ECOSYSTEM ENGINEERS: A STUDY OF THE COMMON HIPPOPOTAMUS (HIPPOPOTAMUS AMPHIBIOUS) AND ITS IMPORTANCE IN SUB-SAHARAN AQUATIC ECOSYSTEMS THROUGH AN ANALYSIS OF THE AFRICAN SHARPTOOTH CATFISH (CLARIAS GARIEPINUS)

Abstract

The common hippopotamus (Hippopotamus amphibious) has long been implicated as a driver of connective dynamics between aquatic and terrestrial Sub-Saharan ecosystems. Hippos are semi-aquatic in nature and spend the majority of their day in pools of water but are known to emerge at night and graze heavily on terrestrial grasses. This nocturnal feeding behavior provides a unique scenario where the majority of hippo feces are submitted as an allochthonous input directly into the water. Aquatic inputs are hypothesized to increase trophic production in rivers by creating nutrient hotspots, whose effects can ripple throughout a food web. To assess the validity of this claim, a population of hippos on the Ewaso Ngiro River in Laikipia, Kenya were studied to assess the effects hippo nutrients could have on the growth of local fish species. Two species of fish were surveyed: an air breathing catfish called the Sharptooth Catfish (Clarias gariepinus) and Ripon barbel (Barbus altianalis); both of which were measured for their health and \(^{13}\)C/\(^{15}\)N stable isotope. All catfish were found in areas of high hippo density, and should in theory have more nutrients available to them than the barbel caught in low hippo density areas. The population of Sharptooth Catfish (n = 21) was found to scale isometrically (b = 3.1358) with an average condition factor of 0.3665; where the population of Ripon Barbel (n = 24) was also found to scale isometrically (b = 2.8409) and have an average condition factor of 0.9533. Scaling constants indicate both species were growing in proportion based off their length and weights with catfish having slightly higher growth parameters but lower overall health. Stable isotope analyses indicate catfish operate at multiple trophic levels depending on age but adults operate as an upper level consumer that feeds on various species. The data on fish species in this study does not provide conclusive evidence toward the beneficial role hippo inputs play in Sub-Saharan ecosystems; however, previous research and theory suggest there is an important contribution being made to aquatic ecosystems and hippos are an essential species in connecting African habitats.