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Grazing ecosystems support more herbivore biomass than any other terrestrial habitat (Sinclair 1975, Detling 1988, McNaughton et al. 1989, 1991, Huntly 1991). A functional consequence of this disparity in trophic structure emerges by comparing the relationship between aboveground production and herbivore consumption in the Serengeti and Yellowstone ecosystems with that in other terrestrial ecosystems [ILLUSTRATION FOR FIGURE 3 OMITTED]. For consumption measurements, we included plant material removed by all important herbivores, both vertebrates and invertebrates. All values were energy equivalents (kJ), converted from biomass measurements using standard conversion factors (Golley 1968). For productivity measurements, we considered only the nonwoody fraction of aboveground productivity - that is, net foliage production (NFP) - because woody production is largely unavailable to herbivores. Plotting plant production against consumption revealed that terrestrial ecosystems fall into two groups that are distinguished by the intensity of herbivory ([F.sub.1,78] = 88.2, P [less than] 0.0001; [ILLUSTRATION FOR FIGURE 3 OMITTED]). The first group includes low-herbivory habitats: desert, tundra, temperate forest, tropical forest, [TABULAR DATA FOR TABLE 1 OMITTED] and small grassland sites lacking large herbivores. The second includes the Serengeti and Yellowstone, which exhibit high herbivory rates. On average, herbivores removed 57% (SE = 3.4, n = 40) of NFP in the Serengeti and Yellowstone, whereas they removed only 9% (SE = 1.4, n = 40) of NFP in other terrestrial ecosystems. For example, only 10% (SE = 2.1, n = 14) of the aboveground production was consumed in temperate grasslands that lack large herbivores, showing that the removal of migratory grazers dramatically affects the energy dynamics of grasslands. Slopes of the relationships did not differ statistically between the two groups (P [greater than] 0.10) and were greater than 1, indicating that the proportion of available primary production consumed increased as NFP increased for both groups of habitats. The low level of dispersion of samples around the regression line characterizing plant productivity and consumption in the Serengeti and Yellowstone grasslands suggests that the relationship describes a continuum from cool, temperate to warm, tropical grazing ecosystems. Primary production is greater in... ...erlag. McNaughton SJ, Milchunas DG, Frank DA. 1996. How can net primary productivity be measured in grazing ecosystems? Ecology 77: 974-977. Meagher M. 1973. The Bison of Yellowstone National Park. National Park Service Scientific Monograph Series 1. Washington (DC): United States Department of Interior. Meagher M, Meyer ME. 1994. On the origin of brucellosis in bison of Yellowstone National Park: A review Conservation Biology 8: 645-653. Milchunas DG, Lauenroth WK. 1993. Quantitative effects of grazing on vegetation and soils over a global range of environments. Ecology 63: 327-366. Morton JK. 1972. Phytogeography of the west African mountains. Pages 221-236 in Valentine DH, ed. Taxonomy, Phytogeography, and Evolution. New York: Academic Press. Oesterheld, M, Sala OE, McNaughton SJ. 1992. Effect of animal husbandry on herbivore carrying capacity at the regional scale. Nature 356: 234-236. Peters RH. 1983. The Ecological Implications of Body Size. Cambridge (UK): Cambridge University Press. Prins HHT. 1996. Ecology and Behavior of the African Buffalo: Social Inequality and Decision Making London: Chapman & Hall. Senft RL, Coughenour MB, Bailey DW, Rittenhouse LR, Sala