Site and Date of Introduction: Because B. tectorum was introduced and became invasive in North America before systematic investigation of plant range and distribution and monitoring for invasives had become the norm, there is some debate about the point and date of introduction of B. tectorum. Cheatgrass was first recorded in New York and Pennsylvania in 1861. However, cheatgrass was not introduced in the western United States until the 1880s. It was first recorded in Ontario in 1986, in Washington in 1893, and in Utah in 1894. It was planted in Washington in 1898 in a search for new grasses for the area.
Mode(s) of Introduction: Since cheatgrass began to appear in the western United States simultaneous with the expansion of cropping and other agriculture, it seems reasonable to suppose that cheatgrass was probably introduced in impure seed, although this is speculation. It was intentionally planted in some areas.
Reason(s) Why it has Become Established: Intermontane areas of the American west had been isolated from intensive use until the 1850s and the advent of the gold and silver rushes. Cattle were driven to and from mines, and the previously ecologically isolated areas became important cattle ranching areas. Cattle trails led to localized intensive overgrazing. Likewise, cattle were introduced into the Great Basin in the 1860s when valuable ores were discovered in parts of Nevada. Overgrazing was identified as a serious threat to ecosystem health in the late 1800s, but by this stage the damage had become so widespread that it was effectively irreversible. Because of a lack of intensive grazing pressure throughout evolutionary history in these areas, grasslands never developed the resilient and competitive native sod-forming grasses that might thrive under conditions of overgrazing, and instead are characterized by a delicate layer of cryptogams covering the soil between shrubs, which are susceptible to damage from cattle. Thus, the alteration of natural ecosystem processes essentially created a new and open niche, prime for occupation. It seems possible that B. tectorum may actually have been introduced into rangelands on purpose to stabilize damaged shrub lands.
Ecological Role:
While the establishment of cheatgrass in the western plains was almost certainly facilitated by the introduction of livestock, its persistence and success also depends on another factor: fire. Many of the shrubby areas that have been converted to cheatgrass monocultures originally had a long fire cycle of about 60 to 110 years in some areas and about 30 to 50 in others. Variation in this fire cycle allows heterogeneity in the establishment of native perennials. However, the unique growing cycle of cheatgrass has completely altered fire cycles. Cheatgrass has a life cycle such that it is regarded as a winter annual. It seeds in late summer and germinates in fall. Its seedlings over winter and those that survive have the advantage in spring of having hearty root and photosynthetic systems while other species are establishing. Early in the season, cheatgrass produces above-ground biomass sooner and in greater bulk than its competitors, so that as it dries after seeding in summer before the autumn rains. This creates large quantities of fuel during the dry season. Moreover, it is distributed more continuously than native vegetation, guaranteeing the faster spread of fire in addition to increased frequency. Thus, cheatgrass can reduce the time between fire events to as little as 10 years, effectively decimating its native competitor’s chances of success.
In addition, and perhaps more subtly than the effect on changing fire season, cheatgrass and native shrubby vegetation have different mycorrhizal requirements. Native shrubs are obligate symbionts with vesicular-arbuscular mycorrhizae (VAM), while cheatgrass is merely a facultative symbiont. Thus, it out competes native vegetation in circumstances in which disturbance and thus the disruption of soil infection with mycorrhizae is more frequent. Thus, here again, cheatgrass has a competitive edge when grazing and frequent fires are introduced into native ecosystems.
Benefit(s): Given the level of disturbance and damage to ecosystems during the latter half of the 19th century which continues today, it seems unlikely that ecosystems that have now been taken over by B. tectorum would have been able to recuperate given the lack of native bunch grasses. While the need for cheatgrass in such a situation is undoubtedly unfortunate, it seems almost more reasonable to site habitat/ecosystem destruction as the demise of the shrub-steppe ecotype rather than the establishment or spread of cheatgrass. Cheatgrass may have actually rescued landscapes from erosion and other types of degradation, as intended by those who intentionally introduced cheatgrass into the intermontane western United States.
Threat(s): B. tectorum is a well-established, possibly even integrated, part of may areas of the intermontane western plains. Nonetheless, there do remain some areas in which native shrub-steppe vegetation persists which merit protecting. Control of cheatgrass could therefore yield some native ecosystem conservation. However, there is some question as to how competitive cheatgrass actually is when natural ecosystem processes have not been severely disturbed by cattle ranching. Thus, the conservation of the remnants of shrub-steppe ecosystem seems to mandate a more ecosystem-oriented approach.
Economically, cheatgrass does present material threats to both cattle ranching and wheat farming. While cheatgrass can be a fair grazing grass early in the season before it has flowered, its value as forage decreases significantly later in the season after it has seeded. Moreover, its long, fine awns can actually injure cattle, horses, and other grazing animals. Wheat fields, on the other hand, often become infested with cheatgrass, which significantly reduces crop yields and thus economic output.
Control Level Diagnosis: Highest Priority
Despite the fact that cheatgrass has become an integral and probably inevitable part of many ecosystems, its economic and potential further ecosystem threats are such that it should be an important priority in invasive species control. It is perhaps the economic argument that is most compelling, however. Because cheatgrass is so pervasive, any efforts at control it would need to be biological and ecosystem oriented to be cost-effective.
Control Method: A variety of innovative approaches to the control of B. tectorum exist, most of which involve targeting particular environmental preferences of the grass rather than more traditional chemical and physical removal methods, although these are still employed. In general, these strategies pertain directly to the agricultural type under treatment:
1. Cattle - reducing grazing pressures in an attempt to allow the regeneration and recuperation of native perennial plants may help mitigate the problems associated with cheatgrass in rangelands.
2. Wheat - crop rotation has been suggested as an effective mechanism for cleansing land of cheatgrass infestation. For instance, corn, grain sorghum, proso millet, and sunflower are substantially better spring competitors than wheat and may be able to succeed even when cheatgrass invasion is intense.
Similar techniques are suggested to rehabilitate non-agricultural lands. Because cheatgrass germinates immediately upon seed release, its seeds do not store or hibernate well. Therefore, if other plants or grasses can out compete cheatgrass for a number of years, it seems possible that this might effectively cleanse an area of its infestation. Planting Sudan and forage sorghum in combination with tillage is often cited as a successful method for achieving this. However, as with biological control, this much be approached with caution. Cheatgrass itself became established under similar circumstances when it was introduced as an ecological fix.
To date no effective biological control agent has been found, although a number of GMO bacteria have proven effective in lab tests.<>
