Direct and indirect effects of ants on a forest-floor food web

Interactions among predators that prey on each other and are potential competitors for shared prey (intraguild [IG] predators) are widespread in terrestrial ecosystems and have the potential to strongly influence the dynamics of terrestrial food webs. Ants and spiders are abundant and ubiquitous terrestrial IG predators, yet the strength and consequences of interactions between them are largely unknown. In the leaf-litter food web of a deciduous forest in Kentucky (USA), we tested the direct and indirect effects of ants on spiders and a category of shared prey (Collembola) by experimentally subsidizing ants in open plots in two field experiments. In the first experiment, ant activity was increased, and the density of ants in the litter was doubled, by placing carbohydrate and protein baits in the center of each plot. Gnaphosa spiders were almost twice as abundant and Schizocosa spiders were half as abundant in baited plots relative to controls. There were more tomocerid Collembola in baited plots, suggesting possible indirect effects on Collembola caused by ant-spider interactions. The second experiment, in which screening of two mesh sizes selectively excluded large and small worker ants from a sugar bait, revealed that the large ants, primarily Camponotus, could alone induce similar effects on spiders. Gnaphosa biomass density was almost twice as high in the plots where large ants were more active, whereas Schizocosa biomass density was reduced by half in these plots. Although tomocerid densities did not differ between treatments, tomocerid numbers were negatively correlated with the activity of Formica, another large ant species. Path analysis failed to support the hypothesis that the ant Camponotus indirectly affected tomocerid Collembola through effects on densities of spiders. However, path analysis also revealed other indirect effects of Camponotus affecting tomocerids. These results illustrate the complexity of interactions between and within two major IG predator groups with disparate predatory behaviors, complexities that will have consequences for functioning of the forest-floor food web.