No 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.

Niche differentiation is a way in which similar species avoid competition. Some species do this by specializing in certain prey items. This review aims to determine why the dire wolf (Canis dirus) went extinct while its similar and less abundant relative, the grey wolf (Canis lupus) did not. Both species were present in North America during the Pleistocene, though only one went extinct during the Quaternary extinction event. Physiological differences existed between the two species, mostly due to a greater focus in hypercarnivory for dire wolves. Dire wolves had more robust frame and skull, greater bite strength, and larger carnasials and canines. These differences in dire wolf morphology all help it to handle and kill larger prey species, while the more lithe grey wolf is better adapted to switching to smaller alternative prey. Dire wolves at have been shown to consume mostly large herbivores while grey wolves can survive with lagomorphs as a primary food source. Larger carnivore body size means reduction in locomotor performance, which means that when many mega-herbivores went extinct at the end of the Pleistocene, dire wolves were not as well adapted to switch to smaller prey as grey wolves are. Their naturally larger body mass also means that they needed higher caloric input to maintain their body condition and fecundity. Overall, Canis dirus specialized in larger prey than Canis lupus, so when this prey became extinct, the dire wolf went extinct along with other hypercarnivores such as the North American lion, Smilodon, and short-faced bear.