Seismic lines are linear clearings created for petroleum exploration and are common in the western Canadian boreal forest. By favouring the growth of early seral vegetation, they can increase the abundance and palatability of woody browse, a ‘browse subsidy’ that may attract ungulates like moose (Alces alces) and white-tailed deer (Odocoileus virginianus), the main prey of wolves. Peatlands traditionally served as refugia from wolves for boreal woodland caribou (Rangifer tarandus caribou), but such subsidies may facilitate disturbance-mediated apparent competition, whereby predator activity associated with deer and moose raises incidental predation risk to caribou. We tested this ’browse subsidy’ hypothesis by comparing availability and proportional use of palatable winter browse on seismic lines and adjacent undisturbed forest plots across a gradient from upland to peatland forest in the Athabasca oil sands region of Alberta, Canada. Seventeen seismic lines were sampled, each with four paired plots across an ecological gradient from upland to peatland. Each plot pair compared the seismic line with the adjacent undisturbed forest. Seismic lines averaged 5.0 more highly palatable stems per 10 m2 (61.5 % more) than adjacent forest plots, while the proportion of stems browsed by deer and moose was 59.2 % higher on lines and increased with browse stem density. However, even after accounting for increases in browse availability, browse use remained higher on seismic lines relative to the adjacent forest, indicating selection of seismic lines by deer and moose. These results suggest that browse subsidy on seismic lines may contribute to disturbance-mediated apparent competition and declines in caribou, particularly where subsidies occur in or adjacent to peatland caribou refugia. Targeted restoration of high-subsidy lines could reduce deer and moose activity in these areas and thus potentially reduce predation risk for caribou.
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