The invasive annual grass cheatgrass (Bromus tectorum) is one of the most severe ecological threats facing North America’s rangelands. Originally from Eurasia, it rapidly established itself across the Western United States after its introduction in the mid-1800s, particularly in the sagebrush steppe and Great Basin regions. Cheatgrass is a winter annual that dominates landscapes across millions of acres. The detrimental effect of this grass stems from its ability to fundamentally alter natural processes, threatening biodiversity, and creating enormous financial costs.
Acceleration of Wildfire Frequency
The presence of cheatgrass radically alters the natural fire regime, creating a self-reinforcing phenomenon known as the grass/fire cycle. Native sagebrush ecosystems historically experienced infrequent fires, often with a return interval of 60 to 100 years or more. Cheatgrass transforms the sparse, patchy vegetation into a dense, continuous carpet of fine fuel that connects the native plants.
This dense fuel ignites easily and burns rapidly due to its fine texture. The plant’s winter annual life cycle means it dries out and “cures” much earlier in the season than native species, often by late April or June, significantly extending the fire season. This abundance of dry, flammable material allows fires to start more easily and spread faster.
In areas heavily infested with Bromus tectorum, the fire return interval can be shortened dramatically, sometimes to as little as three to five years. These frequent, fast-moving fires destroy the slow-to-recover native perennial grasses and shrubs. The cheatgrass seeds, adapted to survive the heat, germinate quickly in the newly cleared ground, resulting in a monoculture that fuels the next fire and permanently transforms the habitat.
Biological Mechanisms of Dominance
Cheatgrass is a successful invader because of specific biological traits that give it a competitive advantage over native perennial plants. Its seeds primarily germinate in the fall or early winter when soil temperatures are low and moisture is available. This early start allows the cheatgrass to establish a root system and begin growth while native perennial species are still dormant.
The grass develops a shallow, fibrous root system that is highly effective at rapidly absorbing water and nutrients, particularly nitrogen, from the upper soil layer early in the spring. By the time native grasses and shrubs emerge from dormancy, much of the season’s limited soil moisture and nitrogen has already been depleted.
Individual cheatgrass plants are highly prolific, capable of producing over 500 seeds under optimal conditions, quickly building a massive, viable seed bank in the soil. The plant is also predominantly self-pollinating, guaranteeing successful reproduction even in isolated environments. These traits enable cheatgrass to quickly outcompete native seedlings and establish dense stands that resist the return of native vegetation.
Displacement of Native Plant Life
The competitive traits of cheatgrass result in a profound loss of biological diversity and a simplification of the entire ecosystem structure. As cheatgrass establishes dense monocultures, it physically replaces the native perennial grasses and the long-lived shrubs, particularly the keystone sagebrush species. This replacement eliminates the complex, layered structure of the native habitat, which is necessary for a variety of wildlife species.
For instance, species like the Greater Sage-Grouse rely on sagebrush for nesting and winter forage, and the pygmy rabbit requires sagebrush for both food and protective cover. When cheatgrass takes over, these animals lose their necessary habitat, leading to population declines.
The shallow-rooted nature of cheatgrass contrasts sharply with the deep root systems of native shrubs, which can extend up to eight feet into the soil. This difference results in the loss of deeper soil-cycling benefits. The annual die-off of cheatgrass roots changes the soil’s microbial community, often favoring nitrifying microorganisms. This alteration leads to an accumulation of nitrate nitrogen, which cheatgrass is better suited to utilize than many native plants, further entrenching its dominance.
Economic Burden on Rangeland
The ecological destruction caused by cheatgrass translates directly into significant financial costs for land managers, ranchers, and taxpayers. Cheatgrass reduces the quality and quantity of forage available for livestock, negatively impacting ranching operations. While the grass is briefly palatable and nutritious early in the spring, it quickly dries out and offers little nutritional value for the majority of the grazing season.
Once the plant matures, its sharp, bristly awns can injure the mouths and eyes of grazing animals, making it undesirable forage. The resulting loss of productive pasture decreases the land’s value, with some reports indicating a reduction in rangeland cash rent values by as much as 80% in heavily infested areas.
The costs associated with managing the invasion are substantial, involving mechanical removal, prescribed burns, and the application of herbicides. Annual economic losses to the grazing industry are estimated to be in the tens of millions of dollars in individual western states, such as \\(32 million in annual losses for Wyoming’s grazing economy. Control efforts using chemical treatments, such as pre-emergent herbicides, can cost anywhere from \\)15 to over \$40 per acre, representing a major financial burden for land managers.