USGS study: ‘Overcrowded wild horse herds drive out sage grouse’

/ In The News, News
Photo by Meg Frederick

As published by E&E News

A new federal study suggests that the uncontrolled growth of one iconic symbol of the American West — wild horses — is jeopardizing the future survival of another: greater sage grouse.

The peer-reviewed study, published in the Journal of Wildlife Management and led by researchers at the U.S. Geological Survey, found sage grouse populations are negatively affected by the overcrowding of wild horses on federal herd management areas.The study released today, and billed by USGS as the first to evaluate “the ecological relationship” between wild horses and sage grouse, estimated that 70% of grouse populations within herd management areas could be lost within 15 years in the study area of Nevada and northeast California unless wild horse herd sizes on federal rangelands are reduced.

The study does not advocate for the removal of wild horses, but does suggest that reducing the number of feral equids in overcrowded herd management areas “could neutralize their negative impacts” on sage grouse.

“The results of the study indicate that coexistence is possible for free-roaming horses and sage grouse if horse populations are maintained” below the so-called appropriate management level, said Peter Coates, a research wildlife biologist with the USGS’s Western Ecological Research Center and the study’s lead author.

The study noted the importance of the sagebrush ecosystem for the chicken-like bird, which can be degraded by horses that trample the habitat as well as eat the vegetation.

There are currently 86,189 wild horses and burros on federal rangelands, or more than three times the appropriate management level of 26,715 animals that the Bureau of Land Management estimates that federal rangelands can sustain without causing damage to vegetation, soils and other resources.

The 86,189 wild horses and burros, as of March 1, marked the first decline in rangewide populations since 2012, and is 9% less than the record 95,114 wild horses and burros counted last year.

But BLM announced today it may have to round up and remove as many as 6,000 additional wild horses and burros by the end of next month due to the extreme drought conditions in the West that threaten the safety of federally protected horses and burros. BLM so far this year has already conducted numerous emergency gathers to remove nearly 1,200 wild horses and burros due to the drought.

Greater sage grouse, meanwhile, are suffering, too.

A USGS report in March revealed that millions of acres of priority sage grouse habitat have been lost in the last two decades, due to human activity and natural threats. The No. 1 reason for grouse population declines is a loss of habitat, and the study found that wildfires alone have destroyed 20% of priority grouse habitat in the Great Basin area of Nevada since 2000.

But there had never been “empirical evidence linking horse abundance to sage grouse population dynamics” until now, according to the study.

The clash between sage grouse and wild horses is most pronounced in the Great Basin — which has by far more wild horses roaming federal rangelands than any other state — and northeast California, according to the latest USGS study. Both states have been greatly impacted by the ongoing drought.

The USGS researchers developed a computer model based on sage grouse male surveys at breeding grounds, called leks, to estimate how grouse population levels changed relative to the number of wild horses that exceeded the appropriate management level.

It found a greater than 99% probability “of sage grouse population decline” when wild horse populations were greater than three times the appropriate management level — as is the case today rangewide.

In Nevada, as of 2019, all herd management areas had populations greater than four times the appropriate management level, the study says.

“If feral horse populations continue to grow at current rates unabated, model projections indicate sage grouse populations will be reduced within horse-occupied areas by [greater than] 70% by 2034,” the study says.

Overall, in the study area of Nevada and northeast California, sage grouse populations are already declining by about 4% annually, the study concludes, with “strong evidence” that the concentration of wild horse populations plays a major role.

If the wild horse and burro populations in the study area could be reduced to the appropriate management level, the study estimated sage grouse populations would increase 12%.

This suggests, among other things, that “there is potential for rebound where sage grouse populations have previously declined,” according to the study.

“While this study highlights the challenges of maintaining biodiversity in sagebrush environments, it also provides valuable insights into the options for wildlife management,” said David Applegate, the USGS’s associate director temporarily leading the agency.

Neda DeMayo, president of Return to Freedom, a wild horse conservation group, said the USGS study underscores the complexity of managing landscapes for all wildlife.

“This is exactly why it’s critical for diverse groups and agencies to develop good communication and trust, so that we can work together on holistic and sustainable conservation that benefits all,” DeMayo said in an emailed statement. “Better understanding of the specific issues within each wild horse Herd Management Area enables us to design the most effective, humane fertility control program to slow down reproduction so our herds can remain healthy even in the more compromised ranges.”

%d bloggers like this: