Spatial variation in Allee effects influences patterns of range expansion

Allee effects are thought to slow range expansion and contribute to stable range boundaries. Recent studies have shown Allee effects to vary spatiotemporally due to influences of environmental heterogeneity on population processes. Gradients in Allee effects might occur as a species' range approaches suboptimal conditions while expanding into new territory. Allee effects could exhibit patchiness if drivers of positive density dependence (e.g. mate finding rates) are influenced by habitat patchiness. However, theoretical studies have largely assumed Allee effects to be spatially constant. The goal of this study was to evaluate how spatiotemporal patterns of range expansion respond to spatial variations in Allee effects. We simulated spread in landscapes that differed in the spatial configuration and range of Allee thresholds. We compared spread with a constant Allee effect to spread in landscapes where the Allee threshold varied along a gradient or in a patchy fashion. Landscape configuration affected patterns of range expansion when Allee thresholds were near or exceeded the number of colonizing immigrants. In gradient landscapes, spread decelerated as the range edge approached higher Allee thresholds. In patchy landscapes, spread advanced quickly through areas with lower Allee thresholds and stalled in areas with higher Allee thresholds. Both focal and neighboring locations influenced spread. Spatial variation in Allee effects may be an underappreciated source of heterogeneity in patterns of range expansion. When Allee effects vary, spread estimates based on a spatially averaged Allee threshold may not accurately predict realized rates of spread. Our findings suggest that spread can occur despite generally high Allee thresholds if Allee thresholds are low in a subset of patches. This result has negative implications for controlling the spread of invasive species, but it also suggests range shifts by native species in response to climate change may be possible with even sparsely distributed refugia from Allee effects.