Research Spotlight: Human Enterprise Brings More Ravens to the Great Basin, Threatening Greater Sage-Grouse

Though ravens are native to the Great Basin, extensive human infrastructure and activities that subsidize resources for ravens, including food, water, and shelter, have led to increases in their abundance since the mid-20^th century. Growth in raven populations as a result of resource subsidies have caused elevated predation rates on nearby sensitive native species like the greater sage-grouse, a phenomenon that can be understood as a form of “hyperpredation.” Though the term “invasive” is typically reserved for non-native species, subsidized ravens and other scavenger species can behave like invasive species—their populations grow and spread quickly as a result of human activity and landscape disturbance, leading to negative economic or ecological consequences.

“Over the past few decades, numerous studies have clearly demonstrated that humans have contributed to the substantial increases in raven numbers,” says USGS WERC scientist and lead author Pete Coates. 

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Recent studies have revealed the profound adverse impacts that this growth is having on highly sensitive species ― which has led to conservation issues that Coates describes as “functionally similar to those of invasive species.”

The greater sage-grouse (Centrocercus urophasianus), a sagebrush-dependent bird, has faced more than 50 years of steady population decline across most of their range. The primary threat to sage-grouse populations has been habitat loss or degradation resulting from wildfire and invasive grass expansion, conifer encroachment, agricultural development, and other human disturbance.

Though raven impacts to sage-grouse have been documented at small scales, the new study is the first to assess this issue across the Great Basin, allowing the authors to map the risk to sage-grouse throughout its range.

Based on nearly 17,000 raven surveys over 10 years, the scientists developed a map of raven density across the Great Basin. They found that raven density was greatest at sites with lower average elevation and with closer proximity to developed areas, agricultural fields, and transmission lines, supporting the idea that human populations subsidize the ravens.

To gather data on sage grouse nest survival, the researchers monitored sage-grouse nests in California and Nevada for seven years, recording whether each nest succeeded at hatching at least one chick or failed to do so.

Then they put the raven data and sage grouse data together, using statistical models to predict how much influence raven density and other environmental factors had on sage-grouse nest survival. Finally, these results were used to build another map, this time showing areas where raven densities were high enough to pose an above average risk to sage-grouse nest survival.

The findings were striking—a majority (64%) of projected sage-grouse breeding concentration areas across the Great Basin and adjoining ecoregions had raven densities associated with below average sage-grouse nest survival. As a point of comparison, less than 10 percent of sage-grouse breeding concentration areas have been impacted by wildfire since the mid-1980s. Wildfire directly destroys sage-grouse habitat and is often considered to have the most profound impact on sage-grouse populations, but the new study suggests that predation as a result of elevated raven numbers is likely more widespread and can impact sage-grouse populations where habitat is still intact.

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The new paper provides both maps and benchmark values that can inform raven management. The researchers estimated a critical raven density of 0.40 ravens per square kilometer, where raven densities exceeding this value corresponded to below average sage-grouse nest survival rates. Though this value is likely a conservative estimate, having a baseline value can help managers identify where sage-grouse are most likely threatened by spillover predation from overabundant ravens. Taking management action for ravens can be difficult, given their status as a native species with cultural value and legal protection under the federal Migratory Bird Treaty Act, as well as their ability to learn quickly and avoid control measures. Maps of raven density and projected risks to sage-grouse are key additions to the toolbox that managers have to make challenging management decisions.

“Because ravens are so common, it is difficult for land and wildlife managers to know exactly where to initiate management strategies to reduce threats to sage-grouse and other wildlife,” says Coates. “Our maps provide an empirically-based decision support tool to help managers make decisions.”

The new study illustrates how sensitive species like sage-grouse can face complex interacting, layered threats—human development simultaneously drives sagebrush habitat loss and subsidizes ravens, which then prey on remaining vulnerable sage-grouse populations. And the greater sage-grouse is not the only sensitive species facing predation by ravens—the federally threatened Mojave desert tortoise and marbled murrelet are among the other bird species studied by USGS that are vulnerable to ravens. The approach used by the USGS WERC team and partners can work for other sensitive species at risk of overabundant ravens.

USGS science illustrates the drivers of population declines for species of concern—and provides tools to help managers navigate multiple, interacting threats to the nation’s wildlife.

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This research spotlight refers to:

Coates, P.S., O'Neil, S.T., Brussee, B.E., Ricca, M.A., Jackson, P.J., Dinkins, J.B., Howe, K.B., Moser, A.M., Foster, L.J. and Delehanty, D.J., 2020. Broad-scale impacts of an invasive native predator on a sensitive native prey species within the shifting avian community of the North American Great Basin. Biological Conservation, 243, p.108409. https://doi.org/10.1016/j.biocon.2020.108409