TROPHIC CONTROL ACROSS A NATURAL PRODUCTIVITY GRADIENT WITH SAP-FEEDING HERBIVORES

Experiments examining the role that predators play in controlling herb ivore damage to plant biomass are common in ecology. The interactive e ffect of productivity and predation on standing plant biomass in terre strial ecosystems, however, remains poorly understood. Here we examine the role of predation and productivity on the damage sap-feeding herb ivores inflict on goldenrod. Sap feeders are the dominant herbivores i n this community. Sap is poor in nutrients and does not carry many of the chemical defense compounds of goldenrod. Sap-feeders should then b e sensitive to changes in productivity and relatively impervious to th e production of chemical defenses. We examined the effect of predation and fertilization on goldenrod across a productivity and successional gradient. We found that the effect of predators on controlling herbiv ore damage increased with increasing productivity. Some of these resul ts are consistent with the Exploitation hypothesis proposed by Oksanen while others seem to reflect alternative mechanisms of trophic contro l across productivity gradients. Top-down control was strong in high p roductivity sites while plant competitive interactions may have obscur ed any pattern of top-down control in intermediate and low productivit y sites.