Variation in Allee effects: evidence, unknowns, and directions forward
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Allee effects, positive effects of population size or density on per-capita fitness, are of broad interest in ecology and conservation due to their importance to the persistence of small populations and to range boundary dynamics. A number of recent studies have highlighted the importance of spatiotemporal variation in Allee effects and the resulting impacts on population dynamics. These advances challenge conventional understanding of Allee effects by reframing them as a dynamic factor affecting populations instead of a static condition. First, we synthesize evidence for variation in Allee effects and highlight potential mechanisms. Second, we emphasize the “Allee slope,” i.e., the magnitude of the positive effect of density on the per-capita growth rate, as a metric for demographic Allee effects. The more commonly used quantitative metric, the Allee threshold, provides only a partial picture of the underlying forces acting on population growth despite its implications for population extinction. Third, we identify remaining unknowns and strategies for addressing them. Outstanding questions about variation in Allee effects fall broadly under three categories: (1) characterizing patterns of natural variability; (2) understanding mechanisms of variation; and (3) implications for populations, including applications to conservation and management. Future insights are best achieved through robust interactions between theory and empiricism, especially through mechanistic models. Understanding spatiotemporal variation in the demographic processes contributing to the dynamics of small populations is a critical step in the advancement of population ecology.
KeywordsAllee threshold Critical density Depensation Extinction Invasion Positive density dependence
We thank Kyle Haynes and Sandy Liebhold for helpful discussions and comments on earlier drafts of this manuscript. Two anonymous reviewers also provided helpful comments. This research was partially funded by an Award of Domestic Cooperative Agreement from the United States Department of Agriculture, Northeastern Area (13-CA-11420004-231) to DMJ. JAW was supported by a United States Department of Agriculture National Institute of Food and Agriculture Postdoctoral Fellowship grant 2016-67012-24694 and KLG was supported by USDA NIFA Fellowship Grant 2014-67012-23539.
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