COS 36-9
Cross-ecosystem linkages created by the common hippopotamus revealed using satellite tracking

Tuesday, August 12, 2014: 4:20 PM
301, Sacramento Convention Center
Tristan Nuñez, Department of Environmental Science, Policy, & Management, University of California- Berkeley, Berkeley, CA
Justin Brashares, Environmental Science, Policy, & Management, University of California Berkeley, Berkeley, CA
Mary E. Power, Department of Integrative Biology, University of California Berkeley, Berkeley, CA
Todd E. Dawson, Department of Integrative Biology, University of California Berkeley, Berkeley, CA
Kelly K. Caylor, Civil and Environmental Engineering, Princeton University, NJ
Mordecai Ogada, Laikipia Wildlife Forum, Nanyuki, Kenya
Jacques C. Finlay, Ecology Evolution and Behavior, University of Minnesota, St. Paul, MN
Rebecca Lewison, Biology Department, San Diego State University, San Diego, CA
Douglas McCauley, Ecology, Evolution, and Marine Biology, University of California at Santa Barbara, Santa Barbara, CA
Background/Question/Methods

The common hippopotamus (Hippopotamus amphibius) is thought to play a key role in African ecosystems by shaping vegetation patterns on land with its nightly grazing forays, and fertilizing aquatic ecosystems by defecating in them during the day. Little is known about the spatial ecology of H. amphibius or the extent at which it collect nutrients and energy on land. In the first study of its kind, we used global positioning system (GPS) technology to track the movements of hippopotamus in Kenya’s Ewaso N’giro River basin. We used local convex hulls and step selection functions to describe the most ecologically important patterns in these observed movements. Data on space use by H. amphibius were coupled with biogeochemical measurements to determine the volume and ecological importance of the nutrient and energy subsidies vectored by H. amphibius into recipient aquatic systems. Macronutrient assays were used to estimate the impact of subsidies of river chemistry and stable isotopes (δ13C) were employed to monitor consumer utilization of H. amphibious­-mediated nutrient additions. 

Results/Conclusions

We calculated the size, shape, and topographical and vegetation correlates of hippopotamus home ranges and habitat use using the GPS tracking data. Hippopotamus movements on land are structured around pools used as thermal refuges during the day, and their movement patterns shift between low directional correlation in grazing areas, to directed walks in moving between refuge pools and grazed areas. These results suggest that hippopotamus habitat requirements are closely tied to water availability and that changes to river hydrology from irrigation offtake and anthropogenic climate change may impact already vulnerable H. amphibius populations. These outputs also explicitly specify the spatial region over which terrestrial materials are collected for transfer in a point source fashion to aquatic ecosytems. Our biogeochemical measurements suggest that the volume of H. amphibious-vectored subsidies are substantial, but are strongly controlled by regional hydrology. Stable isotope results suggest that ecological use of these subsidies is important and greatest during low flow periods when H. amphibius nutrient inputs are more concentrated. Overall our hippopotamus movement and nutrient data provide a first quantification of the spatial domain at which H. amphibius collects terrestrially-derived organic matter and the ecological importance of these subsidies as they are delivered to freshwater ecosystems.

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