A Primary Productivity Hypothesis for Disturbance-Mediated Apparent Competition for Boreal Caribou in Canada

The most widely reported threat to populations of boreal and mountain woodland caribou (Rangifer tarandus caribou) involves what has come to be known as disturbance-mediated apparent competition (DMAC). Here, anthropogenic and natural disturbances that increase the abundance of deciduous-browsing cervids (e.g., moose [Alces alces] and white-tailed deer [Odocoileus virginianus]) are thought to promote predator (especially wolf [Canis lupus]) numbers, in turn heightening predation risk to caribou. We know most about the hypothesis of DMAC as it relates to caribou where the species is under threat by industry; i.e., from relatively productive southern boreal and mountain systems where landscapes are highly managed and multiple species of predators and ungulate prey interact with caribou. Yet almost 2/3 of extant boreal caribou range occurs in poorly productive, wildfire-dominated areas where caribou compete with only one ungulate species (moose) in the context of DMAC. In Ch. 2, using data specific to the Saskatchewan Boreal Shield, I tested for evidence of DMAC with data specific to an area of previously known low primary productivity. I found that the successional dynamics after fire of the low-productivity boreal shield did not allow for flushes in deciduous browse, meaning moose density could not increase and resulting in no evidence for DMAC in this system. To test predictions consistent with DMAC, in Ch. 3, I examined the relationship between net primary productivity (NPP) with calf recruitment and adult female survival at a national scale. I accounted for variables influencing DMAC, including metrics of large mammal richness, alternative prey biomass, and predator biomass. While geographic site played an important role, NPP was the most important variable in beta regressions, visually influenced PCA dimensionality in the dataset, and was a primary causal factor for reduced caribou survival and recruitment in Structural Equation Models (SEM). The results indicate that NPP and anthropogenic disturbance act as an impetus for DMAC, where the phenomenon is unlikely to occur in low-productivity areas. Overall, I postulate that the DMAC phenomenon is dependent on NPP, or energy in the system, where burned areas of low NPP may not create the conditions necessary for DMAC to occur. Understanding what factors influence where DMAC occurs and at what scale will be critical for determining effective conservation strategies for local caribou range-planning and Canada’s federal Recovery Strategy for boreal caribou.

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