it.

The results of our investigation will be published here in this Noozhawk Asks section. Once or twice a month, we plan to do a review of what was asked and answered.

Thanks for asking!

Click here to get started >

Support Noozhawk Today

You are an important ally in our mission to deliver clear, objective, high-quality professional news reporting for Santa Barbara, Goleta and the rest of Santa Barbara County. Join the Hawks Club today to help keep Noozhawk soaring.

We offer four membership levels: $5 a month, $10 a month, $25 a month or $1 a week. Payments can be made through PayPal below, or click here for information on recurring credit-card payments.

Thank you for your vital support.

Reader Comments

Noozhawk is no longer accepting reader comments on our articles. Click here for the announcement. Readers are instead invited to submit letters to the editor by emailing them to [email protected]. Please provide your full name and community, as well as contact information for verification purposes only.

Daily Noozhawk

Subscribe to Noozhawk's A.M. Report, our free e-Bulletin sent out every day at 4:15 a.m. with Noozhawk's top stories, hand-picked by the editors.

Sign Up Now >