Are Aphaenogaster ants keystone mutualists in temperate deciduous forests?

An estimated one-third of the stems in the temperate deciduous forests of the eastern North America produce seeds adapted for dispersal by ants (myrmecochory).  Many ant species have been observed collecting the seeds, hence this interaction is often described as a generalized mutualism.   Here, we combine fieldwork and meta-analyses to test this generalization.  Our meta-analysis demonstrated that Aphaenogaster ants (predominately A. rudis) collect approximately 75 ± 7% (mean ± SE) of the myrmecochorous seeds in forests where they are present, and approximately 60 ± 9% of the seeds in all northeastern forests where seed collection has been monitored.  This remarkable monopolization of myrmecochorous seeds is due to at least two factors: i., Aphaenogaster spp are significantly more likely to discover and collect ant-adapted seeds than are ten other ant genera found in these forests and ii, the densities of Aphaenogaster colonies and myrmecochorous plants are positively correlated at three nested spatial scales (within 20x20m patches, among patches within a forest, and among forests in the eastern United States).  Previous studies have demonstrated that A. rudis both provides and receives substantial benefits from the interaction with the plants (i.e., is a particularly effective mutualist).  Although other ants can collect seeds, we conclude that the most of this temperate ant-dispersed flora are ‘Aphaenogaster specialists’, functionally speaking.  This widespread ant-plant interaction shares the asymmetries demonstrated in other mutualisms.