Induced Responses to Herbivory - Review

Ecology, Jan, 1999 by Arthur R. Zangerl

Karban, Richard, and Ian T. Baldwin. 1997. Interspecific interactions. University of Chicago Press, Chicago, Illinois. ix 319 p. $44.00, [pounds]35.25 (cloth), ISBN: 0-226-42495-2 (alk. paper); $17.95, [pounds]14.25 (paper), ISBN: 0-226-42496-00 (alk. paper).

To say that induction is a hot topic is the understatement of the decade. In the last year alone, more than 140 articles involving jasmonic acid were published; jasmonic acid, you may recall, is the molecule that appears to mediate expression of defense genes in an ever-growing number of plant species. It is thus timely that a book written with a broad perspective is now available to take stock of the accumulated knowledge of inducible defenses from the level of molecular mechanisms to populations. Authors Karban and Baldwin have undertaken the daunting task of assembling a comprehensive review of the physiology and ecology of plant responses induced by herbivores. As one might expect with a book containing over 900 references, a lot of territory is covered. The authors suggest that historically there have been two approaches to the study of induction, one physiological and the other ecological, as exemplified by the work of two of the pioneering giants in inducible defenses, ecologist Erkki Haukioja and physiologist Clarence Ryan. The first three chapters are focused on physiology and define phenomena, identify candidate signals, and detail the mechanisms of induction. The last three chapters explore the ecology, evolution, and agricultural promise of induced responses. No category of response to herbivory is neglected in this treatise - physical, chemical, resistance (damage), and third trophic level agents are all represented. Ecologists will find familiar issues such as whether inducible defenses evolved as cost-saving strategies (according to the authors, the jury is still out on this question), as well as the more novel idea of the "moving target" proposed by Karban and associates.

Throughout their book, Karban and Baldwin clearly intended to do more than simply report what is known about induced responses. As cutting edge researchers in the field, they might be expected to offer a wealth of insights on the subject, and they do; from attempts to make sense out of conflicting data, to identifying gaps in our knowledge, to suggestions of future research approaches, Karban and Baldwin deliver the goods. In one of the more intriguing sections of the book (Chapter 4), the available literature on the taxonomic distribution of induced defenses is examined with a view to gleaning some insights into the relationship between plant life histories and the occurrence of induced defenses. Although a large number of taxa are considered, trends are necessarily speculative due to the fact that the taxa do not necessarily represent a random sample. Nevertheless, this section provides some pleasantly appetizing food for thought.

A recurring theme in the book is that the study of induction has been conducted independently by physiologists and ecologists for too long and that a more integrated approach to the subject is warranted. Ironically, the authors maintained this separation by placing physiology in chapters 1-3 and ecology in chapters 4-6. As a consequence, certain topics are covered twice but with different emphasis; costs of induction, for example, are examined at the physiological level in the first part of the book and at the individual/population level in the second half. Occasionally, phrases are used that are jarringly out of character with the otherwise serious treatment of the subject, as, for example, "... the plant's shoot suddenly gets smaller because part of it walked off in the stomach of an herbivore." These distractions are minor and do not detract substantially from the book.

Induced responses to herbivory will be much appreciated by students and researchers in the field. Most importantly, the book will provide physiologists and ecologists alike a view from the other discipline's perspective of this important but far-from-perfectly understood phenomenon.

ARTHUR R. ZANGERL

University of Illinois at Urbana-Champaign Department of Entomology 505 South Goodwin Urbana, Illinois 61801