Insect-mediated apparent competition in a large mammal food-web: Spruce budworm impacts the distribution, survival and ecological interactions of boreal caribou

Guillemette Labadie, Biology, Université Laval, Québec, QC, Canada, Philip. D. McLoughlin, Biology, University of Saskatchewan, Saskatoon, SK, Canada, Mark Hebblewhite, Wildlife Biology Program, W.A. Franke College of Forestry and Conservation, University of Montana, Missoula, MT and Daniel Fortin, Département de biologie, Université Laval, Québec, QC, Canada

Background/Question/Methods
Wildfires and forest harvesting threaten boreal caribou (Rangifer tarandus caribou) populations by altering trophic interactions. In eastern Canada, disturbed areas are colonized by deciduous vegetation that provides high-quality food for moose (Alces alces). The subsequent increase in moose populations triggers a numerical response in wolves (Canis lupus), which intensifies predation risk for boreal caribou. While the effect of fire and logging on the ecology of caribou is well-studied, other factors can also generate secondary succession in boreal forests. Insect outbreaks in eastern Canada can disturb larger areas than wildfires and logging combined; yet little information exists on how insect outbreaks might impact boreal caribou. We assessed the impact of spruce budworm outbreaks (Choristoneura fumiferana, SBW) on the distribution, survival and trophic interactions of boreal caribou in an area subject to recent and historic outbreaks of SBW. For this, we collated date on annual surveys of SBW severity in the Côte-Nord region (QC, Canada) from 2006 to 2018, and combined these data with locations of 118 radio-collared caribou and 15 radio-collared wolves from 2005 to 2018; and aerial inventories of moose in 2006 and 2018. To account for comparative effects of SBW outbreak on expected responses in moose, wolves, and caribou, we applied our data to a statistical habitat selection and survival analyses for boreal caribou.
Results/Conclusions
Annual surveys of SBW severity combined with field vegetation inventories indicated that stands impacted with high SBW cumulative severity (e.g., after 6–7 years of severe defoliation) had more deciduous vegetation (40% on average), compared to stands less impacted by SBW (8% on average). This increase in deciduous vegetation with SBW severity increased food availability for moose. Aerial inventories showed that moose density increased by 69% between 2006 and 2018, reaching 1.43 moose/10 km² in 2018. Caribou avoided areas with high SBW cumulative severity. Wolves selected salvage logging of SBW infested stands, and caribou that displayed lower avoidance for these areas had higher mortality risk. Critically, predation on caribou post-outbreak was exacerbated by human activity (salvage logging). Our study shows that insect outbreaks can indirectly influence the distribution and survival of boreal caribou by impacting on ecological interactions, through indirect means. In addition to wildfires and forest harvesting, boreal caribou recovery plans need to consider insect outbreaks in the general guidelines for boreal caribou habitat management.