Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

INDUCED RESPONSES 337
Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org
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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.