Estimating the Biomass of Large Mammalian Herbivores in a Tropical Montane Forest: A Method of Faecal Counting That Avoids Assuming a 'Steady State' System

Abstract

1. Faecal counting techniques were used to census five large mammalian herbivores during 1988 and 1989 in the Parc National des Volcans in Rwanda. These five herbivores were the black-fronted duiker (Cephalophus nigrifrons), the bushbuck (Tragelaphus scriptus), the Cape buffalo (Syncerus caffer), the mountain gorilla (Gorilla gorilla beringei) and the African elephant (Loxodonta africana). 2. Population density estimations from standing crop counts of faeces usually correct the counts for the defecation rate of the animal and the decomposition rate of the dung, assuming that the decay of the dung is constant. However, this study found that the dung decay was not constant. For bushbucks, exponential decay curves were found in the wet seasons and sigmoidal decay curves in the dry seasons. Only sigmoidal curves were found for buffalos and elephants. Therefore, measures of dung decomposition throughout the year were used in a computer model which estimated iteratively the population density. 3. Ecological density estimates were obtained for eight habitat types found in the Park. Habitats at the summit of volcanos sustain the lowest biomass of large herbivores at 10-15 kg ha −1 whilst meadows sustain the highest biomass at 45-50 kg ha −1 . The biomass for the whole study area was calculated as 31 kg ha −1 , one of the highest biomasses for any forest yet studied. The bushbuck and the buffalo dominated this biomass. The implications of this for the conservation of the mountain gorilla are discussed and it is concluded that currently these animals have little impact on the gorilla's food supply