Allee effects driven by predation

Summary1 In a population with Allee effects a positive relationship exists between fitness and population size or density. Allee effects may result in extinction thresholds and are therefore crucial in conservation and management. It has been shown theoretically that Allee effects can be driven by predation; however, there are few empirical data. Previous empirical work on Allee effects has emphasized that taxa with life‐history characteristics such as co‐operative breeding may be prone to such effects. Because predation is a general ecological mechanism, Allee effects may be more widespread than previously thought. 2 We used a series of simple heuristic models to develop a theoretical framework for understanding predation‐driven Allee effects as a function of predator functional and aggregative responses. 3 Predators can create an Allee effect if they have a type I (linear) or type II (saturating) functional response without a type III (sigmoid) aggregative response, or vice versa. In addition, predation must be the main driver of prey dynamics, and prey must have little spatial or temporal refuge from predation. 4 We highlighted several, mainly unrecognized, examples of predation‐driven Allee effects from the literature, the majority of which came from systems that had been perturbed by exploitation or introduced predators. 5 Synthesis and applications . Allee effects can arise from a general ecological process under a variety of different combinations of functional and aggregative responses. Allee effects may thus be present in a broad spectrum of different taxa with different types of life history, not only those taxa, such as broadcast spawners and co‐operative breeders, on which empirical work has focused thus far. Conservation biologists and managers working with heavily exploited or threatened populations, or attempting reintroductions, should be aware of the possibility of a threshold population size or density below which extinction is likely. These thresholds can occur regardless of species life history, if predation is a major source of mortality and spatial and temporal predation refuges are limited.