Larger, more frequent wildfires across the Great Basin have contributed significantly to a decline in greater sage-grouse, according to a new study that also indicates that if this trend continues unabated, it could reduce the population of this indicator species to 43 percent of its present numbers.
The culprit, researchers say, is the influx of exotic annual grasses such as cheatgrass that establish after wildfire removes the native plant community, including sagebrush, the plant upon which sage-grouse are dependent for survival.
Results of the study are being published this week in the journal Proceedings of the National Academy of Sciences.
The Great Basin of North America is a vast landscape that is larger than 75 percent of the countries worldwide. It is comprised primarily of a "sagebrush sea" that is threatened by this cycle of wildfire and cheatgrass, according to Christian Hagen, a senior research associate at Oregon State University and a co-author on the study.
"Our modeling indicates that there are long-lasting effects from wildfire that negate increases in sage-grouse population growth that typically occur after years of higher precipitation," said Hagen, a researcher in OSU's Department of Fisheries and Wildlife. "So even when we have a good precipitation year in the Great Basin, the sage-grouse don't recover as they once did because the habitat to support them has been lost to cheatgrass."
The study area encompassed the hydrographic and vegetation boundaries of the Great Basin and included parts of six western states: Nevada (43 percent of the area), Utah (17 percent), Idaho (16 percent), Oregon (14 percent), California (10 percent) and Wyoming (less than 1 percent).
"Wildfire is increasing in the region because the invading cheatgrass is much more prone to burn and re-establishes orders of magnitude quicker than the native sagebrush," Hagen said. "That isn't necessarily news – Aldo Leopold recognized that more than a half-century ago. But the impact on an indicator species like the sage-grouse had not been so clearly documented."
Sagebrush has slow growth rates and does not re-sprout after wildfires; it must re-establish from the seed bank. This delay in succession opens the door for cheatgrass to establish. The researchers say that understanding "R&R" – resilience to wildfire and resistance to cheatgrass – is key to focusing the right land management practices in the right places.
Cheatgrass hails from warmer regions of Eurasia and generally does not persist in relatively cold and moist climates. Colder, moist soils tend to be the most productive ecological sites in the Great Basin, and these sites promote the growth of perennial bunch grasses, native forbs and shrubs – plants that tend to be more resilient to fire, and resistant to invasive grasses.
These also are some of the most productive sites for the sage-grouse. The sage-grouse is a large gallinaceous, or ground-feeding, bird that can be an indicator for ecological health in sagebrush ecosystems because it requires distinct ecological states to meet its diverse life-history needs. Thus, the population dynamics of the species are considered an ideal metric for assessing disturbances to sagebrush disturbance.
The sage-grouse has been considered several times for protection under the Endangered Species Act, the most recent of which triggered massive changes to land management policy on millions of acres of public land.
The cycle of habitat decline is linked to the introduction of cheatgrass to the ecosystem, the researchers say. Land management practices led to a burning regime as often as every 2-3 years, which allowed fast-growing cheatgrass to establish at the expense of slow-growing sagebrush.
"If you can imagine an area the size of a football field infested with cheatgrass, but surrounded by native vegetation," Hagen said. "If a lightning strike sets the cheatgrass on fire, it likely will consume some of the native vegetation and the burned area doubles or triples, and by next year the cheatgrass infestation has spread.
"It's a classic positive feedback loop that promotes cheatgrass and becomes a negative situation for native plants – and ultimately, for sage-grouse."
The key to reversing this decline, the researchers say, may be to further enhance fire prevention and suppression effectiveness in targeted areas of intact sagebrush that have the highest densities of breeding sage-grouse. The study shows that 90 percent of sage-grouse are concentrated on less than 10 percent of the Great Basin.
"Our study illustrates a path toward stabilizing sage-grouse populations through highly focused wildfire management," Hagen said. "New federal wildland fire policies and priorities in sagebrush steppe, combined with improved collaboration with rural communities through rangeland fire protection associations, greatly increases the odds of curbing population declines due to fire."
Explore further: Q&A: What is a sage grouse and why is the bird imperiled?
Peter S. Coates et al. Wildfire, climate, and invasive grass interactions negatively impact an indicator species by reshaping sagebrush ecosystems, Proceedings of the National Academy of Sciences (2016). DOI: 10.1073/pnas.1606898113