The Response of Native Vertebrate Populations to Crested Wheatgrass Planting and Grazing by Sheep

Abstract

Native vertebrate population levels were examined in grazed and ungrazed habitats dominated by big sagebrush (Artemisia tridentata) and crested wheatgrass (Agropyron cristatum) in southeast Idaho. Our objective was to determine the species diversity and relative density of birds, mammals, and reptiles in these habitats with and without grazing pressures by sheep. In a habitat dominated by sagebrush, grazing did not significantly alter the species diversity or the density of reptiles or nesting birds. However, both the diversity and the relative density of small mammals were significantly reduced. Crested wheatgrass plantings, regardless of sheep use, supported fewer nesting bird species and a lower density of birds, mammals, and reptiles than did areas dominated by sagebrush. The synergistic effects of planting with crested wheatgrass followed by grazing were most evident in (1) a significant reduction in the relative density of small mammals, and (2) the occurrence of only one nesting bird species: the horned lark (Eremophila alpestris). At one time over 100 million ha of the western rangelands in the United States were covered with sagebrush, mostly big sagebrush (Artemisia tridentata) (Beetle 1960). Historically, the primary use of sagebrush rangeland has been for livestock grazing. Many areas formerly dominated by sagebrush have been burned, chained, disced, plowed, or sprayed and reseeded with grasses to improve forage for livestock. Additionally, crested wheatgrass (Agropyron cristatum) is being planted in the Intermountain West in an attempt to reclaim strip-mined lands. Braun et al. (1976) estimated that a minimum of 10% of the native sagebrush vegetation in western U.S. has been treated in one way or another. In Idaho alone, 650 thousand ha of public land (BLM document 1974) and 200 thousand ha of private land (L. Sharp, pers. comm.), mostly dominated by sagebrush, have been reseeded with crested wheatgrass. Until recently, little concern was expressed about the potential effects of alteration of sagebrush range on wild animals. The objective of this study was to determine the response of native vertebrate populations (birds, mammals, and reptiles) to different land use practices on the Idaho National Engineering Laboratory (INEL) Site.