Multiple regression analyses were performed on worldwide 236-site data set compiled from studies that compared species composition, aboveground net primary production (ANPP), root biomass, and soil nutrients of grazed vs. protected, ungrazed sites. The authors' objective was to quantitatively assess factors relating to differential sensitivities of ecosystems to grazing by large herbivores. Changes in species composition with grazing were primarily a function of ANPP and the evolutionary history of grazing of the site, with level of consumption third in importance. Changes in species composition increased with increasing productivity and with longer, more intense evolutionary histories of grazing. These three variables explained >50% of the variance in the species response of grasslands or grasslands plus shrublands to grazing. Sensitivities of change in dominant species were greater to varying ecosystem-environmental variables than to varying grazing variables, from low to high values. Percentage differences in ANPP between grazed and ungrazed sites decreased with increasingly long evolutionary histories of grazing and increased with increasing ANPP, levels of consumption, or years of treatment. Counter to the commonly held view that grazing negatively impacts root systems, there was no relationship between difference in ANPP with grazing and differences in root mass; as many positive as negative difference occurred, even though most of the ANPP differences were negative. The authors conclude current management of much of the world's grazing lands based on species composition criteria may lead to erroneous conclusions concerning the long-term ability of a system to sustain productivity.
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