A replicated factorial experiment was designed to test the hypothesis that manipulating inputs of water and mineral nitrogen to a semiarid grassland would disrupt existing interactions resulting in alteration of the structure of the primary producer community. Alteration of community structure was measured as either changes in growing season average biomass of 6 functional groups of plants or their relative contribution to total biomass.Additions of water greatly increased total biomass and resulted in the replacement of one of the dominant functional groups by a subordinate group. The water plus nitrogen treatment resulted in large biomass increases in two of the dominant functional groups, elimination of succulents as an important component of community structure, and establishment of several introduced weedy species. Continuation of the experiment will likely result in complete dominance of the water plus nitrogen treatment by these introduced species.Despite the large changes in community structure observed as a result of water- and nitrogen-induced stresses we conclude that the shortgrass prairie in northcentral Colorado is asymptotically stable with respect to these influences.

There exists considerable uncertainty whether differences between bison () and cattle (Bos taurus) should be exploited in natural area stewardship. Because foraging ecology may prescribe the fundamental relevance of either herbivore, we describe the foraging of bison and cattle on a northern mixed prairie natural area and discuss management implications. As generalist foragers, bison and cattle exhibited seasonal differences in selection for or against C"3 graminoids, C"4 grasses, forbs, and browse. Forty-eight hour in vitro dry matter digestibilities of C"3 graminoids and C"4 grasses were greater by bison than cattle, but equal digestive efficiencies were exhibited for forbs and browse. Dietary in vitro dry matter digestibility and crude protein differed little between herbivores. Cattle allocated more time to grazing than bison did during summer. Our study suggests that the relationship between feeding-time investment and forage patchiness is important in determining differences in diet choice between bison and cattle. In contrast to cattle, it appears that bison balance nutrient and time demands during the rut by consuming almost exclusively graminoids. Apparently, bison and cattle may achieve similar dietary quality through different foraging behavior. We suggest that specific scale-dependent programmatic conditions exist where either herbivore may be the most appropriate for natural area management.

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The herbivore grasing optimixation hypothesis predicts an increase in aboveground net primary productivity (ANPP) at a moderate grnxing intensity. The hypothesis was tested by gruing controlled densities (0 to 145 individuals/m2) of bigheaded grasshoppers (Aukera &otti Thomas) for short time spans (7 to 13 days) on enclosed swards (0.7 mZ) of blue gramr [(Boutcloucr gruc&j (Willd. ex H&K.) Lag. cx Griffithsl. ANPP of each of 257 experimental enclosures was esthtmted following regrowth by using a standing crop index (the product of mean total blade length per tiller and percent basal cover) after the grazing period and clipping after the regrowth period. ANPP was not signiKcantly reduced by graxing in any of the S short-dumtion grazing experiments. In 2 of the S experiments, ANPP increased significantiy with graxing. In 1 of the other 3 experiments there was evidence for the graxhtg optimization hypothesis.

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