9628929 Cornell Whether top-down effects due to enemies or bottom-up effects due to resources control the dynamics of herbivore populations has been a venerable and divisive controversy in ecology. With the realization that natural systems are diverse and heterogeneous the argument has become sterile, and the focus must shift to the relative importance of these two forces and what controls them in different environments. Moreover, it has recently become obvious that exploring these interactions thoroughly requires differentiation between direct and indirect effects. Indirect effects mean that the intensity and symmetry of an interaction between two species may be altered by the presence of a third. For example, plant defenses might be able to reduce herbivore abundances but only in the presence of natural enemies because the primary effect of the defense is to make the herbivore more susceptible to attack. In this study, the investigators are employing a multi-pronged approach using a combination of experiments and statistical path analysis to study top-down and bottom-up effects on the distribution and abundance of the holly leaf miner on American holly. A combination of path analysis and experiments provides a new way of increasing inferential power beyond what is possible when either technique is used separately. The investigators have measured heterogeneity of plant quality and enemy abundance for the holly leaf miner and its host and are now setting up factorial experiments to test their direct and indirect effects on miners. This will be the first study to use such an integrated approach to answer fundamental questions about the structure and function of terrestrial food webs. It should have special relevance to theories of trophic dynamics as well as to the applied areas of forest pest management and biological control.
