LARAMIE, Wyo.— A scientific study by sage grouse scientists, published in the December issue of Wildlife Biology, confirms that the height of grass cover in nesting habitat is a key factor in determining greater sage grouse nest success. Livestock eats native vegetation that sage grouse require for hiding their nests from predators.
“Because livestock grazing determines how much grass height remains, this study demonstrates that livestock grazing is often a major factor in sage grouse population declines by causing lower nest survival,” said Erik Molvar, wildlife biologist for WildEarth Guardians. “This study is proof that irresponsible livestock grazing practices make it difficult for sage grouse populations to survive.”
The long-term study compares nest survival rates in the Montana portion of the Powder River Basin with the drier habitats farther south in Wyoming over a five-year period, finding that “grass height is a strong predictor of nest survival inside intact landscapes and increasing hiding cover can increase nest success.”
Specifically, Montana and Wyoming populations showed significantly higher nest survival rates with higher average grass height. When grass heights averaged 7 inches, grouse nests in the Wyoming part of the study had a 75 percent chance of survival, while Montana nests had only a 47 percent survival rate when grass heights averaged 7 inches. The Montana nests had a 60 percent survival rate at 10.2 inches of grass cover, but didn’t reach the 75 percent survival threshold until grass heights topped 15 inches.
This research is consistent with previous research that indicated that land managers should maintain at least 7 inches of grass height in sage grouse nesting and chick-rearing habitats in drier parts of the sage grouse range. Along the eastern fringes of the species’ range, where sagebrush are smaller and sparser, a minimum of 10.2-inch grass height is recommended.
“This research confirms earlier studies conducted in eastern Oregon where shorter grass height also had negative effects on sage-grouse nesting success,” said Michael Connor of Western Watersheds Project. “These impacts from grazing livestock are widespread across the range of the sage-grouse and federal agencies need to specifically address this threat in their land management plans if sage-grouse populations are to recover.”
“The more grass cows eat, the fewer sage grouse survive on public lands,” said Randi Spivak with the Center for Biological Diversity. “The livestock industry no longer has any defense that cows on public lands do not hurt sage grouse.”
Federal agencies and states have been resistant to adopt specific standards for grazing to maintain adequate grass height to provide hiding cover for sage grouse during the nesting and brood-rearing seasons.
“In some parts of the West, ranchers blame predators for sage grouse declines, but when livestock grazing strips away the cover that sage grouse need to hide, the birds and their nests become easy pickings,” said Molvar. “This new study demonstrates the importance of strict regulation of livestock grazing to protect sage grouse habitat.”
Authors of the study are widely published sage-grouse scientists, including Kevin Dougherty of the U.S. Fish and Wildlife Service, Dave Naugle and Brett Walker of the University of Montana, Jason Tack of Colorado State University and Jeff Beck of the University of Wyoming.
The full study is available online at http://www.uwyo.edu/esm/faculty-and-staff/beck/_files/docs/publications/doherty-et-al-2014-wildlife-biology.pdf