Invaders Unhinge an Ecosystem

Ironically, an exotic grass introduced by human beings now threatens the sagebrush ecosystem that expanded so dramatically as a result of human beings and their cattle. The sagebrush populations exploded after overgrazing removed the cold-adapted native grasses. Those grasses used to carry periodic, low-intensity fires that would burn up encroaching brushes. The grass, on the other hand, usually came back quickly after a fire, taking advantage of the nutrients the fire scattered across the soil in the form of ashes. But once the cattle ate most of the grass and eliminated the brushfires, the sagebrush occupied millions of acres of former grassland.

Then, in about 1900, ranchers introduced an Asian grass called cheatgrass, which sprouts in the fall and produces a huge number of seeds. By June, the portion of the grass above the ground dies, providing a ready host for fire. When lightning or human-caused fires ignite the cheatgrass, the flames spread rapidly and kill the sagebrush without hurting the dormant grass roots. The grass sprouts back from the roots and eventually forms dense stands that crowd out almost all other plants. Unfortunately, few of the native animals and insects are adapted to the cheatgrass, which has only a brief growing season when deer, cattle, pronghorns, rodents, and birds like chukars and partridges can digest it. Once the cheatgrass puts out seeds that are covered in a sharp, barbed coat, hardly any animals can eat it.