© 2000 by The Society for Integrative and Comparative Biology
BOOK REVIEWS
1 Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada and Bamfield Marine Station, Bamfield, British Columbia, V0R 1B0, Canada. E-mail: arsen{at}bms.bc.ca. E-mail: rich.palmer{at}ualberta.ca
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 thought-provoking 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 précis 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. Brönmark 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. J
remo 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 Grünbaum (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.
In their concluding chapter, Tollrian and Harvell (Ch. 17) return to the original four criteria required for inducible defenses to evolve and comment on how convincingly they were demonstrated across the various organismal systems examined: i) variable selection pressures (shown in all systems), ii) reliable chemical cues (shown in most systems), iii) defenses are beneficial (shown in most systems), and iv) defenses incur a cost (may or may not be demonstrable; "the role of costs ... continues to be a complex issue" p. 320). To their credit, they also note how this compilation of examples rejects one of Harvell's earlier predictions: that inducible defenses should be most prevalent among modular as opposed to solitary organisms.
For those unfamiliar with induced defenses, this book offers a convenient overview of the many and varied forms such defenses take and the impressive variety of taxa in which they occur. Both European and North American authors are well represented (20 and 12, respectively), reflecting the editors' geographic affiliations, and many contributors offer up-to-date summaries of what is known about relevant chemical cues: vascular plants (Ch. 1, 3), phytoplankton (Ch. 5), rotifers (Ch. 7), protozoans (Ch. 8), cladocera (Ch. 10), crucian carp (Ch. 11), and bryozoans (Ch. 13). Dicke (Ch. 4) presents a similar summary of the chemicals plants release that attract predators of the attacking herbivores.
Outside of the sometimes fascinating natural history, though, there seems little here to lure new players to the field. With the exception of Agrawal and Karban (Ch. 3), Gilbert (Ch. 7), and Tollrian and Harvell (Ch. 17), few authors acknowledge pressing technical or analytical problems needing to be addressed. One of the major conclusions, that costs are often difficult to demonstrate convincingly, hardly seems an appealing invitation to newcomers. Are there no controversies lurking beneath a patina of perhaps uncomfortable consensus, or exciting new challenges to reigning orthodoxies?
Most disappointing to us is the limited phylogenetic perspective. Few authors compare taxa exhibiting induced defenses to related taxa with similar, but fixed, defenses. In this era of exuberant phylogenetic analysis, the complete absence of cladograms or formal comparative analyses seems a great oversight, particularly given the fine study of spider-mite resistance in cotton plants by Thaler and Karban (1997)
. As the editors note briefly (Ch. 17), comparative data on the incidence of inducible characters in several related taxa are increasingly available and should permit phylogenetic analyses of inducible characters (e.g., among Cladocera). So why weren't some detailed phylogenetic analyses presented here? Only then can we really begin to understand how often, and under what circumstances, inducibility arises. In many respects, Schlichting and Pigliucci's (1998) book takes a more stimulating approach to the evolution of phenotypic plasticity, including induced defenses.
From a technical perspective, the format of the references and index are not as useful as they might be. All 1000+ references are at the end of the book, even though fewer than 10% were probably cited in more than one chapter. In addition, readers can not, unfortunately, trace references to the pages where they are cited. The index covers seven pages, but in our two tests of its utility, we were disappointed. Under kairomone—"chemicals in interspecific signal transmission which are exclusively advantageous to the receiving organism" (p. 195)—some relevant pages are identified but a number of others are overlooked. For example, someone wishing to locate such specific information quickly would miss a) the surprising induction of colony formation in the diatom Scenedesmus by exudates from the gut flora of Daphnia (pp. 95–96), b) the discussion of chemical cues from predators that alter patterns of depth selection behavior and diel vertical migration in Daphnia (p. 162), and c) the evidence that carp respond directly to chemical cues released by pike (pp. 209–211). These three examples are also not included under "cue - chemical." Even some pages bearing the word kairomone are not listed (e.g., p. 140, 171, 311). Similarly, ESS models appear three places in the book (pp. 40–44, 249–256, 272–278) but the index indicates only one. Perhaps a bit tongue in cheek, the index does direct inquisitive readers to pages "1–321" to find information on "defenses-inducible." Typographical errors are few, but the one in Table 13.1 (p. 235), listing "predatory" bryozoans that induce morphological changes in ‘prey’ bryozoans, certainly made us smile.
This book is probably best read one chapter at a time, maybe over breakfast, to savor the sometimes fascinating twists of natural history without having to rummage through the primary literature. But students will likely find Travis's (1994) fine chapter on adaptive plasticity a more compact and stimulating synthesis of the central issues.
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101 * Author and address for correspondence.
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References |
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Schlichting, C., and M. Pigliucci. 1998. Phenotypic evolution: A reaction-norm perspective. Sinauer Associates, Sunderland, Massachusetts.
Thaler, J. S., and R. Karban. 1997. A phylogenetic reconstruction of constitutive and induced resistance in Gossypium. Am. Nat, 149:1139-1146.[CrossRef]
Travis, J. 1994. Evaluating the adaptive role of morphological plasticity. In P. C. Wainwright and S. M. Reilly (eds.), Ecological morphology. Integrative organismal biology, pp. 99–122. University of Chicago Press, Chicago.
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