Agriculture Reference
In-Depth Information
Box 4.4 (continued)
2. Chemical defense
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Quantitative chemical defense involving relatively large pools of chem-
icals such as phenolics that reduce tissue quality for herbivores
Qualitative chemical defense involving small amounts of poisonous
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chemicals such as alkaloids that are toxic to many herbivores
Induced chemical defenses that are produced only after herbivore
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attack to discourage continued feeding
3. Biological defenses involving diverse mutualisms
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Production of specialized food bodies or extrafloral nectaries on the
leaf lamina or petiole to attract ants that in turn attack caterpillars
which might feed on the leaf
Production of volatile chemical signals to attract predators and para-
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sites of an herbivore
Specialized structures under the veins on the lower surface of a leaf for pred-
-
atory mites that act as guards against herbivorous mites or infecting fungi
4. Other methods to avoid herbivores
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Open leaves synchronously with other plants to satiate herbivores and
reduce the risk of damage
Reduce apparency to herbivores by mimicking less palatable tissues or
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species
Despite these substantial and diverse investments in defense against herbi-
vores, it still is not entirely clear why levels of terrestrial herbivory are so low
relative to those in aquatic systems. The defenses enumerated here fall into a
bottom-up, escape-in-time explanation for the low level of herbivory in
terrestrial systems: basically, that mature plant tissues are well defended and
of little value as a food resource for herbivores except in the brief period when
the tissues are developing. An alternative, top-down explanation is that preda-
tory animals, parasites, and disease keep herbivore numbers low and plant
defenses have relatively little to do with the outcomes. In fact, it is likely that
both top-down and bottom-up controls play a role in terrestrial as well as aquatic
systems, but the relationships are complex and remain to be fully understood.
Directions for Future Theory
There are at least two main lines along which theories for leaf longevity can usefully
be advanced. We have already alluded to one, the consolidation of theory developed
at the canopy level with that developed at the leaf level. Hikosaka (2005) has taken a
 
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