INDUCIBLE DEFENSES AND TROPHIC STRUCTURE

Abstract

Resource edibility is a crucial factor in ecological theory on the relative importance of bottom-up and top-down control. Current theory explains trophic structure in terms of the relative abundance and succession of edible and inedible species across gradients of primary productivity. We argue that this explanation is incomplete owing to its focus on inedibility and the assumption that plants and herbivores have fixed defense levels. Consumer-induced defenses are an important source of variation in the vulnerability of prey and are prevalent in natural communities. Such induced defenses decrease per capita consumption rates of consumers but hardly ever result in complete inedibility. When defenses are inducible a prey population may consist of both undefended and defended individuals. Here we use food chain models with realistic parameter values to show that variation in consumption rates on different prey types causes a gradual instead of stepwise increase in the biomass of all trophic levels in response to enrichment. Such all-level responses have been observed in both aquatic and terrestrial ecosystems and in microbial food chains in the laboratory. We stress that, in addition to the known food web effects of interspecific variation in edibility, intraspecific variation in edibility is another form of within-trophic-level heterogeneity that also has such effects. We conclude that inducible defenses increase the relative importance of bottom-up control.