Population cycles produce periodic range boundary pulses

Abstract

Classical theories of biological invasions predict constant rates of spread that can be estimated from measurable life history parameters, but such outcomes depend strongly on assumptions that are often unmet in nature. Subsequent advances have demonstrated how relaxing assumptions of these foundational models results in other spread patterns seen in nature, including invasions that accelerate through time, or that alternate among periods of expansion, retraction, and stasis of range boundaries. In this paper, we examine how periodic population fluctuations affect temporal patterns of range expansion by coupling empirical data on the gypsy moth invasion in North America with insights from a model incorporating population cycles, Allee effects, and stratified diffusion. In an analysis of field data, we found that gypsy moth spread exhibits pulses with a period of 6 yr, which field data and model simulations suggest is the result of a 6‐yr population cycle in established populations near the invasion front. Model simulations show that the development of periodic behavior in range expansion depends primarily on the period length of population cycles. The period length of invasion pulses corresponded to the population cycle length, and the regularity of invasion pulses tended to decline with increases in population cycle length. A key insight of this research is that dynamics of established populations, behind the invasion front, can have strong effects on spread. Our findings suggest that coordination between separate management programs targeting low‐density spreading and established outbreaking populations, respectively, could increase the efficacy of efforts to mitigate gypsy moth impacts. Given the variety of species experiencing population fluctuations, Allee effects, and stratified diffusion, insights from this study are potentially important to understanding how the range boundaries of many species change.

Outbreak of an undetected invasive species triggered by a climate anomaly

“…The early-stage dynamics of invasions remain a key challenge to invasion ecologists (Severns et al 2015) and managers. For example, at their invasion front, gypsy moth populations cycle through periods of high and low abundance, which affects both dispersal and the detection of new outbreaks (Walter et al 2015). Similarly, in cases where outbreaks are triggered by anomalies, the ecology of endemic and epidemic equilibria may shed light on these dynamics (Anderson and May 1992).…”

Predicting the spread of all invasive forest pests in the United States

“…; Walter et al . ), potentially lessening the efficacy of management, for instance when implementing containment, rapid response and/or eradication efforts of novel species (Lovett et al . ).…”

Management of invasive Allee species

“…Finally, a recent study shows that spatial variation in Allee effects around a mean tends to facilitate more rapid invasion than a homogeneous mean Allee effect (Walter et al, 2015). Together these studies illustrate the important role Allee effects can play in the colonization, persistence and expansion of invasive species.…”

“…An example of this type of management includes localized pesticide or pathogen applications on infested areas. Recent theoretical work demonstrates how suppressing established populations behind an invasion front can reduce the invasion rate (Walter, Johnson, Tobin, & Haynes, 2015). Manual removal of a pest or its resource in infested areas may also fall in this category (Johnson, Moran, & Driml, 1990).…”

“…Suppressing high density populations is more effective through focused application of either pesticides or a natural enemy such as the Bacillus thuringiensis virus. While, this method is primarily used to reduce negative impacts on local forests, it can have a secondary effect of reducing the number of dispersers moving from the established range to beyond the invasion front, which is thought to be a driver of gypsy moth range expansion (Walter et al, 2015). Finally, the number of dispersers can also be reduced by restricting the anthropogenic movement of gypsy moth life stages, such as egg masses laid on timber products, through a combination of governmental restrictions, public information campaigns and monitoring stations.…”