An understanding of the manifold impacts of exotic plant species on ecosystems is necessary to justify and guide efforts to limit their spread, restore natives, and plan for conservation. Invasion by annual grasses such as Bromus tectorum, B. rubens, B. hordeaceus, and B. diandrus (hereafter collectively referred to as Bromus) result in major transformations in the structure and function of the ecosystems they dominate. Invasion and domination by exotic Bromus can alter fuel bed conditions and thus fire cycles, microbial communities and thus nutrient cycling and carbon storage, water infiltration and use, invertebrate and vertebrate community structure, and more. While experiments that prove cause-and-effect impacts of Bromus are rare, inferences can be gleaned from the combination of Bromus-ecosystem associations, studies of ecosystem conditions of paired invaded versus uninvaded sites, and an understanding of underlying mechanisms. The goal of this presentation is not to thoroughly review all aspects of ecosystem change following Bromus invasion, but rather to selectively highlight specific changes that demonstrate the breadth of alterations in ecosystem processes and then to emphasize the effects of Bromus invasion on two critical components of the animal community, ants and rodents.
Bromus typically establishes in bare soil patches and can eventually replace perennials such as woody species or bunchgrasses, creating a homogeneous annual cover. Plant productivity and cover are less stable when Bromus dominates. Bromus’ “flash” of growth followed by senescence early in the growing season, combined with shallow rooting and annual habit, may lead to incomplete use of deep soil water, reduced C sequestration, and accelerated nutrient cycling. Litter produced by Bromus alters nearly all aspects of ecosystems and increases wildfire occurrence. Changes in litter, fire, and soil properties appear to feedback to reinforce Bromus’ dominance in a pattern that portends desertification. Conversion to Bromus domination has been shown to increase ant abundance up to 10-fold while altering community structure; conversion appears to greatly increase the abundance of common species/functional groups but decrease the abundance of less common species and functional groups. Bromus invasion also impacts rodent communities, but differently than for ants. Conversion to Bromus resulted in substantial decreases in rodent abundances (an order of magnitude) and species richness (9 to 5 species). Further, we have evidence that the longer a site is Bromus-dominated, the greater the degradation of the rodent community.