Are defoliators beneficial for their host plants in terrestrial ecosystems—a review?

The impact of herbivory on food plants has engaged mankind for millennia. The detrimental impact of browsing by cattle on the performance of seedlings and young trees of species other than crop plants was noticed long ago, e.g. for Scotch pine on Surrey heaths (Darwin 1859). For more than a century physiologists have studied the responses of defoliated plants (Vochting 1884, Reinke 1884, Kny 1894). Since the seventies ecologists have developed a number of hypotheses to describe the responses of plants to defoliation under field conditions. It has been hypothesized that under certain conditions herbivory may result in better growth and/or higher fitness (Harris 1973, Owen & Wiegert 1976, Owen 1980, McNaughton 1983a). In this paper these hypotheses, especially those on the possible beneficial impact of defoliation, are discussed, a survey of possible responses is given, and finally the conditions under which beneficial effects may occur are summarized.