Modeling micro-scale ecological processes and emergent patterns of mountain pine beetle epidemics

Forest insect outbreaks can impose significant tree mortality across vast forested landscapes. The current epidemic of mountain pine beetle, Dendroctonus ponderosae Hopkins, for example, has led to the mortality of pine trees in western Canada and the U.S. spanning tens of millions of hectares. The ecological processes driving mountain pine beetle outbreaks are governed by multiple feedback mechanisms, thresholds, and external constraints that exist along a spatial continuum from individual insect–tree interactions to landscape level change. These components of mountain pine beetle epidemics need to be explicitly parameterized in modeling efforts that aim to predict where insect disturbance will occur in a forest each year and the amount of tree mortality that will ensue as a result. However, do date, minimal efforts exist that examine how local level interactions between beetles and trees translate into broader patterns of tree mortality, and those that do are limited to relatively local scales. In this study, we present an agent-based model that simulates how tree mortality results from the combination of beetle–tree interactions, beetle-to-beetle communication, tree defense to beetle attack, beetle density dynamics, host tree availability, dispersal behavior, and landscape heterogeneity. Our model is tested using data from an area in central British Columbia, Canada, that is near the center of the current outbreak in that region. The model simulates both overall tree mortality and spatial patterns of tree mortality, producing results that are similar to those observed in aerial surveys of tree health. Moving forward, the computational efficiency of our model demonstrates the capability to be applied to large, regional landscapes when implemented with sufficient computing resources.