How do sap-feeding insects modify the trophic ecology of ants? Examining the community-level consequences of a food-for-protection mutualism

Food-for-protection mutualisms between ants and sap-feeding herbivores can alter the structure of insect food webs.  By providing a food reward to ants, sap-feeding insects can modify ant interactions with other species through changes in ant behavior, density, or community structure.  This research tests mechanisms underlying the trophic consequences of ant-sap-feeder mutualisms through a series of field experiments. First, we selectively excluded ants and removed sap-feeding membracids in a factorially designed experiment.  Through this selective exclusion we isolated the effects of each ant species and membracids on leaf-chewing caterpillars.  Second, we directly observed the behavioral interactions between competing ant species and caterpillars in the presence of sap-feeder rewards.  From these two experiments, we observed the following:  Formica neogogates was numerically dominant on host plants, but subdominant to Camponotus spp. in the presence of membracid food rewards.  Our results show that in communities with either Camponotus or Formica tending sap-feeders, caterpillar suppression still occurs, but the ecological mechanism is different depending on the species of ant.  For Camponotus the presence of sap-feeder rewards increased abundance, leading to density-dependent changes in their effects on caterpillars.  Conversely, Formica neogagates behavior was modified by the mutualism with membracids, and in the presence of sap-feeder rewards they were more likely to take caterpillar prey.