Prevalence of social parasitism in ant populations: Modeling energetics, demography and space in the Polyergus/Formica system

Obligatory interspecific brood raiding is a unique form of parasitism in which one ant species steals pupae from another species and raises them into workers to perform duties within its nest. Although this strategy can support very large colonies, the relative species abundance of these social parasites is always low. Using fully parameterized mathematical models of the growth and reproduction of the well-studied interaction between brood raider Polyergus and its hosts in the genus Formica, we aim to discover the mechanisms that limit brood raider abundance. These mathematical models explain the range of observed relative species abundance of these social parasites and provide a criterion for Polyergus persistence within a patch of hosts. In particular, Polyergus colony survival depends on the number of host colonies between 23 and 73 meters from their nest—close enough to raid but distant enough to survive raiding. The number sets the upper bound of Polyergus abundance to be less than 10% of the community. Furthermore, we quantify the fitness costs imposed by brood raiding on nearby host colonies, which can be effectively castrated by the constant drain on their worker resources. These findings provide a mechanistic framework for understanding the ecological constraints on social parasitism, its role in shaping ant community dynamics and its connection to the evolution of host defense strategies.