Abstract
Species of conservation concern are increasingly threatened by multiple, anthropogenic stressors which are outside their evolutionary experience. Greater sage-grouse are highly susceptible to the impacts of two such stressors: oil and gas (energy) development and West Nile virus (WNv). However, the combined effects of these stressors and their potential interactions have not been quantified. We used lek (breeding ground) counts across a landscape encompassing extensive local and regional variation in the intensity of energy development to quantify the effects of energy development on lek counts, in years with widespread WNv outbreaks and in years without widespread outbreaks. We then predicted the effects of well density and WNv outbreak years on sage-grouse in northeast Wyoming. Absent an outbreak year, drilling an undeveloped landscape to a high permitting level (3.1 wells/km2) resulted in a 61% reduction in the total number of males counted in northeast Wyoming (total count). This was similar in magnitude to the 55% total count reduction that resulted from an outbreak year alone. However, energy-associated reductions in the total count resulted from a decrease in the mean count at active leks, whereas outbreak-associated reductions resulted from a near doubling of the lek inactivity rate (proportion of leks with a last count = 0). Lek inactivity quadrupled when 3.1 wells/km2 was combined with an outbreak year, compared to no energy development and no outbreak. Conservation measures should maintain sagebrush landscapes large and intact enough so that leks are not chronically reduced in size due to energy development, and therefore vulnerable to becoming inactive due to additional stressors.
Highlights
Population response to multiple stressors is among the top ten priorities for science to inform conservation and management policy in the United States [1]
In the absence of energy development, we predicted a West Nile virus (WNv) outbreak year would cause the total count to decline by 55% (4537 [CI: 3668, 5507] to 2037 [CI: 1318, 3062]), which is similar to the 61% reduction achieved by drilling to 3.1 wells/km2
Energy development and WNv are both outside the sage-grouse evolutionary experience, and our work is the first to quantify their combined effects, across a 20 million ha region encompassing a wide range of stressor intensities
Summary
Population response to multiple stressors is among the top ten priorities for science to inform conservation and management policy in the United States [1]. Oil and gas (energy) development is an ongoing stressor to wildlife populations on lands throughout the western United States [4], and in 2002, West Nile virus (WNv) emerged as an additional stressor to these populations [5]. The greater sagegrouse (Centrocercus urophasianus; sage-grouse) in northeast Wyoming provides a case study demonstrating the potential consequences of multiple stressors, such as energy development and disease, on a species of conservation interest. Breeding sage-grouse populations are severely impacted at commonly permitted oil and gas well densities [12]. West Nile virus is a known stressor which may compound the impacts from oil and gas development [14]. In response to sage-grouse population declines in Wyoming, conservation areas (termed ‘core areas’) in which new energy development is restricted were delineated in 2008 [16]
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