Few systems have data available for quantifying the complex spatial dynamics taking place at range edges or across invasions, primarily due to the challenges of studying low density populations. The depression of population growth rates at low densities, termed the Allee effect, has received a surge of interest over the last decade because of the importance of these dynamics to extinction risk. Empirical data to test predictions from theoretical models is extremely limited, but a large-scale trapping scheme to monitor the spread of an invasive forest pest has provided one of the best opportunities to study Allee effects in natural populations. The Slow the Spread program for minimizing range expansion of gypsy moth (Lymantria dispar) has resulted in over two decades of spatiotemporal abundance data. The gypsy moth system is a noteworthy exception where demographic Allee effects can be empirically quantified and has significantly advanced our ability to test predictions about low-density population dynamics. We examined spatiotemporal variation in Allee effects using 20 years of data spanning the geographic extent of the gypsy moth invasion in the United States from Wisconsin to North Carolina. We also identify a new metric for quantifying demographic Allee effects, termed the Allee slope.
Results/Conclusions
Previous work has shown geographic variation in the Allee threshold for gypsy moth and the relationship between the strength of this effect and spread rates. New region-specific estimates for the southern invasion front show strong relationships between Allee effects and spread rate on a finer spatial scale. We quantified extremely low Allee thresholds for population replacement of 4.4 and 2.2 moths per trap in West Virginia and the Virginia Mountains. Populations in the Virginia Piedmont and Coastal Plain did not have detectable thresholds, as populations did not achieve greater than 50% replacement across any low density population size. We also illustrate the use of the Allee slope to quantify temporal variation, which is more closely linked to the underlying mechanisms leading to variation in Allee effects and provides a continuous variable capable of describing spatiotemporal variation in far greater detail than the Allee threshold. This slope represents the rate at which increases (or decreases) in the population size or density translate into increases (or decreases) in the population growth rate. These results highlight the important role of data from the gypsy moth invasion front for investigating the role of Allee effects in population dynamics.