TRENDS in Ecology & Evolution Vol.17 No.11 November 2002 507 Opinion Although reproduction is the overriding theme of the drama of life, feeding and being fed on, are the key subplots. For this reason, ecologists and evolutionary biologists are well aware of the importance of predation and are increasingly considering the role of parasitism in the control of populations and the structure of communities. A barrier to fully incorporating concepts about parasitism has been the daunting diversity of parasitic strategies that exist; this can impair our modeling efforts, blur theoretical predictions and retard basic communication about parasitism. Early population modelers recognized a need to define four types of distinct NATURAL ENEMY (see Glossary): PREDATORS, PARASITOIDS, MICROPARASITES and MACROPARASITES. It is about this diversity of trophic strategies that we ask: are these distinct trophic strategies? Are there any other strategies that these modeling categories do not adequately cover? What life-history traits of a natural enemy define the axes of the adaptive landscape of trophic strategy? In addition, what are the ecological consequences and corollaries of these strategies? Our conclusion morphs the old adage ‘you are what you eat’ into ‘you are how you eat’. Evolutionary concepts of trophic strategies Natural enemies take nourishment from a victim (host) or victims (prey) using a variety of trophic strategies. The particular strategy that an individual natural enemy uses can vary from one victim to the next (we treat asexually produced progeny in a host as equivalent to a single genetic individual). For instance, a PARASITE might interact very differently with an intermediate host compared with a definitive host. Our goal is to determine the underlying key factors that distinguish different trophic strategies used by natural enemies. We do this with a factorial application of four dichotomies, each of which describes an aspect of enemy–victim interactions (expanding here on earlier work [1]*). A logical trophic strategy emerges for ten of the potential 16 cells (Fig. 1). These dichotomies apply to all taxa, including animals, plants, PATHOGENS, phages, bacteria, microbes and helminths, enabling us, for example, to evaluate herbivores as types of parasites or predators of plants. Nonetheless, our approach does not supplant existing terminologies based on the taxonomy of the victim (herbivore or carnivore). We intentionally avoid creating new terminology by adapting existing terms that, although they might refer traditionally to specific taxa, apply conceptually to other organisms. The first dichotomy: does the enemy attack more than one victim? How do parasites differ from predators? In his authoritative work on parasite ecology and evolution, Combes [2] considers that it is the durability of the http://tree.trends.com 0169-5347/02/$ – see front matter Published by Elsevier Science Ltd. PII: S0169-5347(02)02615-0 Trophic strategies, animal diversity and body size Kevin D. Lafferty and Armand M. Kuris A primary difference between predators and parasites is the number of victims that an individual attacks throughout a life-history stage. A key division within natural enemies is whether a successful attack eliminates the fitness of the prey or the host. A third distinctive axis for parasites is whether the host must die to further parasite development. The presence or absence of intensity-dependent pathology is a fourth factor that separates macroparasites from microparasites;this also distinguishes between social and solitary predators. Combining these four dichotomies defines seven types of parasitism, seven corresponding parasites, three forms of predation and, when one considers obligate and facultative combinations of these forms, four types of predator. Here, we argue that the energetics underlying the relative and absolute sizes of natural enemies and their victims is the primary selective factor responsible for the evolution of these different trophic strategies. Published online: 11 September 2002 Kevin D. Lafferty* US Geological Survey, University of California, c/o Marine Science Institute, Santa Barbara, CA 93106, USA. *e-mail: Lafferty@lifesci.ucsb.edu Armand M. Kuris Dept of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, USA. 33 Heinz Center (1999) Designing a Report on the State of the Nation’s Ecosystems, H. 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