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 341<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 />
among parts of plants, and among different plants within a population. The<br />
responses <strong>to</strong> damage of different plants and different parts within an individual<br />
plant may be quite idiosyncratic. The plant may not simply be in an<br />
"induced state." Rather, induction likely involves differential changes occurring<br />
in different types of organs of a plant, and in different organs of the same<br />
type (leaves on a tree). <strong>Induced</strong> responses include many traits that affect<br />
herbivores, all of which can change, rather than the turning on of a single<br />
"defensive chemical." Each of these traits in each plant part may respond with<br />
its own rate of induction and relaxation. A changing, heterogeneous target<br />
may allow for a more rapid response and may retard or prevent the adaptation<br />
of herbivores or diseases <strong>to</strong> the plant defense (105). A changing plant<br />
phenotype may allow the plant <strong>to</strong> respond more rapidly <strong>to</strong> herbivores and<br />
other parasites than it could if it relied on constitutive defenses that changed<br />
only in evolutionary time. Constitutive defenses have no lag time at all, but<br />
also no ability <strong>to</strong> change when herbivores circumvent them. <strong>Induced</strong> responses<br />
may allow the plant <strong>to</strong> respond <strong>to</strong> unpredictable environmental<br />
variability (65). Phenotypic plasticity in resistance is expected <strong>to</strong> be more<br />
effective than genetic adaptation in response <strong>to</strong> selective fac<strong>to</strong>rs, such as<br />
herbivores and plant pathogens, that may vary during the life span of an<br />
individual plant. This hypothesis predicts that induced resistance will be most<br />
common for plants that experience unpredictable selective pressures from<br />
herbivores sporodically in relation <strong>to</strong> the generation time of the plant. This<br />
argument would be strengthened if we knew that phenotypic plasticity in<br />
resistance <strong>to</strong> herbivores was heritable, as plasticity of some other plant traits<br />
appears <strong>to</strong> be (92).<br />
<strong>Induced</strong> Defenses Are Less Costly<br />
Much of the recent theory concerning the evolution of plant defenses has<br />
centered around the notion that defenses are costly (18, 27, 76, 85, 88).<br />
Accordingly, plants should allocate resources <strong>to</strong> defenses only when and<br />
where such allocation will result in increased fitness.<br />
This leads <strong>to</strong> several testable predictions: (a) <strong>Herbivory</strong> should reduce plant<br />
fitness and induced plants should have greater fitness than noninduced plants<br />
when herbivores are present. (b) <strong>Plant</strong>s without induced defenses should have<br />
higher fitness in environments without herbivores. (c) <strong>Plant</strong>s that are well<br />
defended by constitutive defenses against a particular herbivore should not<br />
allocate resources <strong>to</strong> induced defenses against that same herbivore. If constitutive<br />
defenses are effective, induced defenses, which are presumably<br />
costly, would be redundant. In other words, these two should be negatively<br />
correlated.<br />
These predictions have not been tested adequately. Cot<strong>to</strong>n plants that were<br />
induced at the cotyledon stage supported smaller populations of spider mites