Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
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INDUCED RESPONSES 337<br />
Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org<br />
by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only.<br />
caterpillar. Less mobile herbivores, such as leaf miners, gall formers, and<br />
bark beetles are more likely <strong>to</strong> be affected by localized responses than are<br />
herbivores that constantly move.<br />
The distinction between responses that influence the attacking organisms<br />
and those that influence only later challengers <strong>to</strong> the plant is important<br />
because, in theory, the consequences of these two effects should be quite<br />
different. <strong>Induced</strong> resistance effective against the organisms causing the<br />
response is more likely <strong>to</strong> reduce the local population of this herbivore species<br />
(37). <strong>Induced</strong> resistance activated only after the attacker has left works as a<br />
negative fac<strong>to</strong>r with a time delay and is much less likely <strong>to</strong> have a stabilizing<br />
effect (37, 73). However, increased instability caused by a delay in the<br />
induced response could still be accompanied by a reduction in mean herbivore<br />
density. Using simple models of induced resistance involving mobile nonselective<br />
herbivores with continuous generations, Edelstein-Keshet &<br />
Rausher (22) argued that increasing the rate at which plants respond or<br />
decreasing the rate of decay of the response make it more likely that induced<br />
resistance will affect an herbivore population.<br />
Both common sense and mathematical theory suggest that the rates of<br />
induction and relaxation will influence the consequences on herbivores.<br />
Nonetheless, we know relatively little about these rates because the appropriate<br />
experiments are difficult, involving several treatments that must be subsampled<br />
at several different time intervals. Most of the studies that followed<br />
the time course of the induced response have found that the organism that<br />
causes the damage also suffers the consequences [caterpillars on birch trees<br />
(9, 41, 109), beetles on cucurbits (16, 96), spider mites on cot<strong>to</strong>n plants (55),<br />
caterpillars on <strong>to</strong>ma<strong>to</strong> plants (10, 23), mites on avocado trees (75), beetles<br />
and fungi on pines (83), aphids on cot<strong>to</strong>nwoods (106), cicada eggs in cherry<br />
trees (50), and caterpillars on oaks (89)]. However, three studies which<br />
showed evidence of induced resistance found that the response was delayed so<br />
that it had less chance of affecting the individuals (not the species) that caused<br />
the induction [mammals on acacias (111), hares on birches (13), and caterpillars<br />
on larches (6)]. The extent <strong>to</strong> which these individual herbivores are<br />
terri<strong>to</strong>rial or otherwise feed on the same individual plants in successive years<br />
determines the likelihood that they will suffer the consequences of their<br />
previous feeding. Some induced effects can accumulate if the stress continues<br />
for several years. For instance, performance of gypsy moth caterpillars on<br />
black oak trees decreased as the number of years that the trees had been<br />
defoliated increased from 0 <strong>to</strong> 3 (101). Several studies have found that<br />
induced resistance increased as the level of injury <strong>to</strong> the plant increased [mites<br />
on citrus (44), mites on cot<strong>to</strong>n (Figure 4 in 53), caterpillars on birch (Figure 1<br />
in 40)]. These results suggest that induced resistance should probably be<br />
thought of as a graded response rather than as an on/off process.