Demographic Allee effects, the positive dependence of population growth on density at low densities, can result from multiple mechanisms, including mate limitation, predator satiation and cooperative behavior. When present, Allee effects increase the risk that small populations will decline, with important implications for conservation, invasion biology and general understanding of population dynamics. A growing body of theory predicts Allee effects will also interact with other aspects of life history to have important effects on population stability and persistence. A deterministic model developed by Lewis and Karieva (1993) predicts that populations of an invading species subject to an Allee effect require not only a critical density for persistence, but must also occupy a critical geographic area. Populations in small areas will lose a higher proportion to dispersal, depressing density and leading to population contraction and extinction. We tested this prediction with data from the gypsy moth Slow the Spread program which monitors the leading edge of gypsy moth invasion across a large area of the U.S. Spatial data on gypsy moth abundance was used to create a Voronoi tesselation that defined 500-1900 population patches for each of 12 years.
Results/Conclusions
In eleven of twelve years, population persistence depended on both population area and density as predicted. A generalized additive model was used to account for geographic variation in the strength of the Allee effect, proximity to established populations, and variations in sampling effort. Additional evidence for the predicted effect of area came from a negative interaction between area and density in six years. This is the first empirical confirmation of this theoretical prediction about the spatial dynamics of populations subject to an Allee effect. Evidence for a critical area provides support for the importance of understanding the combined effects of the Allee effect and other population processes and suggests population area may be an important factor in both establishment of invasive species and conservation of rare species. This study also highlights the value of large amounts of monitoring data for detecting processes in highly variable and hard to detect low-density populations.