Herbivory in Forested Ecosystems

Abstract

Forest canopies support a complex assemblage of herbivores that are usually inconspicuous except when population outbreaks of one or more species pro­ duce noticeable defoliation or other signs of herbivore activity. Little is known about the ecological impacts of herbivores during non-outbreak phases (52, 109, 137, 141). Foliage consumption during non-outbreak periods has been measured as 5-15 % of leaf area production in temperate forests (109), but the techniques used may underestimate actual herbivory (88). Foliage loss may reach 100% of foliage production during herbivore population outbreaks (33, 89, 168). In the last five years there has been an explosion of information on both the process of herbivory and the nature of inseCt/plant relationships. Recent com­ pendia include volumes by Ahmad (2), Bell & Carde (10), Crawley (34), Denno & McClure (37), Edwards & Wratten (42), Hedin (65), Rosenthal & Janzen ( 130), and Strong et al (147). Ecological chemistry has been a fun­ damental organizing theme for many of these works. As recently as 1960, Hairston et al (57) based their world hypothesis on the premise that all green foliage was equally available to herbivores. Secondary metabolites were considered to be metabolic by-products with no known function. The emergence of ecological chemistry can be traced from Fraenkel' s (49) concept of plant chemical defense to Feeny's (46) classic study of plant regulation of winter moth (Operophtera brumata L.) populations. More recently, the induc-