Population phase-dependent niche requirements may limit outbreaks of the range-expanding mountain pine beetle in the western boreal forest

Climate warming has exacerbated hyperepidemics by irruptive species in habitats outside their historic range. However, range expansion during epidemics may not result in long-term persistence or proliferation in novel habitats if requirements of sub-outbreak (endemic) populations differ from those of epidemic populations. Mountain pine beetle (Dendroctonus ponderosae), an irruptive bark beetle in western North American lodgepole pine forests, recently breached the Rocky Mountain geoclimatic barrier and expanded into evolutionarily naïve lodgepole pine and jack pine forests. We examined its potential to naturalize in newly invaded habitats by quantifying biotic interactions with trees known to predispose hosts to endemic beetle populations in 16 stands along a transect from the native range to eastern edge of the invasion front. The density of trees susceptible to endemic mountain pine beetle was 3 × greater in native and naïve lodgepole pine than in jack pine. The quality of the endemic niche in novel habitats for D. ponderosae, especially in jack pine, was further constrained by greater competition with other subcortical species. In newly invaded jack pine forests, 50 % of defensively compromised trees were co-occupied by woodboring beetle species versus 20 % in naïve lodgepole and < 1 % in historical lodgepole pine forests, and when woodborers were present, 17 × , 6 × , and 0.25 × more phloem was consumed, respectively. Thus, the niche available to endemic D. ponderosae in western boreal jack pine forests appears to be constrained by both lower resource availability and higher competition, suggesting recurrent outbreaks originating there are unlikely. In contrast, novel lodgepole pine habitats appear suitable for endemic populations, and may generate outbreaks during favorable conditions.