Breach of the northern Rocky Mountain geoclimatic barrier: initiation of range expansion by the mountain pine beetle

Aim Our aim is to examine the historical breach of the geoclimatic barrier of the Rocky Mountains by the mountain pine beetle (Dendroctonus ponderosae Hopkins). This recent range expansion from west of the North American continental divide into the eastern boreal forest threatens to provide a conduit to naive pine hosts in eastern North America. We examine the initial expansion events and determine potential mechanism(s) of spread by comparing spread patterns in consecutive years to various dispersal hypotheses such as: (1) meso-scale atmospheric dispersal of insects from source populations south-west of the Rocky Mountains in British Columbia (i.e. their historical range), (2) anthropogenic transport of infested plant material, and (3) spread of insect populations across adjacent stands via corridors of suitable habitat. Location British Columbia, Canada. Methods We explore potential mechanism(s) of invasion of the mountain pine beetle using spatial point process models for the initial 3 years of landscape-level data collection, 2004-2006. Specifically, we examine observed patterns of infestation relative to covariates reflecting various dispersal hypotheses. We select the most parsimonious models for each of the initial 3 years of invasion using information criteria statistics. Results The initial range expansion and invasion of the beetle was characterized by aerial deposition along a strong north-west to south-east gradient, with additional aerial deposition and localized dispersal from persisting populations in following years. Main conclusions Following deposition of a wave front of mountain pine beetles parallel to the Rocky Mountains via meso-scale atmospheric dispersal, the areas of highest intensity of infestations advanced up to 25 km north-east towards jack pine (Pinus banksiana) habitat in a single year. There appeared to be no association between putative anthropogenic movement of infested materials and initial range expansion of the mountain pine beetle across the continental divide.