Research Spotlight: New Research Indicates that Greater Sage-Grouse are Struggling to Adapt to Wildfire-Induced Changes in the Great Basin

Sources/Usage: Public Domain. View Media Details

The greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse) is an iconic western species, a symbol of remote landscapes. Sage-grouse populations in the Great Basin have been in decline over the past 30 years. While the decline has been linked to an accelerated cycle of wildfire and invasion of cheatgrass (annual grass), exactly how these landscape changes drive sage-grouse losses has been uncertain. Results of a new study, published in Global Change Biology, indicate that changes in land cover as a result of wildfire is influencing nesting success, which may be driving population declines.

Using the locations of 786 sage-grouse nests monitored from 2009 to 2018, USGS researchers and partners identified which environmental factors influenced where sage-grouse selected a nest site and used this information to create maps of preferred sage-grouse nesting habitat across the region.

Most of the environmental factors that were associated with nest site selection by sage-grouse were those that seem to reflect a healthy environment for eggs and chicks. Overall, the birds preferred rugged, high-elevation sites with plenty of sagebrush, sites that provide cover from nest predators like ravens. But a surprisingly large portion of nests were in areas that did not seem well-suited for hatching chicks: 23 percent of selected nests were in areas that had burned in recent wildfires and were often dominated by invasive cheatgrass.

The fact that so many nests appeared to be in less ideal habitat underscores an issue with many studies of sensitive species and their habitats.

“Just knowing that an animal is there doesn't tell you much about the quality of the habitat, only that it meets some bare minimum requirement for the animal to exist there in the first place,” says lead author Shawn O’Neil.

To address the issue of habitat quality, the scientists needed a way to measure whether the sage-grouse nesting habitats they mapped in the Great Basin had thriving, growing populations—what ecologists call “source” habitats—or whether they were just hanging on, with populations in decline (“sink” habitat). They selected nest survival—the probability that a nest fledges at least one chick. Over the 10-year monitoring period, the researchers tracked the fate of every nest. These data were then used to map the predicted probability of nest survival for sage-grouse habitat throughout the Great Basin.

By combining the two maps—one of the types of nesting habitats the sage-grouse selected, one of nest survival--the researchers could determine whether sage-grouse were actually selecting high quality nesting sites for their young.

Sources/Usage: Public Domain. View Media Details

The results revealed a striking mismatch: many sage-grouse were selecting nest sites where their eggs and chicks were likely to have poor rates of survival. Sixteen percent of the area was classified as “maladaptive”, where highest selection categories intersected lowest survival estimates. Below-average survival was predicted for 58% of nesting habitat within 17 kilometers of active breeding areas, known as leks.

“That area was larger than we expected,” said O’Neil, “The birds continue to nest in those areas, even when they seem to be low quality habitat.”

The study did not explore exactly why sage-grouse chose such poor quality nesting sites, but the researchers believe that it is related to site fidelity. Greater sage-grouse tend to choose the same nesting sites they chose in years past, and where previous generations of sage-grouse have nested. Historically, this may have been a good strategy—if a habitat was good in the past and little has changed, it will likely be a good nesting site again in the future, and returning to a prior site saves the energy of finding a suitable location.

But in recent years, the Great Basin has undergone rapid transformation. Invasive cheatgrass has spread across sagebrush ecosystems. The cheatgrass dries out and burns easily, and these wildfires facilitate further spread of cheatgrass, in a self-perpetuating process that scientists call the “grass-fire cycle.” Heavy winter precipitation followed by drought can exacerbate the grass-fire cycle.

The grass-fire cycle has transformed large areas of sage-grouse habitat, burning the sagebrush and perennial grasses that the birds use for cover from predators and replacing these plants with cheatgrass. About half (51%) of the area classified as high-selection, low-survival was associated with recent burns or more than 10% annual grass cover.

Sources/Usage: Public Domain. View Media Details

The new publication suggests that greater sage-grouse may not be adapting quickly enough to the changes driven by the grass-fire cycle and other types of disturbance in the Great Basin, leading to population declines.

“Although we have previously concluded that the grass-fire cycles adversely impact sage-grouse population growth rates, these new findings reveal the nesting-stage process behind these patterns to better inform management practices aimed at reducing these negative effects,” says co-author Peter Coates.

In other words, better understanding of the “why” of population declines can help land managers select and tailor strategies for sage-grouse conservation. While the combined effect of the grass-fire cycle and nest selection is only one factor in sage-grouse decline, the results of the new study can provide new insight into sage-grouse management. The maps produced in this study can identify high-quality habitat, with both high levels of site selection and nest survival, as well as areas that have been degraded and where sage-grouse may be in jeopardy.

O’Neil cautions that locations on the map with lower quality habitat should not be written off—these are still areas that sage-grouse are using for nesting and may represent opportunities to improve habitat through restoration. Further disturbance to these areas will only add to the problem.

Sources/Usage: Public Domain. View Media Details