Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

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336 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. nitrogen fertilization and carbon stress could produce the result predicted by the model for many reasons having nothing to do with the hypothesized nutrient stress. Nitrogen fertilization could cause ratios of specific amino acids to become unnaturally lopsided or levels of nitrogen to become higher than optimal for herbivores (82). More convincing tests of the model would cause changes in nutrient ratios by means other than herbivory (by plant crowding or more careful fertilization treatments). The effects of these treatments on both plant chemistry and plant quality for herbivores could be measured. Tissue removal by herbivores may alter the plant physiologically, making it more resistant in the process. Pruning commonly causes shoots to exhibit juvenile characters compared to unattacked shoots of similar plants. Juvenile growth is often characterized by greater concentrations of secondary chemicals or physical resistance (14, 79). Although these responses cause an increase in less palatable tissue, they are probably examples of generally high protection of the juvenile stage. INFLUENCE OF INDUCED RESPONSES ON HERBIVORES Field studies on the effects of induced responses on herbivores have yielded extremely variable results among plants within a population, and among populations (reviewed in 26, 34). Much of this variation may be the result of differences in species, age, genotype, history, and environmental factors (17, 26, 48). Despite this variability, we can make preliminary generalizations about the timing and spatial extent of induced responses, and specificity of their effects on herbivores. Timing of Induced Responses The rate at which induced changes occur and the rate at which they are relaxed determines whether they affect particular herbivores. The critical distinction between rapid or short-term responses versus long-term responses is neither the rate at which the response occurs nor the rate of relaxation of the response. Rather, these rates must be compared to the relevant events of attack and resultant damage. Short-term responses occur during the attack such that the attacking individuals experience the consequences of the changes they induce. Long-term responses occur following the attack and have little effect on the attacking individuals but can influence herbivores that attempt to use the plant at later times. The effect of an induced response must be considered in terms of the life history and mobility of particular herbivores. The same plant response may affect only subsequent generations of short-lived herbivores such as spider mites, or it may affect the attacker in the case of a longer lived
INDUCED RESPONSES 337 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. caterpillar. Less mobile herbivores, such as leaf miners, gall formers, and bark beetles are more likely to be affected by localized responses than are herbivores that constantly move. The distinction between responses that influence the attacking organisms and those that influence only later challengers to the plant is important because, in theory, the consequences of these two effects should be quite different. Induced resistance effective against the organisms causing the response is more likely to reduce the local population of this herbivore species (37). Induced resistance activated only after the attacker has left works as a negative factor with a time delay and is much less likely to have a stabilizing effect (37, 73). However, increased instability caused by a delay in the induced response could still be accompanied by a reduction in mean herbivore density. Using simple models of induced resistance involving mobile nonselective herbivores with continuous generations, Edelstein-Keshet & Rausher (22) argued that increasing the rate at which plants respond or decreasing the rate of decay of the response make it more likely that induced resistance will affect an herbivore population. Both common sense and mathematical theory suggest that the rates of induction and relaxation will influence the consequences on herbivores. Nonetheless, we know relatively little about these rates because the appropriate experiments are difficult, involving several treatments that must be subsampled at several different time intervals. Most of the studies that followed the time course of the induced response have found that the organism that causes the damage also suffers the consequences [caterpillars on birch trees (9, 41, 109), beetles on cucurbits (16, 96), spider mites on cotton plants (55), caterpillars on tomato plants (10, 23), mites on avocado trees (75), beetles and fungi on pines (83), aphids on cottonwoods (106), cicada eggs in cherry trees (50), and caterpillars on oaks (89)]. However, three studies which showed evidence of induced resistance found that the response was delayed so that it had less chance of affecting the individuals (not the species) that caused the induction [mammals on acacias (111), hares on birches (13), and caterpillars on larches (6)]. The extent to which these individual herbivores are territorial or otherwise feed on the same individual plants in successive years determines the likelihood that they will suffer the consequences of their previous feeding. Some induced effects can accumulate if the stress continues for several years. For instance, performance of gypsy moth caterpillars on black oak trees decreased as the number of years that the trees had been defoliated increased from 0 to 3 (101). Several studies have found that induced resistance increased as the level of injury to the plant increased [mites on citrus (44), mites on cotton (Figure 4 in 53), caterpillars on birch (Figure 1 in 40)]. These results suggest that induced resistance should probably be thought of as a graded response rather than as an on/off process.
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