Differential dispersal and the Allee effect create power‐law behaviour: Distribution of spot infestations during mountain pine beetle outbreaks

Spatiotemporal outbreak dynamics of bark and wood-boring insects

Bark and wood-boring insects (BWI) can cause important disturbances in forest ecosystems, and their impact depends on their spatiotemporal dynamics. Populations are usually at stable, low densities but can be disrupted by stochastic perturbations that trigger a transition to an epidemic phase. For less aggressive species, outbreaks die out quickly once the perturbation is removed, while aggressive species exhibit density-dependent feedback mechanisms facilitating persistent landscape-scale outbreaks. The interactions of attributes of tree, stand, landscape- and regional-scale stressors, and insect life history and behavior determine system-specific dynamics. However, most of our knowledge is based on a few species of mainly Holarctic bark beetles. With global change, it is becoming increasingly important to improve our understanding of the frequency and severity of BWI outbreaks.

The Signature of Endemic Populations in the Spread of Mountain Pine Beetle Outbreaks

The mountain pine beetle (MPB) is among the most destructive eruptive forest pests in North America. A recent increase in the frequency and severity of outbreaks, combined with an eastward range expansion towards untouched boreal pine forests, has spurred a great interest by government, industry and academia into the population ecology of this tree-killing bark beetle. Modern approaches to studying the spread of the MPB often involve the analysis of large-scale, high-resolution datasets on landscape-level damage to pine forests. This creates a need for new modelling tools to handle the unique challenges associated with large sample sizes and spatial effects. In two companion papers (Koch et al. in Environ Ecol Stat. https://doi.org/10.1007/s10651-020-00456-2 , 2020a; J R Soc Interface 17(170):20200434, 2020b), we explain how the computational challenges of dispersal and spatial autocorrelation can be addressed using separable kernels. In this paper, we use these ideas to capture nonstationary patterns in the dispersal flights of MPB. This facilitates a landscape-level inference of subtle properties of MPB attack behaviour based on aerial surveys of killed pine. Using this model, we estimate the size of the cryptic endemic MPB population, which formerly has been measurable only by means of costly and time-intensive ground surveys.