ESTIMATING THE RELATIVE ROLES OF TOP-DOWN AND BOTTOM-UP FORCES ON INSECT HERBIVORE POPULATIONS - A CLASSIC STUDY REVISITED

Although most ecologists agree that both top-down and bottom-up forces (predation and resource limitation, respectively) act in concert to i nfluence populations of herbivores, it has proven difficult to estimat e the relative contributions of such forces in terrestrial systems. Us ing a combination of time-series analysis of population counts recorde d over 16 Sears and experimental data, we present the first estimates of the relative roles of top-down and bottom-up forces on the populati on dynamics of two terrestrial insect herbivores on the English oak (Q uercus robur). Data suggest that temporal variation in winter moth, Op erophtera brumata, density is dominated by time-lagged effects of pupa l predators, By comparison, spatial variation in O. brumata density is dominated by host-plant quality. Overall, top-down forces explain 34. 2% of population variance, bottom-up forces explain 17.2% of populatio n variance, and 48.6% remains unexplained. In contrast, populations of the green oak tortrix, Tortrix viridana, appear dominated by bottom-u p forces, Re: source limitation, expressed as intraspecific competitio n among larvae for oak leaves, explains 29.4% of population variance. Host quality effects explain an additional 5.7% of population variance , Wc detected no major top-down effects on T. viridana populations. An unknown factor causing a linear decline in T. viridana populations ov er the 16-year study period accounts for most of the remaining unexpla ined variance, We discuss the observed differences between the insect species and the utility of time-series analysis as a tool in assessing the relative importance of top-down and bottom-up forces on herbivore populations.