Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

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338 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. Many ecologists became interested initially in induced responses because they provided a potential mechanism to explain multiyear population cycles of forest insects. The hypothesis presented by Haukioja & Hakala (38) and Benz (8)-that plant quality decreases after defoliation and then increases gradually after a lag of several years-provides a delayed density-dependent mechanism that could potentially drive population cycles of herbivores (11, 36, 87). To explain regional synchrony of population fluctuations of forest Lepidoptera, we must test whether host trees respond in a consistent manner to insect attack. This basic premise does not seem to be supported: Induced responses of trees have been found to vary among species, among populations, among years, and across environmental gradients (81a). On the other hand, changes in the fecundity and survival of fluctuating populations of forest Lepidoptera often show consistent patterns through the cycle, even when caterpillars feed on different species of host plant, in different areas, and following different histories of attack (77, 78). Although the variation in response of trees to herbivore damage seems to make inducible changes in food quality an unlikely explanation for the cyclic population dynamics of forest Lepidoptera, we list in Table 2 further predictions of the hypothesis that can be tested. Observations on cyclic populations of tent caterpillars and other forest Lepidoptera do not support these predictions (77, 78). The importance of inducible changes in food plant quality to population dynamics of nonoutbreak species has not been studied. Table 2 Testable predictions arising from the hypothesis that population cycles of forest Lepidoptera are driven by deterioration in food plant quality following feeding damage from increasing numbers of herbivores. Species and populations of host trees must respond in a consistent manner to herbivore damage for the fluctuations of different populations of insects to remain in synchrony within a region. 1. Fecundity and survival of herbivores will be related to the history of attack on trees. 2. If the response of trees is density dependent, fecundity and survival of herbivores will decline with increasing density (level of attack) and deterioration in food quality. 3. Decreasing fecundity and survival of herbivores following damage to host plants will be translated into a decline in the population density. 4. Cropping of herbivore density to reduce damage will prolong the outbreak phase of the population. 5. Introduction of herbivores to suitable foodplants in sites with no previous herbivore damage will lead to an outbreak out of synchrony with natural populations.
INDUCED RESPONSES 339 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. The Specificity of Induced Resistance Most vertebrate immune responses are highly specific. We can ask two questions regarding the specificity of induced resistance against herbivores and other plant parasites: (a) Are plant responses triggered specifically by particular parasites or injuries? and (b) do plant responses have activity only against specific challengers? Many studies have found that artificial damage causes responses in plants that affect herbivores. However, these tell us little about whether the responses caused by artificial damage are physiologically the same and similar in strength to those caused by herbivores. Studies that include at least three treatments (plants damaged by herbivores, plants damaged artificially, and undamaged controls) are more informative. Several such studies found that artificial damage caused effects similar to those resulting from actual herbivory (30, 51, 81). However, several studies found that the effects of injury inflicted by herbivores and by artificial means were different in extent (42, 39,25, 2) or in quality (33, 81). In a particularly elegant experiment, Hartley & Lawton (34) found that insect feeding stimulated increased concentrations of PAL and phenolics more than cutting the leaves with scissors. Fungi or some component of insect saliva may stimulate the response since cutting with scissors and applying caterpillar regurgitate produced a response similar to that of insect damage. When designing experiments of induced responses, investigators should not assume that artificial damage will produce results similar to actual herbivory, unless this hypothesis is experimentally tested. Induced responses in plants can influence a variety of different herbivores. The inducer and the affected species may belong to very different feeding guilds and be taxonomically unrelated. For instance, cotton seedlings damaged by spider mites become more resistant to the symptoms of a fungal disease (56). Similarly, seedlings that had been infected by the fungus became less suitable hosts for spider mites. Many studies have found "crossresistance" between different herbivore species [many different herbivores on cotton (58, 52, 54), insects on larch (5), caterpillars on lupines (31), insects on oaks (103, 25, 45)]. However, several studies found that different species reacted idiosyncratically to induced plant changes. For example, birch leaves damaged by leaf mining were avoided by four species of caterpillars, whereas leaves damaged by chewing caterpillars were avoided by one caterpillar species but were equally preferred by another two species; leaves damaged artificially were preferred by two caterpillar species and were preferred equally by another two species (33). Unlike the antibody-antigen model of immune responses in vertebrates, induced resistance in plants against herbivores is characterized by low specificity. Interestingly, plant pathologists have reached the same conclusions about the lack of specificity of induced resistance against pathogenic organisms (61, 62).
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