Effects of Livestock Grazing and Well Construction on Prairie Vegetation Structure Surrounding Shallow Natural Gas Wells

Abstract

Short and sparse vegetation near shallow gas wells has generally been attributed to residual effects from well construction, but other mechanisms might also explain these trends. We evaluated effects of distance to shallow gas wells on vegetation and bare ground in mixed-grass prairies in southern Alberta, Canada, from 2010 to 2011. We then tested three hypotheses to explain why we found shorter vegetation and more bare ground near wells, using cattle fecal pat transects from 2012, and our vegetation quadrats. We evaluated whether empirical evidence suggested that observed patterns were driven by (1) higher abundance of crested wheatgrass (Agropyron cristatum) near wells, (2) residual effects of well construction, or (3) attraction of livestock to wells. Crested wheatgrass occurrence was higher near wells, but this did not explain effects of wells on vegetation structure. Correlations between distance to wells and litter depth were the highest near newer wells, providing support for the construction hypothesis. However, effects of distance to wells on other vegetation metrics did not decline as time since well construction increased, suggesting that other mechanisms explained observed edge effects. Cattle abundance was substantially higher near wells, and this effect corresponded with changes in habitat structure. Our results suggest that both residual effects of well construction and cattle behavior may explain effects of shallow gas wells on habitat structure in mixed-grass prairies, and thus, to be effective, mitigation strategies must address both mechanisms.

Effects of shallow natural gas well structures and associated roads on grassland songbird reproductive success in Alberta, Canada

“…Despite shorter and exotic vegetation near gas wells [22, 39], these changes did not affect nest success. This suggests that sufficient vegetation remains on the landscape to allow individuals to select sites with suitable cover and protection from predators [64, 65].…”

“…As is typical of mixed-grass prairies in this area, all our sites were grazed by cattle ( Bos taurus ). Livestock density and density of well pads were not correlated ( r = -0.19, p = 0.28; [22]). Site topography was flat to gently rolling.…”

“…For example, oil wells in the western prairies require permanent access roads, are noisy (56–81 dB(C) at 10 m; n = 64), and are visited by 1–15 vehicles per week [21]. In contrast, shallow gas wells average only 1.4 m in height, are silent [22], and disturbed areas (e.g., bare ground) associated with individual conventionally drilled shallow gas wells in this area are each only 1% of the footprint of oil wells. Further, shallow gas wells are typically accessed by trails rather than permanent gravel roads, and are visited once annually (usually) for maintenance [23].…”

“…Well infrastructure and associated roads may create edge effects, alter vegetation structure near wells, and contribute to the spread of exotic plant species [22, 25], which may lower nest or reproductive success of grassland songbirds [26]. Nest and reproductive success have a significant impact on avian population trends [27, 28], and thus one reason for declining grassland bird populations might be effects of energy development on their reproductive success.…”

“…We also evaluated whether there were residual effects from well construction, as habitat removal, alteration, and/or restoration methods were used during well installment [3, 22]. Because exotic vegetation was briefly used for re-vegetation in the 1980s and early 1990s [29], and topsoil removal was common practice until the early 2000s (“full-build” construction methods; [30]), we predicted that the greatest ecological effects might be from intermediate-aged (15–30 years old) wells.…”