Novel insights on population and range edge dynamics using an unparalleled spatiotemporal record of species invasion

Abstract

Quantifying the complex spatial dynamics taking place at range edges is critical for understanding future distributions of species, yet very few systems have sufficient data or the spatial resolution to empirically test these dynamics. This paper reviews how data from a large-scale pest management programme have provided important contributions to the fields of population dynamics and invasion biology. The invasion of gypsy moth (Lymantria dispar) is well-documented from its introduction near Boston, Massachusetts USA in 1869 to its current extent of over 900,000 km² in Eastern North America. Over the past two decades, the USDA Forest Service Slow the Spread (STS) programme for managing the future spread of gypsy moth has produced unrivalled spatiotemporal data across the invasion front. The STS programme annually deploys a grid of 60,000-100,000 pheromone-baited traps, currently extending from Minnesota to North Carolina. The data from this programme have provided the foundation for investigations of complex population dynamics and the ability to examine ecological hypotheses previously untestable outside of theoretical venues, particularly regarding invasive spread and Allee effects. This system provides empirical data on the importance of long-distance dispersal and time-lags on population establishment and spatial spread. Studies showing high rates of spatiotemporal variation of the range edge, from rapid spread to border stasis and even retraction, highlight future opportunities to test mechanisms that influence both invasive and native species ranges. The STS trap data have also created a unique opportunity to study low-density population dynamics and quantify Allee effects with empirical data. Notable contributions include evidence for spatiotemporal variation in Allee effects, demonstrating empirical links between Allee effects and spatial spread, and testing mechanisms of population persistence and growth rates at range edges. There remain several outstanding questions in spatial ecology and population biology that can be tested within this system, such as the scaling of local ecological processes to large-scale dynamics across landscapes. The gypsy moth is an ideal model of how important ecological questions can be answered by thinking more broadly about monitoring data.

Keywords: Lymantria dispar; Allee effects; biological invasions; gypsy moth; invasion front; invasive species management; low-density population dynamics; range border; spread rate.