Abstract
BackgroundAllee effects may arise as the number of individuals decreases, thereby reducing opportunities for cooperation and constraining individual fitness, which can lead to population decrease and extinction. Obligate cooperative breeders rely on a minimum group size to subsist and are thus expected to be particularly susceptible to Allee effects. Although Allee effects in some components of the fitness of cooperative breeders have been detected, empirical confirmation of population extinction due to Allee effects is lacking yet. Because previous studies of cooperation have focused on Allee effects affecting individual fitness (component Allee effect) and population dynamics (demographic Allee effect), we argue that a new conceptual level of Allee effect, the group Allee effect, is needed to understand the special case of cooperative breeders.ResultsWe hypothesize that whilst individuals are vulnerable to Allee effects, the group could act as a buffer against population extinction if: (i) individual fitness and group fate depend on group size but not on population size and (ii) group size is independent of population size (that is, at any population size, populations comprise both large and small groups). We found that both conditions apply for the African wild dog, Lycaon pictus, and data on this species in Zimbabwe support our hypothesis.ConclusionsThe importance of groups in obligate cooperative breeders needs to be accounted for within the Allee effect framework, through a group Allee effect, because the group mediates the relationship between individual fitness and population performance. Whilst sociality is associated with a high probability of Allee effects, we suggest that cooperative individuals organized in relatively autonomous groups within populations might be behaving in ways that diminish extinction risks caused by Allee effects. This study opens new avenues to a better understanding of the role of the evolution of group-living on the probability of extinction faced by social species.
Highlights
Cooperative behaviour and Allee effects Cooperative behaviour, how it evolved and why it persists, are fundamental questions in the theory of natural selection [1]
Both, Allee effects and negative density dependence can be present in the population dynamics of a given species [5], here we focus at processes of population dynamics occurring at low population densities such as the ones resulting from the benefits of cooperation
Allee effect A general overview of the analyses for Allee effects revealed several component Allee effects caused by pack sizes but not by population sizes: some traits of individual fitness were positively related to pack sizes but none of them was correlated to population sizes (Table 3)
Summary
Cooperative behaviour and Allee effects Cooperative behaviour, how it evolved and why it persists, are fundamental questions in the theory of natural selection [1]. Explanations of cooperative behaviour were formulated in the mid-XXth century and based on benefits to groups and populations [2,3]. While these authors did not offer evolutionary explanations [4], Allee proposed a theoretical explanation of collaboration at dynamics occurring at low population densities such as the ones resulting from the benefits of cooperation. Allee effects in some components of the fitness of cooperative breeders have been detected, empirical confirmation of population extinction due to Allee effects is lacking yet. Because previous studies of cooperation have focused on Allee effects affecting individual fitness (component Allee effect) and population dynamics (demographic Allee effect), we argue that a new conceptual level of Allee effect, the group Allee effect, is needed to understand the special case of cooperative breeders
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