Review of Public Broadcasting Nature film The Sagebrush Sea

This past week, Public Broadcasting’s Nature film series featured the Sagebrush Sea. The film’s main focus was on the Greater Sage Grouse which is the emblematic creature found in this vast landscape that covers the bulk of many western states including substantial parts of New Mexico Wyoming, Colorado, Utah, Nevada, Oregon, California, Montana and Idaho...While the movie’s producers did mention fragmentation of habitat resulting from oil and gas development as one major threat, it failed to articulate the greatest threat coming from livestock production. Like everyone from Secretary of Interior Sally Jewell on down to some conservation organizations, there is a disconnect between the proximate causes of sage grouse decline and the ultimate cause. For example, the movie did mention cheatgrass wildfires as a threat to the grouse. Cheatgrass is an annual grass that dries out early and becomes highly flammable fuel for fires. While sage brush ecosystems are adapted to fire, the historic fire regime burned these landscapes at intervals of decades to hundreds of years. This gave sage brush plenty of time to recover from any fire. However, with the advent of cheatgrass fires, the interval has shortened dramatically with some Sagebrush Sea sites burning every few years so that the only plant that can survive on cheatgrass infested sites is cheatgrass. Where the film failed, however, is making the connection between livestock grazing and the spread of cheatgrass. Cheatgrass does not magically appear. Its spread is exacerbated by livestock production. The natural sage brush grassland is dominated by perennial bunchgrasses that are widely spaced with the intervening soil covered by lichens, mosses, and algae collectively called “biocrusts”. These biocrusts serve many purposes. By capping the soil, they prevent wind and water erosion. They capture atmospheric nitrogen and transfer it to the soil, enriching the site for all plants. However, what might be the most important factor in the health of the Sagebrush Sea ecosystem is these biocrusts hinder the establishment and germination of cheatgrass seeds. So when the biocrust is intact and healthy, cheatgrass has a difficult time becoming established and competing with the native grasses on the site. Livestock hooves, however, trample and break up the soil biocrusts giving cheatgrass the added advantage it needs to colonize the landscape. Cattle further tip the balance in favor of cheatgrass by selectively grazing the native perennial grasses. Heavy grazing can reduce the production of seeds by such natives, and reduce their vigor. Over time the loss of perennial grasses favors even greater colonization by cheatgrass. It must be mentioned that historically over most of the Sagebrush Sea ecosystem bison or other large herding herbivores were absent. The dominant hoofed animals were highly migratory bands of pronghorn and deer so the soils of these sagebrush ecosystems are not adapted to concentrated heavy trampling by native wildlife. Finally if the annual cheatgrass burns, its seeds, buried in the soil, quickly germinate, and outcompete the native perennial grasses which tend to produce successful seed crops and germination at irregular and long intervals...more