The Resource Limitation of Trophic Levels in Tropical Grassland Ecosystems

Abstract

One feature of terrestrial ecosystems that is becoming widely recognized is the small proportion of primary production, measured on an annual basis, that is consumed by herbivores.t In grassland systems Wiegert & Evans (1967) found that consumption rarely exceeded 10% of the primary production and the only exceptions appeared to be in the East African tropical grasslands supporting large ungulate populations, where it was roughly estimated that between 30 and 60% of the primary production was consumed. But with further information Phillipson (1973) put forward a tentative model for the Serengeti ecosystem in Tanzania, and in this model only 18% of the net primary production was utilized by the herbivores. In woodland systems similar results have emerged: herbivores consumed less than 70 of the tree production in a deciduous woodland (Varley 1970), and Reichle et al. (1973) found that insects consumed only 2-6% of the primary production in a forest canopy. The more restricted studies of the energy flow in small mammal communities have all found that less than 20% and sometimes only 2% of the primary production is utilized (Grodzinski 1963, 1971; Grodzinski et al. 1970; Ryszkowski 1970). Slobodkin, Smith & Hairston (1967) have summarized other similar results all of which show that less than 10% of the primary production is consumed.