Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

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342 KARBAN & MYERS Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only. during the remainder of that field season (52). However, growth and yield of these induced plants did not differ from plants that were not induced, contrary to prediction (a). Either spider mites did not reduce these aspects of plant fitness or else the reduction in fitness to control plants caused by greater herbivory was offset by the costs of inducing resistance. Since cotton has undergone intense selection as an agricultural crop this may not be an appropriate model system. In native tobacco plants, both constitutive levels of alkaloids and increases in alkaloid titers induced by damage were negatively correlated with seed output, suggesting a cost to this presumed defense (3). The best examples of estimates of costs of induced resistance come from small invertebrates in fresh water and marine environments. Some of these organisms respond to predators through morphological modifications such as the production of helmets in daphnia (43), heavier shells in barnacles (67), and spines in rotifers (29) and bryozoans (35). Induced resistance for rotifers did not reduce survival, fecundity, or population growth, but for barnacles, daphnia, and bryozoans, these induced morphological changes reduced growth and/or fecundity. When predators are not present, unarmored individuals have the fitness advantage. CONCLUSIONS The initial observations of changes in chemical composition of plants following stress or damage seemed obvious examples of plant adaptations against herbivores. If, in a bioassay, the quality of foliage was reduced (as indicated by poorer survival and fecundity of the herbivore), then an impact on the future density of the herbivore seemed an obvious conclusion. Many studies have now found that induction causes changes in performance of bioassay herbivores. However, all stages in the interactions between plants and herbivores have been found to vary; insects vary in their choice of damaged and undamaged foliage and in their growth and survival on damaged and undamaged tissue. Some plants respond to damage, some do not; some improve as hosts following damage, others deteriorate. After a decade of work, there are few generalities concerning the effects of induced plant responses on population dynamics. The hypothesis outlined by Haukioja (36) and Rhoades (87), in which changes in food pilnt chemistry were proposed as the driving mechanism behind large-scale cyclic fluctuations in folivorous insects, has met with equivocal support. In some instances, variation among populations of trees is too great to provide the consistent impact on the insects sufficient for widespread cyclic declines. More work is needed to examine the effects of induced host changes on populations of herbivores in natural and agricultural systems. The variation that may have surprised ecologists searching for simple
INDUCED RESPONSES 343 Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only. answers and general patterns will perhaps come as little surprise to plant physiologists. Recognizing that fast- and slow-growing trees will respond to defoliation in different ways and that loss of buds in the winter or spring will cause different patterns of foliage quality has greatly helped interpretations of conflicting findings. However, still controversial is whether chemical changes following damage can be wholly attributed to passive changes by damaged plants, or if active defensive processes must be invoked. The role of microparasites, fungi, bacteria, or viruses in eliciting active responses in damaged plants following contamination by herbivores will be an exciting area for future research and one that may help answer questions about the mechanisms of induction. The controversy between active and passive responses of plants to herbivore damage will almost certainly be resolved by the realization that a combination of mechanisms are involved. We must find out what is happening, where, why, and how often. If, as ecologists, we wish to understand induced changes we should be prepared to devote ourselves to long-term and multidimensional studies. If we aim to understand the chemical mechanisms of induced resistance, we should consider all of the chemicals within a plant with potential activity against herbivores, rather than specializing on a particular subset that are easy to work with or are thought to bc important. Certainly, we should seek experimental evidence that allows us to vary only one constituent, using artificial diets and isogenic lines, when available. This careful experimentation must be conducted for all of the plausible mechanisms. At the same time we should keep in mind that the effects we observe in these highly artificial experiments may be very different from effects experienced by herbivores dealing with the chemicals in plants, where interactions and synergisms arc likely to be important. We have now learned that many plants change in response to herbivory and that no single mechanism will explain all of these diverse plant responses. At the other extreme, we must extend our bioassay results to field experiments on natural populations of herbivores. Rather than asking whether induced responses can be shown to affect the performance or behavior of herbivores we should assess the relative importance of induced plant resistance compared to other ecological factors that may also affect the population dynamics of herbivores. Induced responses should not be assumed to be defenses. Instead, we must observe whether they defend plants by comparing fitness of induced and un induced plants in an environment that includes herbivores. Fitness will be most easily measured on small, short-lived plants which show evidence of induced responses following low levels of herbivore damage [e.g. cucurbits (96), wild tobacco (3), crucifers (93)]. It should be kept in mind that results with these systems may have little relevance to what is happening with trees. Even after an induced response is shown to provide resistance against a
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