Disturbance‐Mediated Apparent Competition Decouples in a Northern Boreal Caribou Range

Abstract

The most widely reported threat to boreal and mountain populations of woodland caribou (Rangifer tarandus caribou; caribou) involves habitat‐ or disturbance‐mediated apparent competition (DMAC). With DMAC, natural and anthropogenic disturbances that increase the abundance of deciduous‐browsing cervids (e.g., moose [Alces alces], deer [Odocoileus spp.]) are thought to promote predator (especially wolf [Canis lupus]) numbers, which heightens predation risk to caribou. We know most about the effects of DMAC on caribou where the species is under threat by anthropogenic activities in relatively productive southern boreal and mountain systems. Yet, >60% of extant boreal caribou range in North America consists of northern shield and taiga ecoregions of low productivity where caribou may compete with only 1 ungulate species (moose) in the context of DMAC. In this environment, we know very little of how DMAC acts as a limiting factor to caribou. In Saskatchewan, Canada, from 2014–2018, using a combination of vegetation sampling, aerial surveys, and telemetry data (n = 38 wolves), we searched for evidence of DMAC (trends in data consistent with the hypothesis) in an 87,193‐km2 section of the Western Boreal Shield, a poorly productive but pristine region (0.18% of land cover classed as an anthropogenic feature) with a historically high fire‐return interval (47% of stands aged <40 years). Despite the high levels of disturbance, moose density was relatively low (47 moose/1,000 km2), likely because of the scarcity of deciduous or mixed‐wood stands and low abundance of deciduous browse in the young conifer stands that dominated the landscape. In contrast, boreal caribou density was relatively high for the species (37 caribou/1,000 km2). Wolf density (3.1 wolves/1,000 km2) and pack sizes ( = 4.0 wolves/pack) were low and resident (established) territories were large ( = 4,360 km2; 100% minimum convex polygon). The low density of wolves mirrored the low (standardized) ungulate biomass index (UBI; moose + boreal caribou) of the study area (0.36 UBI/km2). We conclude that wolf and hence caribou populations were not responding in accordance with the outcomes generally predicted by DMAC in our study area because the requisite strong, positive response to fire of deciduous‐browse and alternate‐prey abundance was lacking. As a limiting factor to caribou, DMAC is likely modulated at a macroecological scale by factors such as net primary productivity, a corollary to the general hypothesis that we advance here (i.e., primary productivity hypothesis of DMAC). We caution against managing for caribou based on the presumption of DMAC where the mechanism does not apply, which may include much of boreal caribou range in the north. © 2020 The Wildlife Society.

What do caribou eat? A review of the literature on caribou diet

“…In most cases, caribou separate themselves from other ungulates and consequently also wolves and black bears by selecting mature boreal forests and less productive wetlands (Bowman et al 2010). However, early seral stage forests with hardwood and mixed wood stands are favourable for sympatric ungulates, i.e., moose and white-tailed deer, and thus these habitats tend to have higher predator abundances (Neufeld et al 2021). Mature boreal forests tend to have productive lichen communities (Greuel et al 2021), while early seral stage forests tend to have higher primary productivity and higher vascular plant and graminoid abundance.…”

Calf/female ratio and population dynamics of wild forest reindeer in relation to wolf and moose abundances in a managed European ecosystem

“…To control predation by bears and lynx, regional license allocation for leisure hunting, which is the primary method for regulating bear and lynx abundances in Finland [ 21 ], could constantly inform the vulnerability of wild forest reindeer within the coming decades. However, the managing of wild forest reindeer population should preferably be cautious whenever the mechanism is not identified [ 64 ]. For example, habitat mediated apparent competition appears to decouple in northernmost ranges of woodland caribou where moose and wolf densities are low [ 64 ] likewise in our study area.…”

“…However, the managing of wild forest reindeer population should preferably be cautious whenever the mechanism is not identified [ 64 ]. For example, habitat mediated apparent competition appears to decouple in northernmost ranges of woodland caribou where moose and wolf densities are low [ 64 ] likewise in our study area.…”

Global change impacts a threatened species through alteration of predator-prey dynamics

“…For example, deciduous vegetation growing in logged boreal forests provides high-quality food for moose that can increase their abundance, which in turn can trigger a numerical response of their predators, with a subsequent increase in caribou mortality 28 . Recent work suggests, however, that such strong effect of apparent competition may not occur over most of the northern limits of the boreal caribou range because post-disturbance growth of deciduous vegetation would be insufficient to trigger such numerical responses (e.g., 29,30 ). Global change could alter the situation by enhancing primary productivity and vegetation growth (resource availability) through increased temperature and disturbance rates 31 .…”