Rabbit Valley is the last exit on I70 as you drive west through Colorado. It is in the McInnis Canyons National Recreation Area and it has several places to camp, so I decided to visit. But as I drove from I70 to Knowles Overlook on the Colorado River I was disappointed to see miles and miles of cheatgrass.
Cheatgrass, Bromus tectorum, is native to North Africa, Southwestern Asia and Europe, but is introduced and solidly established in North America. It was introduced an unknown number of times with ship ballast and as a contaminant in shipments of seed, and was intentionally introduced for an experiment in Pullman, Washington in 1898. We know that it was established in Pennsylvania and New York as early as 1860. It swept through the Great Basin and portions of the Columbia River basin like wildfire, essentially establishing its present distribution in the early 1900s with extensive populations in OR, ID, WA, NV, UT and the eastern edge of California.
Cheatgrass has largely decimated the unique plant communities in the Great Basin, which were known for a variety of perennial bunchgrasses. These communities were quite different from the grassland communities of the Great Plains, which were composed primarily of perennial grasses whose large, intertwined root and rhizome systems formed a matrix so thick that early settlers used it to build sod houses. In contrast, the bunchgrasses of the Great Basin grew literally as clumps of grass, often with bare dirt between them. Sadly, the bunchgrasses have been largely displaced by cheatgrass, which can form monocultures like the one in the accompanying photograph.
A series of characteristics make cheatgrass a superior competitor: remarkable ability to change the timing of the life cycle, production of numerous seeds, extremely dense populations and its propensity to support fires.
On any site, cheatgrass can germinate in the fall to grow as a winter annual, germinate in the spring with the bunchgrasses, or skip years altogether to grow as an ephemeral. It usually germinates in the fall and grows quickly through winter and early spring, gaining a competitive edge in the race to occupy space.
Cheatgrass produces phenomenal amounts of seed. Direct counts of the number of seeds produced per square meter range up to 5,000 and 15,000 in the Great Basin and to 17,000 in Idaho.
High production of seeds leads to dense grass populations. The number of mature cheatgrasses in a square meter in the Great Basin varies from 10 to 13,000.
Cheatgrass dries out earlier in the growing season than the bunchgrasses that it is displacing, and the dense, dry expanses of cheatgrass catch fire easily, burn quickly and decrease time intervals between fires. It is said that fire loves cheatgrass and cheatgrass loves fire, for fire sustains the dominance of cheatgrass.
More frequent fires are changing the ecology of the Great Basin. For example, more frequent fires can greatly reduce sagebrush, which sage grouse use both for food and cover. Deprived of shrubs to hide in, sage grouse are more susceptible to predation from eagles and hawks, so populations of sage grouse in Idaho are declining.
Under most conditions, cheatgrass develops seeds after cleistogamy, a type of selfing in which the flowers do not even open. But after a grass fire, cheatgrass can alter its mating system to allow a low level of outcrossing. This change has the benefit of producing a greater variety of genotypes to enhance adaptation to that specific environment. The predominance of selfing means that a single seed reaching a new habitat can germinate and successfully invade the space, displacing most of the native species.
Previously, my personal experiences had led me to believe that tamarisk was the most destructive of natural populations in the west, for they displace cottonwoods from the edges of creeks and rivers. But now that I have seen monocultures of annuals in valleys and pastures once dominated by perennials, I feel that cheatgrass is the most destructive invader in the west.