  The Mountain pine beetle (MPB) is a destructive forest pest that undergoes
boom-bust cycles every 20-40 years. It also has unusual life-history traits,
including a chemosensory affinity for large pine trees, coordinated attacks
facilitated by aggregation and anti-aggregation pheromones, and variable
dispersal that increases in the absence of trees. Using an empirically
calibrated model, we show that MPB's distinctive life-history characteristics
are responsible for its distinctive population dynamics. Specifically, three
life-history traits are necessary for episodic boom-bust dynamics:
density-dependent dispersal, where beetles disperse once most of the large
trees have been killed; an Allee effect, which requires a threshold number of
beetles to overcome tree defenses; and a short generation time, notably in
comparison with their pine tree hosts. In addition to explaining the
qualitative behavior of MPB dynamics, these three ingredients explain residual
tree density, the duration of outbreaks, and the average waiting time between
outbreaks. The peak number of beetles, believed to be the primary factor
driving range expansion, is mostly a consequence of MPB's constitutive high
fecundity. However, two life history traits -- MPB's size-dependent fecundity
and preference for large trees -- are responsible for 25% of the peak number of
beetles. By identifying patterns across the extensive MPB literature, and by
integrating data across multiple sources, we develop a highly accurate
description of MPB's density-dependent interactions with lodgepole pine. We
conclude with a simplified mechanistic model, distilled to two difference
equations, effectively capturing the essence of MPB outbreak dynamics.
