Ungulate Grazing in Sagebrush Grassland: Mechanisms of Resource Competition

Abstract

In many areas of western North America, populations of Rocky Mountain elk (Cervus elaphus canadensis) avoid snow at high elevations during winter by migrating to sagebrush grassland communities in mountain valleys, communities that are used by cattle in the spring and early summer. 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 variation in population density of elk on the availability and use of forage resources by cattle in a randomized complete block experiment conducted in sagebrush grassland during four years. We manipulated elk numbers to achieve four levels of population density (0, 9, 15, and 31 elk/km2), replicated each level three times, and observed responses of vegetation and cattle to these manipulations. At high densities (31 animals/km2), elk annually removed 57% of the standing crop of dead perennial grass and 12% of the total annual production of live perennial grass. Standing crops of dead perennial grass in early spring declined in direct proportion to increasing elk density (linear effect F1,6 = 10.0, P = 0.02) from a mean of 8.7 g/m2 in the controls (0 elk/km2) to 3.3 g/m2 in the high density (31 elk/km2) treatment. Early spring standing crops of live perennial grass also declined as elk population density increased, but these trends only approached significance (linear effect F1.6 = 3.4, P = 0.12). Effects of elk grazing on herbaceous aboveground net primary production were not significant (minimum P > 0.38), but the total supply of herbaceous dry matter available to cattle (standing dead + primary production) declined in linear relation to elk density (linear effect F1.6 = 7.7, P = 0.03). Canopy cover of shrubs was least and canopy cover of grass was greatest at intermediate levels of elk density (quadratic effect F1.6 = 9.4, P = 0.03). We found weak enhancing effects of elk populations on nutritional quality of spring forage. Elk grazing caused linear increases in the digestibility (F1.6 = 5.0, P. = 0.07) and nitrogen content (F1.6 = 15.1, P = 0.008) of perennial grass available to cattle. Nitrogen content of cattle diets increased in the moderately grazed treatments (control vs. 15 elk/km2, F1.6 = 4.3, P = 0.06), but dietary digestibility and fiber content did not change significantly with treatment. Daily forage intake by cattle (kilograms of dry matter per cow per day) declined in direct relation to elk density (F1.6 = 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 F1.6 = 5.1, P = 0.06) and nitrogen (control vs. others F1.6 = 5.4, P = 0.06) declined as elk population density increased. The mechanism responsible for this decline was a Type II functional response of cattle to forage biomass. We conclude that effects of elk on cattle represent a composite of facilitative and competitive effects. When forage production is low and cattle density is high, competition is a much stronger force than facilitation.