Effects of the hippopotamus on the chemistry and ecology of a changing watershed

Abstract

Cross-boundary transfers of nutrients can profoundly shape the ecology of recipient systems. The common hippopotamus, <i>Hippopotamus amphibius</i>, is a significant vector of such subsidies from terrestrial to river ecosystems. We compared river pools with high and low densities of <i>H. amphibius</i> to determine how <i>H. amphibius</i> 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 <i>H. amphibius</i> 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 <i>H. amphibius</i> Likewise, the diversity and abundance of fish and aquatic insect communities were indistinguishable. During the dry season, however, high-density <i>H. amphibiu</i>s 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 <i>H. amphibius</i> pools. Scaled up to the entire watershed, we estimate that <i>H. amphibius</i> 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 <i>H. amphibius</i>, altering their influence on ecosystem diversity and functioning in a fashion that may be more severe than presently appreciated.