In this study, prairie dogs (Cynomys ludovicianus), bison (Bison bison), or both were excluded from a mixed grass prairie in southwest South Dakota for two years. Only modest vegetational changes were observed, and herbivore species impacts seemed to be similar and additive (no interactions or synergetic effects). Total above ground biomass, current year growth, and graminoid biomass the second year of the study increased by 32%,37%, and 50 %, respectively, in bison excluded plots, and 36%,36%, and 43%, respectively, in prairie dog excluded plots; the increase when both were excluded was approximately the sum of these values. Proportion of forbs decreased significantly the second year when either species was excluded, while graminoids (proportion) increased in the bison exclosures only. The authors suggest that rate of vegetation change, following removal of grazers, depends upon weather conditions, plant species composition, and prior intensity and duration of grazing.
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