Terrestrial trophic cascades: How much do they trickle?

Although more consensus is now emerging on the magnitude and frequency of c ascading trophic effects in aquatic communities, the debate over their terr estrial counterparts continues. We used meta- analysis to analyze field exp eriments on trophic cascades in terrestrial arthropod- dominated food webs to evaluate the overall magnitude of trophic cascades and conditions affect ing their occurrence and strength. We found extensive support for the prese nce of trophic cascades in terrestrial communities. In the majority of expe riments, predator removal led to increased densities of herbivorous insects and higher levels of plant damage. Cascades in which removing predators le d to decreased herbivory also were detected but were less frequent and weak er, suggesting a predominantly three-trophic- level behavior of arthropod- dominated terrestrial food webs. Despite the clear evidence that cascades o ften decreased plant damage, residual effects of predation produced either no or only minimal changes in overall plant biomass. Agricultural systems a nd natural communities exhibited similarly strong effects of predation on h erbivore abundance. However, resulting effects on plant damage and communit y- wide effects of trophic cascades on plant biomass usually were highly va riable, and only in the managed agricultural systems did predators occasion ally have strong indirect effects on plant biomass. Our meta- analysis sugg ests that the effects of trophic cascades on the biomass of primary produce rs are weaker in terrestrial than aquatic food webs.