Date of this Version
Human–Wildlife Interactions 15(3):270–288, Winter 2021 • digitalcommons.usu.edu/hwi
The common raven (Corvus corax; raven) is native to North America and has increased in abundance, especially throughout western North America, during the last century. Human subsidies have facilitated raven dispersal into less suitable habitats and enabled these populations to maintain higher annual survival and reproduction. Concomitantly, overabundant raven populations are impacting other native at-risk species such as the greater sage-grouse (Centrocercus urophasianus) and potentially the Gunnison sage-grouse (C. minimus). Using Breeding Bird Survey data from 1995–2014, we evaluated raven count data to quantitatively describe changes in abundance and expansion into sagebrush (Artemisia spp.) ecosystems, specifically sage-grouse habitat. We focused our analyses on the 7 sage-grouse management zones (MZs) delineated across 11 western U.S. states and 2 Canadian provinces. We assessed the effects of land cover and anthropogenic disturbance on instantaneous growth rate (r) or carrying capacity (K) of ravens. Abundance of ravens in western and southeastern MZs was greater than northeastern MZs within the greater sage-grouse range. While raven abundance was lower in MZ I and II (Alberta, Canada; Dakotas, Montana, and northwestern Colorado, USA; Saskatchewan, Canada; and Wyoming, USA), raven expansion and percent increase were equivalent or greater than all other MZs. High abundance in MZ VII indicated Gunnison sage-grouse have been exposed to increased raven populations for several decades. Areas with greater electric power transmission line density had higher r; higher K was positively related to proportion of urban land cover within 25 km and burned area within 3 km and negatively related to greater distance from landfills and proportion of forest land cover within 15 km. Ravens have capitalized on human subsidies to increase abundance and expand into sagebrush ecosystems that did not historically support high raven populations. As such, managers are now faced with a new dilemma of reducing populations of a native species to benefit other native sagebrush obligate species.
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