UNGULATE GRAZING IN SAGEBRUSH GRASSLAND - MECHANISMS OF RESOURCE COMPETITION

In many areas of western North America, populations of Rocky Mountain elk (Cervus elaphus canadensis) avoid snow at high elevations during w inter by migrating to sagebrush grassland communities in mountain vall eys, communities that are used by cattle in the spring and early summe r. As a result of these patterns of habitat use, the impact of elk on forage supplies for cattle has emerged as an important issue in range and wildlife management throughout the West. We examined effects of va riation in population density of elk on the availability and use of fo rage resources by cattle in a randomized complete block experiment con ducted in sagebrush grassland during four years. We manipulated elk nu mbers to achieve four levels of population density (0, 9, 15, and 31 e lk/km(2)), replicated each level three times, and observed responses o f vegetation and cattle to these manipulations. At high densities (31 animals/km(2)), elk annually removed 57% of the standing crop of dead perennial grass and 12% of the total annual production of live perenni al grass. Standing crops of dead perennial grass in early spring decli ned in direct proportion to increasing elk density (linear effect F-1, F-6 = 10.0, P = 0.02) from a mean of 8.7 g/m(2) in the controls (0 elk /km(2)) to 3.3 g/m(2) in the high density (31 elk/km(2)) treatment. Ea rly spring standing crops of live perennial grass also declined as elk population density increased, but these trends only approached signif icance (linear effect F-1,F-6 = 3.4, P = 0.12). Effects of elk grazing on herbaceous aboveground net primary production was not significant (minimum P > 0.38), but the total supply of herbaceous dry matter avai lable to cattle (standing dead + primary production) declined in linea r relation to elk density (linear effect F-1,F-6 = 7.7, P = 0.03). Can opy cover of shrubs was least and canopy cover of grass was greatest a t intermediate levels of elk density (quadratic effect F-1,F-6 = 9.4, P = 0.03). We found weak enhancing effects of elk populations on nutri tional quality of spring forage. Elk grazing caused linear increases i n the digestibility (F-1,F-6 = 5.0, P = 0.07) and nitrogen content (F- 1,F-6 = 15.1, P = 0.008) of perennial grass available to cattle, Nitro gen content of cattle diets increased in the moderately grazed treatme nts (control vs. 15 elk/km(2), F-1,F-6 = 4.3, P = 0.06), but dietary d igestibility and fiber content did not change significantly with treat ment. Daily forage intake by cattle (kilograms of dry matter per cow p er day) declined in direct relation to elk density (F-1,F-6 = 5.2, P = 0.06), primarily as a result of reductions in intake of standing dead grass. Consequently, cattle daily intake of digestible energy (linear effect F-1,F-6 = 5.1, P = 0.06) and nitrogen (control vs, others F-1, F-6 = 5.4, P = 0.06) declined as elk population density increased, The mechanism responsible for this decline was a Type II functional respo nse of cattle to forage biomass. We conclude that effects of elk on ca ttle represent a composite of facilitative and competitive effects. Wh en forage production is low and cattle density is high, competition is a much stronger force than facilitation.