Ecology and Evolution of Inducible Defenses, The

American Zoologist, Sep 2000 by Arsenault, David J, Palmer, A Richard

The Ecology and Evolution of Inducible Defenses. RALPH ToLLRIAN AND C. DREw HARVELL, eds. Princeton University Press, Princeton, 1999. 383 pp., $29.95 US (paperback). ISBN 0-691-00494-3.

It's hard not to be impressed by the striking forms that inducible defenses sometimes take in organisms as diverse as protists, plants and animals. Chemical cues from both predators and competitors can induce dramatic and seemingly adaptive changes in morphology, in chemistry, and in behavior. Tollrian and Harvell have responded to the growing interest in these phenomena by assembling an overview of the many taxa in which induced defenses occur and the various factors that might favor their evolution. Given our own interests in predator-induced defenses, we both received this book with anticipation.

Tollrian and Harvell clearly encouraged authors to focus on a common theme. As they note, four criteria must be met for inducible defenses to evolve: i) agents of selection (e.g., predators or competitors) must vary in space or time, ii) cueing mechanisms must be reliable, iii) induced defenses must yield a benefit, and iv) induced defenses must incur a cost, otherwise they should become fixed. Most authors adhere to this theme, but as an unfortunate consequence the later data chapters begin to sound repetitive because, although the taxa and traits change, the script remains more or less the same. Clearly, if inducible defenses do exist in a taxon, then all four criteria must have been met for them to have evolved. So the only real surprises are how the criteria are met by different organisms, or how unexpected are the forms that costs or trade-offs take.

Chapters fall roughly into five groups. The first few focus on plants. Berenbaum and Zangerl (Ch. 1) detail the multi-faceted work on the biochemistry and genetics of inducible chemical defenses in wild parsnip. Agrawal and Karban (Ch. 3) discuss several thoughtprovoking alternative benefits-as opposed to costs-- to inducibility, such as: lowered reliability of host-finding by specialist herbivores, avoidance of unnecessary defense pathways that might reduce the effectiveness of others, avoidance of autotoxicity, and reduced likelihood of inadvertently deterring pollinators. It seemed to us that some of these phenomena could profitably be explored in animal systems as well. Dicke (Ch. 4) summarizes many striking examples of induced indirect defenses in plants: volatile signals that attract predators of the attacking herbivores.

Two of the more fascinating chapters summarize recent discoveries in "protists." Van Donk et al. (Ch. 5) describe some wonderful cases of zooplankton-induced changes in phytoplankton morphology (e.g., the induction of colonies) and the experimental confirmation that a) chemical signals from the zooplankton are clearly involved, and b) induced forms are less likely to be eaten. Kuhlmann et al. (Ch. 8) illustrate some intriguing examples of induced morphological defenses in ciliate protozoans, including changes in cell shape, cell size, locomotory behavior, and the development of defensive spines.

Five chapters examine different groups of invertebrates. Gilbert (Ch. 7) provides a nice summary of his own and others extensive work on induced morphological defenses in rotifers, and offers some sobering and instructive reflections on the difficulties of measuring the "cost" of induced defenses. De Meester et al. (Ch. 9) outline what is known about predator-induced changes in depth selection and diel vertical migration by zooplankton. Tollrian and Dodson (Ch. 10) review the costs and benefits of induced morphological defenses, behavioral defenses, and life-history shifts in cladocera. Harvell (Ch. 13) enumerates examples of predator- and competitor-induced morphological changes in colonial marine invertebrates. And Lively (Ch. 14) provides a precis of his work on what is perhaps the most fully understood induced morphological defense: the "bent" form of the acorn barnacle Chathamalus anisopoma induced by contact with predatory snails.

Two chapters outline examples from vertebrates. Bronmark et al. (Ch. 11) concisely summarize the costs and benefits of changes in body shape in crucian carp in response to kairomones from piscivorous fish, and Frost (Ch. 6) strives to view the vertebrate immune system from the perspective of an inducible defense, but arrives at few new insights.

The remaining chapters emphasize theory. Jaremo et al. (Ch. 2) develop a largely heuristic model of the costs of inducible versus fixed defenses in plants, and conclude that localized defenses should evolve before systemic ones, which in turn should evolve before inter-plant communication. Anholt and Werner (Ch. 12) examine the impact of predator-induced changes in prey behavior on food web dynamics, but this chapter seemed to be a bit out of place between the covers of this book. Lively (Ch. 14), in addition to summarizing his work on barnacles, develops a thought-provoking evolutionarily-stable-strategy (ESS) model suggesting that the conditions permitting a stable genetic dimorphism are considerably more restrictive than those for a stable inducible dimorphism. Adler and Grunbaum (Ch. 15) take an ESS approach that incorporates predator movement between patches, variable emission of cues by the predator, and sensitivity of prey to the predator cues (the coevolutionary "cues race"), and conclude that changes in the strategy of a predator can greatly alter the outcome of models of inducible defenses; but we wondered why a predator that could restrict its emission of cues would not do so all the time. Similarly, Gabriel (Ch. 16) models the relative merits of reversible versus non-reversible induced changes, and argues that reversible changes may precede irreversible ones evolutionarily.