From the recent literature:
The Journal of Experimental Biology has a lab study by Dussutour et al documenting how leafcutter ants avoid traffic jams under crowded trail conditions. Apparently, unladen ants increase a narrow trail’s efficiency by following the leaf-carrying ants instead of trying to pass their slower sisters. See also commentary by JEB and Wired.
source: Dussutour, A., Beshers, S., Deneubourg, J. L., Fourcassie, V. 2009. Priority rules govern the organization of traffic on foraging trails under crowding conditions in the leaf-cutting ant Atta colombica. J Exp Biol 2009 212: 499-505.
In the journal PLoS One, Youngsteadt et al document that the seeds of the neotropical ant plant Peperomia macrostachya are dispersed by just a single species of Camponotus in spite of a high ant diversity at the study site.
source: 2009. Species-Specific Seed Dispersal in an Obligate Ant-Plant Mutualism. PLoS ONE 4(2): e4335. doi:10.1371/journal.pone.0004335