Abstract
The sagebrush ecosystem and greater sage-grouse are in danger of degradation and population decline due to anthropogenic land use. Some people have attributed sage-grouse habitat degradation, in part, to livestock grazing on sagebrush rangelands. Greater sage-grouse are dependent on sagebrush for shelter, food, and protection from predators. Arthropods, an important component of the greater sage-grouse diet, may also be impacted by livestock grazing in this ecosystem. Understanding the effects of livestock grazing is important for the conservation of the sagebrush ecosystem and for greater sage-grouse. Proper management of grazing could allow preservation of this ecosystem, conservation of greater sage-grouse, and continued use for livestock grazing. The work presented here examines the effect of no-grazing and deferred spring grazing (hereafter spring grazing) on arthropod community structure and its implications for greater sage-grouse conservation.This work took place on three sites in Idaho: Big Butte, Brown’s Bench, and Jim Sage, in the Butte, Twin Falls, and Cassia counties, respectively. Over three summers, arthropods were trapped using pitfall traps serviced weekly. Insects and spiders were identified to taxonomic family and non-insect arthropods were identified to taxonomic order. A total of 99,832 specimens from 1040 weekly pitfall trap deployments were catalogued. Communities were compared between spring grazing paddocks and those that were not grazed during the study (“no-grazing”). The effect of grazing on arthropod communities differed among the three sites. At Big Butte, a site with lower elevation and lower average daily temperature throughout the year, average abundance and taxa richness were higher under no-grazing while average biomass and H’ were higher under spring grazing. At Brown’s Bench, a site with intermediate elevation and temperature and lower precipitation, average abundance was higher under spring grazing while average biomass, taxa richness, and H’ were higher under no-grazing. At Jim Sage, a site with high elevation, precipitation, and temperature, average abundance, average biomass, H’, and taxa richness were all higher under spring grazing. The effect of grazing appears to be variable based on environmental factors, such as elevation, precipitation, temperature, and vegetation but spring grazing does not appear to be detrimental to greater sage-grouse in terms of arthropod prey.
Ants, Formicidae, were the most abundant family of arthropods collected and are considered an important component of sage-grouse diets. At Brown’s Bench, ants that had the highest abundances, Formica neogagates species group, Pogonomyrmex, Lasius, Formica integra species group, and Formica sanguinea species group, were more abundant in spring grazing compared to no-grazing. Ant functional groups respond to grazing differently. Homopteran tenders appear to prefer spring grazing, likely due to increased aphid abundance under spring grazing. Seed harvesters appear to prefer spring grazing, likely due to decrease in shrub cover and increased cheatgrass. Generalists appear to prefer no-grazing, likely due to the reduced vegetation cover and increased bare ground resulting from grazing. Liquid feeders prefer no-grazing, likely due to higher vegetation cover and more foraging habitat. Slave makers appear to prefer spring grazing, likely due to the preference of spring grazing by workers of other species they are dependent on.
Based on this study, spring grazing relative to no-grazing is not detrimental to epigaeic arthropods, including sage-grouse food arthropods, and among those, ants in Idaho sagebrush systems. In at least one site in this study, spring grazing enhanced arthropod abundance relative to no-grazing. Future work should delve into fully understanding the diet of greater sage-grouse regarding arthropod components to allow community surveys like this to be more applicable to the conservation of sage-grouse. Replicating this study, or studies like this, at more locations with more levels of grazing could potentially provide answers about why the response of arthropod communities to grazing is so variable by site. Surveying these arthropod communities pre and post treatment would also provide a more accurate look into how they are impacted by grazing treatment.