Stratified dispersal explains mountain pine beetle's range expansion in Alberta

TL;DR

This study models mountain pine beetle dispersal in Alberta using fat-tailed kernels, revealing that rare long-distance flights significantly influence their range expansion.

Contribution

The paper introduces a dispersal model based on fat-tailed Student's t-distribution kernels, providing a biologically interpretable and accurate fit for MPB dispersal data.

Findings

Fat-tailed kernels best fit the dispersal data.

Median dispersal distance is 60 meters, with some beetles traveling nearly 5 km.

Long-distance dispersal events are crucial for range expansion modeling.

Abstract

The mountain pine beetle (MPB), a destructive pest native to Western North America, has recently extended its range into Alberta, Canada. Predicting the dispersal of MPB is challenging due to their small size and complex dispersal behavior. Because of these challenges, estimates of MPB's typical dispersal distances have varied widely, ranging from 10 meters to 18 kilometers. Here, we use high-quality data from helicopter and field-crew surveys to parameterize a large number of dispersal kernels. We find that fat-tailed kernels -- those which allow for a small number of long-distance dispersal events -- consistently provide the best fit to the data. Specifically, the radially-symmetric Student's t-distribution with parameters {\nu} = 0.012 and {\rho} = 1.45 stands out as parsimonious and user-friendly; this model predicts a median dispersal distance of 60 meters, but with the 95th percentile of dispersers travelling nearly 5 kilometers. The best-fitting mathematical models have biological interpretations. The Student's t-distribution, derivable as a mixture of diffusive processes with varying settling times, is consistent with observations that most beetles fly short distances while few travel far; early-emerging beetles fly farther; and larger beetles from larger trees exhibit greater variance in flight distance. Finally, we explain why other studies have found such a wide variation in the length scale in MPB dispersal, and we demonstrate that long-distance dispersal events are critical for modelling MPB range expansion.