The influence of grass biomass production on hippopotamus population density distribution along the Luangwa River in Zambia

Abstract

Hippopotamus is a selective nocturnal grazer consuming 50 kg of grass. Due to its large body size, it requires large areas of grass often exceeding 5 hectares within 2-5 km of the water body to maintain good body condition. In this study, hippo population size and density, grass biomass and grazing capacity were assessed in March 2008 along a 165 km stretch of the Luangwa River. The study area was subdivided into study blocks, A, B, C, D, E and F. Methods used were: river bank total counts, quadrat sampling and identification of all grass species, clipping of grass in a quadrat, drying of clipped grass at constant temperature of 70oC and weighing to obtain dry weight. Grazing capacity of the hippo was estimated based on grass biomass values obtained during the study. Hippo population density was then matched with grass biomass distribution along the study blocks A-H. Total primary production was 62, 800 kg with mean biomass per study block of 7, 850 kg /ha-1 (in 2008). Grazing capacity was 1 hippo/6 ha-1. Grass biomass varied significantly between river segments being higher in study blocks A, B and E and lower in study blocks C, D, F and G. Mean hippo density was 33 individuals/km stretch of the river. Hippo density was found to be above 33/km in study blocks A, B and E which had higher biomass and lower than 33/km in study blocks C, D, F and G. Hippo population density distribution was found to be influenced by grass species diversity and amount of biomass produced which also determined grazing capacity. In light of global climate change and changing rainfall patterns, more studies are required to determine the influence of rainfall on primary production and how this would affect the increase or decrease in hippo density and grazing capacity in the long-term. Key words: Hippo, density, grazing capacity, grass biomass.