Conditions favouring B romus tectorum dominance of endangered sagebrush steppe ecosystems

Ecosystem invasibility is determined by combinations of environmental variables, invader attributes, disturbance regimes, competitive abilities of resident species and evolutionary history between residents and disturbance regimes. Understanding the relative importance of each factor is critical to limiting future invasions and restoring ecosystems. We investigated factors potentially controlling B romus tectorum invasions into A rtemisia tridentata ssp. wyomingensis communities across 75 sites in the G reat B asin. We measured soil texture, cattle grazing intensity, gaps among perennial plants and plant cover including B . tectorum , biological soil crusts ( BSC s) and bare soil. Using a priori knowledge, we developed a multivariate hypothesis of the susceptibility of A rtemisia ecosystems to B . tectorum invasion and used the model to assess the relative importance of the factors driving the magnitude of such invasions. Model results imply that bunchgrass community structure, abundance and composition, along with BSC cover, play important roles in controlling B . tectorum dominance. Evidence suggests abundant bunchgrasses limit invasions by limiting the size and connectivity of gaps between vegetation, and BSC s appear to limit invasions within gaps. Results also suggest that cattle grazing reduces invasion resistance by decreasing bunchgrass abundance, shifting bunchgrass composition, and thereby increasing connectivity of gaps between perennial plants while trampling further reduces resistance by reducing BSC . Synthesis and applications . Grazing exacerbates Bromus tectorum dominance in one of North America's most endangered ecosystems by adversely impacting key mechanisms mediating resistance to invasion. If the goal is to conserve and restore resistance of these systems, managers should consider maintaining or restoring: (i) high bunchgrass cover and structure characterized by spatially dispersed bunchgrasses and small gaps between them; (ii) a diverse assemblage of bunchgrass species to maximize competitive interactions with B . tectorum in time and space; and (iii) biological soil crusts to limit B . tectorum establishment. Passive restoration by reducing cumulative cattle grazing may be one of the most effective means of achieving these three goals. Grazing exacerbates Bromus tectorum dominance in one of North America's most endangered ecosystems by adversely impacting key mechanisms mediating resistance to invasion. If the goal is to conserve