Grasslands and Savannas: Regulation of Energy Flow and Nutrient Cycling by Herbivores

Abstract

Although the importance of herbivores in some terrestrial ecosystems has been questioned, results of research conducted in grasslands and savannas suggest that herbivores may have pervasive effects on ecosystem structure and function. In contrast to many terrestrial ecosystems that support relatively low herbivore loads and annually lose 5-10% or less of their net primary production (NPP) to herbivores (Chew 1974; Wiegert and Evans 1967; Owen and Wiegert 1976), grasslands and savannas typically support heavy herbivore loads. As discussed below, these herbivores may consume half or more of the annual aboveground net primary production (ANPP) and one fourth or more of the annual belowground net primary production (BNPP). As a result of these relatively high rates of consumption, it has been suggested that the structure of grasslands is the result of numerous interactions, many of which are either direct effects of, or mediated by, herbivores (McNaughton 1983). A simplified conceptual model of carbon, mineral nutrients, and water, and a few of the factors affecting their flows in a grazing system, is shown in Figure 7.1.