Allee effects in biological invasions

Abstract

Understanding the dynamics of small populations is obviously important for declining or rare species but is also particularly important for invading species. The Allee effect, where fitness is reduced when conspecific density is low, can dramatically affect the dynamics of biological invasions. Here, we summarize the literature of Allee effects in biological invasions, revealing an extensive theory of the consequences of the Allee effect in invading species and some empirical support for the theory. Allee effects cause longer lag times, slower spread and decreased establishment likelihood of invasive species. Expected spatial ranges, distributions and patterns of species may be altered when an Allee effect is present. We examine how the theory can and has been used to detect Allee effects in invasive species and we discuss how the presence of an Allee effect and its successful or unsuccessful detection may affect management of invasives. The Allee effect has been shown to change optimal control decisions, costs of control and the estimation of the risk posed by potentially invasive species. Numerous ways in which the Allee effect can influence the efficacy of biological control are discussed.

Allee effects and population spread in patchy landscapes

“…Allee effects, defined as positive density dependence of growth rates at low densities [1], are increasingly being recognized as important factors determining the success of species invasion [26]. Several mechanisms that operate at small population density may induce an Allee effect, for example, reduced fertilization of sessile organisms, reduced mate finding, reduced defence against a predator, reduced hunting efficiency of predators who forage in groups [3].…”

A model for a spatially structured metapopulation accounting for within patch dynamics

“…Similarly, for the successful reintroduction of a species, the release size needs to be sufficiently large for the population density to exceed the critical threshold [37]. The Allee effect can also be exploited to create more efficient strategies for managing invasive pests [38].…”

Allee Effect and Control of Lake System Invasion

“…For the invader population size p in a lake we use the classical differential equation population model [28] dp…”

“…Therefore the problem of invasion stopping may be solvable only if there is a critical population size, or equivalently, a critical invader flow, below which the invader population cannot establish. This means that there must be Allee effect for the invader [6,28,29], and we shall consider the class of models satisfying this condition.…”