THE INFLUENCE OF ELEVATION ON TRI-TROPHIC INTERACTIONS - OPPOSING GRADIENTS OF TOP-DOWN AND BOTTOM-UP EFFECTS ON A LEAF-MINING MOTH

While the relative importance of host-plant and natural-enemy effects on insect herbivores varies among systems, such variation has seldom b een documented within a plant-herbivore natural-enemy system. Furtherm ore, sources of variation both within and among systems in the relativ e strength of these trophic impacts are poorly understood. In an effor t to account for variation in the relative importance of bottom-up and top-down trophic effects on herbivores, Hunter & Price (1992) present ed the template/trophic cascade model, which ascribes an important rol e to the effects of environmental heterogeneity on tri-trophic interac tions. We evaluate this component of the template/trophic cascade mode l by describing the effects of abiotic heterogeneity associated with e levation on tri-trophic interactions involving Phyllonorycter sp., a l eaf-mining moth. Mortality of the moth associated with host-plant qual ity increased with elevation in both 1992 and 1993. In contrast to thi s first-trophic level effect, Phyllonorycter was not attacked by paras itoids at the highest elevation sites in both years. A trophic index d escribing the relative contribution of natural enemies, parasitoids an d predators, to Phyllonorycter mortality showed a decrease in enemy-ca used mortality with increasing elevation. These effects of elevation p roduce systematic changes in the nature of herbivore mortality within the range of a single population. They also emphasize the deed to incl ude the effects of abiotic heterogeneity in models of trophic interact ions.