Cross-boundary transfers of nutrients can profoundly shape the ecology of recipient systems. The common hippopotamus, Hippopotamus amphibius, is a significant vector of such subsidies from terrestrial to river ecosystems. We compared river pools with high and low densities of H. amphibius to determine how H. amphibius subsidies shape the chemistry and ecology of aquatic communities. Our study watershed, like many in sub-Saharan Africa, has been severely impacted by anthropogenic water abstraction reducing dry-season flow to zero. We conducted observations for multiple years over wet and dry seasons to identify how hydrological variability influences the impacts of H. amphibius. During the wet season, when the river was flowing, we detected no differences in water chemistry and nutrient parameters between pools with high and low densities of H. amphibius. Likewise, the diversity and abundance of fish and aquatic insect communities were indistinguishable. During the dry season, however, high-density H. amphibius pools differed drastically in almost all measured attributes of water chemistry and exhibited depressed fish and insect diversity and fish abundance compared with low-density H. amphibius pools. Scaled up to the entire watershed, we estimate that H. amphibius in this hydrologically altered watershed reduces dry-season fish abundance and indices of gamma-level diversity by 41% and 16%, respectively, but appears to promote aquatic invertebrate diversity. Widespread human-driven shifts in hydrology appear to redefine the role of H. amphibius, altering their influence on ecosystem diversity and functioning in a fashion that may be more severe than presently appreciated.
|Original language||English (US)|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - May 29 2018|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. We thank the Director General of Tanzania National Parks, the Tanzanian Wildlife Research Institute, Tanzania Commission for Science and Technology, the Chief Park Warden of Ruaha National Park Dr. C. D. Timbuka, and Ruaha National Park Ecologist Paul Banga for allowing us to conduct our research in Ruaha National Park. We also thank Robert Philemon Kivuyo and Melissa H. Schmitt for invaluable assistance in the field and Sandra Brovold and Michelle Rorer for their assistance with laboratory analyses. This research was funded by National Science Foundation (NSF) International Research Fellowship Program Office of International Science and Engineering Grant 1064649, NSF Division of Environmental Biology Grant 1146247, the Safari Club International Foundation, and a University of California Faculty Research Grant.
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