JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Monographs.Abstract. Primary productivity and herbivory were studied in the Serengeti National Park, Tanzania, and Masai Mara Game Reserve, Kenya, during the annual cycle of 1974-1975, and wet-dry season transitions in 1976-1979. Basic state variables measured were aboveground plant biomass inside permanent and temporary fences, and outside fences. Productivity was calculated as the sum of positive plant biomass increments. Control productivity (cPn) was calculated from biomass dynamics inside permanent fences. Temporary fences were moved in concert with grazing by the region's abundant ungulates to estimate actual aboveground primary productivity (aPn). Primary productivity was highly stochastic with productive periods poorly synchronized even among nearby sites. Shortterm productivities could be extremely high, exceeding 30 g. mi-2 d-l. Grazing animals adjusted their densities in relation to grassland productivity. The average proportion of annual aPn that was consumed by herbivores was 0.66, with a minimum of 0.15 and a maximum of 0.94. Green forage was available everywhere late in the wet season in May but was available only at high rainfall sites in the northwest late in the dry season in November. By the end of the dry season, the residual plant biomass outside fences averaged only 8% of cPn. Nomadic grazers moved seasonally in response to grassland productivity. The growing season ranged from 76 d in low rainfall areas to virtually continuous in high rainfall areas.Annual cPn was linearly related to rainfall and averaged 357 g m-2 yr-1 over the year and 1.89 g.m-2 d-I during the growing season. Actual aPn was substantially greater than cPn at most sites, averaging 664 g.m-2 yr-1. Growing season aPn averaged 3.78 g.m-2 d-l. Grazing stimulated net primary productivity at most locations, with the maximum stimulation at intermediate grazing intensities. Stimulation was dependent upon soil moisture status at the time of grazing. Rain had a diminishing effect on primary productivity as the wet season progressed and plant biomass accumulated. Part of the stimulation of grassland productivity by grazing was due to maintenance of the vegetation in an immature, rapidly growing state similar to that at the beginning of the rainy season. Since grazers overrode rainfall-determined productivity patterns, aPn was more closely related to grazing intensity than to ranfall. Grazing was heavier on grasslands that were intrinsically more productive. Rate of energy flow per unit of plant biomass was much higher in grazed vegetation. Grazers ate green leaves almost exclusively during...

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ecologists have tended to view plants as relatively passive participants in short-term interactions at the plant-herbivore interface, suffering tissue reduction from herbivory, and responding in evolutionary time through the evolution of novel antiherbivore chemicals and structures (Fraenkel 1959;Ehrlich and Raven 1964;Levin 1971Levin , 1973Caswell et al. 1973;Freeland and Janzen 1974). Agronomists, botanists, foresters, and range managers, however, have frequently emphasized the direct plant responses to herbivory of compensatory growth and assimilate reallocation (

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