The Allee Effect: Its History and Future Importance

The role of mutually beneficial interactions (++, cooperation) is a rapidly growing research field in population dynamics, microevolution, and conservation biology. Such positive influences cause destabilizing pressures in population dynamics (anti-regulating factors), and can generate Allee effects. Not only can large demes benefit from such cooperation, but the loss of cooperation in small demes can produce a minimum threshold density. Interest in these phenomena grew rapidly to the middle of the 20th century, followed by about four decades in which interest waned. In the last 20 years attention to Allee effects has burgeoned once again. This renewal has produced new perspectives, including a more realistic framework for the way populations and communities are organized. A core concept for Allee effects emerges from the historical record and current views on population dynamics: Allee effects are demographic consequences of the collective actions of anti-regulating influences. Recent developments, including proposals for much new terminology, are reviewed and found to be helpful in building mechanistic understanding of the core concept. Support for the growing relevance of Allee effects to conservation biology as well as population and community dynamics is emphasized. Some new avenues for future research directions include improving our abilities to predict life history and environmental features that favor strong anti-regulation and hence Allee effects, the role of mutually positive interspecific relations in community function, and possible role of anti-regulation in restoration ecology.