Uncommon specialization in a mutualism between a temperate herbaceous plant guild and an ant: are Aphaenogaster ants keystone mutualists?

Abstract

Ant‐dispersed herbs (myrmecochores) can account for more than one‐third of the stems in the temperate deciduous forests of eastern North America. Because many ant species have been observed collecting the seeds, this interaction is often described as a generalized mutualism. Here, we combine fieldwork and meta‐analyses to test this assumption. Our meta‐analysis demonstrated that Aphaenogaster ants (predominantly A. rudis) collect approximately 74±26% (mean±SD) of the myrmecochorous seeds in eastern North American forests where any encounters with Aphaenogaster were reported, and approximately 61±37% of the seeds in all the eastern forests where any seed collection has been monitored. This remarkable monopolization of seeds is due to at least two factors: 1) Aphaenogaster are significantly more likely to collect the ant‐adapted seeds they discover than are ten other ant genera found in these forests and 2) the densities of Aphaenogaster and myrmecochorous plants are positively correlated at three nested spatial scales (within 20×20 m patches, among patches within a forest, and among 41 forests in the eastern United States). Although other ants can collect seeds, our analyses demonstrate that A. rudis is the primary seed dispersal vector for most of this rich temperate ant‐dispersed flora. The low levels of plant partner diversity for myrmecochores demonstrated here rivals that of tropical ant‐plants (myrmecophytes) and well exceeds that typically observed in temperate plant–frugivore and plant–pollinator mutualisms and myrmecochory in other biomes.