EVIDENCE FOR THE PROMOTION OF ABOVEGROUND GRASSLAND PRODUCTION BY NATIVE LARGE HERBIVORES IN YELLOWSTONE-NATIONAL-PARK

We examined the effect of native large herbivores on aboveground primary production of nonforested habitat in Yellowstone National Park, Wyoming. Productivity of vegetation grazed by elk (Cervus elaphus) and bison (Bison bison) was compared with that of ungrazed (permanently fenced) vegetation at four sites. Two methods were used that, we believed, would provide the most accurate measurements under the different grazing regimes encountered in the study. Production of ungrazed vegetation in permanent exclosures (10 x 10 m or 15 x 15 m, 3 per site) and that of vegetation that was grazed only in the winter was taken as peak standing crop. Production of vegetation grazed during the growing season was the sum of significant increments (P<0.05) in standing crop inside temporary exclosures (1.5 x 1.5 m, 6 per site) moved every four weeks to account for herbivory. Aboveground productivity of grazed vegetation was 47% higher than that of ungrazed vegetation across sites (P<0.0003). This result could be explained by either a methodological or grazer effect. We believe it was the latter. Results from a computer simulation showed that sequential sampling with temporary exclosures resulted in a slight underestimation of production, suggesting that the reported differences between treatments were conservative. We suggest that stimulation of aboveground production by ungulates may be, in part, due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecosystem.