Now, for the first time, a University of Georgia ecologist has been able to assign numerical estimates to the two approaches.
"It has been difficult to attach numerical values to the relative importance of predation and plant quality for a number of reasons," said Dr. Mark Hunter. "Although this kind of study has been done in lakes, we believe it is the first for any terrestrial species."
The research was published today in the Proceedings of the National Academy of Sciences.
Hunter's work has been an inventive combination of experimental analysis and detective work. It began with a re-examination of data collected by two British scientists, George Varley and George Gradwell. They collected data for some 16 years on the interactions of two insect species, the winter moth and the green oak leaf roller moth, that feed on English oaks in Wytham Woods near Oxford, England.
The research published by Varley and Gradwell has long been considered a classic of population ecology. And yet they left some tantalizing puzzle-parts with their long-term study. First, although they collected information on five different trees, they published their information as averages for all the trees and didn't consider each tree as an individual. Second, and most interestingly, they also collected information on the green oak leaf roller moth for some 18 years but never published their results.
Hunter speculates that one reason for the latter may be that the data was extremely difficult to analyze, because it showed a steady and marked decline in the green oak leaf roller moth that the British researchers could not explain. Whatever the reason, Varley died in 1983 and Gradwell in 1974, leaving a classic study and an opening for further study.
"I had access to the data when I was a Ph.D. student at Oxford," said Hunter, "and I have been studying it since 1985. While the result of my study was partly due to advances in analytical techniques, it was mostly because we just didn't know what to look for."
Varley and Gradwell based their work on larval density estimates for the trees in Wytham Woods, using water-filled trays under each tree. Larvae were captured in the trays as they spun down to pupate in the soil.
A reliable way to understand the relative importance of top-down or bottom-up forces in natural systems could be invaluable to scientists, wildlife managers and many others. Hunter used a technique called time-series analysis, along with years of his own experimental data, to arrive at the relative importance of the forces. He believes that combination is crucial to proper analysis.
Strangely enough, Hunter found that the winter moth's population was regulated largely by top-down forces, while the green oak leaf roller moth was affected almost exclusively from the bottom up. In fact, there were no measurable top-down effects at all for the green oak leaf roller moth. Years ago, these results would have seemed odd, since both species feed at the same time on the same kind of oak tree. But ecologists are increasingly rejecting a model that bases changes in insect populations mostly on natural enemies or on the limits of plant quality.
"More recently, a balanced view has emerged in which both natural enemies and plant quality or quantity interact to influence the population ecology of herbivores," said Hunter. "Data is often not collected at an appropriate scale to estimate the effects of plant quality on herbivore populations."
Hunter's study argues for the consistent collection of long-term data sets, without which such analysis may be close to impossible. Though Varley and Gradwell did not themselves make complete use of the data they gathered, the information was a gold mine for Hunter.
Still, he said that time-series analysis of existing data sets alone cannot provide enough information for accurate estimates of top-down or bottom-up forces in insect-plant systems. Only by combining this analysis with experimental work can a researcher be sure what is causing an insect population to flourish or decline.
Hunter's work could open new fields of inquiry into the relative effects of predators, parasites, diseases and the quality or quantity of the food supply on insects. For instance, he is not sure what caused the precipitate decline in the numbers of the green oak leaf roller moth that the British authors noted in their unpublished data. It could, for instance, be only a part of a very long cycle of rise and fall, or it could, in fact, be part of an extinction curve.
(Editors/writers: Mark Hunter will be out of his office until August 23, but he will be checking his voice mail every day and will call you back. Please call the number listed at the top of this release and leave a message.)