Christine A. Johnson, The American Museum of Natural History and Joan M. Herbers, The Ohio State University.
Competition is an important evolutionary force behind population regulation and community structures. The degree of competition symmetry (competition hierarchies) between species determines coexistence, exclusion or niche differentiation. Intra-specific competition, however, is an important component in dictating levels of symmetry/asymmetry between species and should be accounted for when attempting to understand inter-specific evolutionary or ecological outcomes. Some social parasites compete for access to a common host and, thus, adhere to both parasite-host and predator-prey co-evolutionary and ecological dynamics. Here we examine tripartite evolutionary dynamics with respect to intra-specific competition of two sympatric social parasites in the family of ants that conduct ‘brood-raids’ on a shared host species and use the captured brood as a functional work force. Social parasites were challenged with either a conspecific or heterospecific parasite and the impact on hosts and parasites was evaluated. Within-species raiding occurred frequently in the ‘prudent’ parasite but was almost non-existent in the ‘virulent’ parasite. Although intra-specific raiding led to larger single prudent parasite colonies, the preservation of two independent virulent parasite colonies had an early and a more devastating effect on the host. However, having effaced raiding phenologies, the prudent parasite emerged as the better direct competitor against the virulent parasite. Sex ratios were male-biased in the virulent parasite colonies but numerically equal (hence female-biased) in prudent parasite colonies. Combined, our results suggest inter-specific parasite interactions have produced ecological shifts in both parasites and attenuated the co-evolutionary arms race between the prudent parasite and the shared host in sympatric populations.
