Allee effect in social species
The benefits of aggregation of conspecific individuals are central to Allee effects, and these benefits are enhanced by cooperation. Therefore, social species have been hypothesised to be more vulnerable to Allee effect-related extinction. Social species are characterised by nested levels of organisation (e.g., castes, social groups, populations), which can interact in complex ways. For example, most cooperation occurs between individuals from the same social group, while mating often occurs between individuals of different groups, and competitive interactions may be stronger between individuals from different groups. These fundamental differences imply that Allee effects can occur at different levels of organisation, depending on the level at which aggregations are beneficial. Interactions among multiple Allee effects depend on the strength of negative versus positive density dependence. This complexity increases with increasing levels of organisation, making predictions challenging without a conceptual framework. There is also confusion and inconsistency in the literature about how different types of Allee effects in social species are defined and related. Allee effects are classically divided into component and demographic Allee effects. Recently, several studies on social species have also identified group Allee effects, and group-level and population-level Allee effects, giving rise to a suite of concepts that have yet to be integrated into a coherent framework.
We propose a framework to integrate and clarify the different types of Allee effects in social species and causal mechanisms for each level of social grouping. Our proposal is based on examples across varied taxa with different social systems. We distinguish three different levels of organization where Allee effects can occur in social species: subgroup, group, and population. Our framework provides testable predictions about whether Allee effects at a certain level of organisation can lead to significant population impacts (e.g., extinction vortex) depending on the interactions occurring within and between these levels of organisation, which it turns may depend on species social structure. Many recent studies address the Allee effect dynamics on evolution, conservation of threatened species, and invasive species management, which are topics for which social species are often at the forefront. Without considering these issues, research on social species will continue to fall short on understanding the ecological and evolutionary implications of cooperation in social species.