Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

338 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.
Many ecologists became interested initially in induced responses because
they provided a potential mechanism to explain multiyear population cycles
of forest insects. The hypothesis presented by Haukioja & Hakala (38) and
Benz (8)-that plant quality decreases after defoliation and then increases
gradually after a lag of several years-provides a delayed density-dependent
mechanism that could potentially drive population cycles of herbivores (11,
36, 87).
To explain regional synchrony of population fluctuations of forest Lepidoptera,
we must test whether host trees respond in a consistent manner to insect
attack. This basic premise does not seem to be supported: Induced responses
of trees have been found to vary among species, among populations, among
years, and across environmental gradients (81a). On the other hand, changes
in the fecundity and survival of fluctuating populations of forest Lepidoptera
often show consistent patterns through the cycle, even when caterpillars feed
on different species of host plant, in different areas, and following different
histories of attack (77, 78).
Although the variation in response of trees to herbivore damage seems to
make inducible changes in food quality an unlikely explanation for the cyclic
population dynamics of forest Lepidoptera, we list in Table 2 further predictions
of the hypothesis that can be tested. Observations on cyclic populations
of tent caterpillars and other forest Lepidoptera do not support
these predictions (77, 78). The importance of inducible changes in food
plant quality to population dynamics of nonoutbreak species has not been
studied.
Table 2
Testable predictions arising from the hypothesis that population cycles of forest
Lepidoptera are driven by deterioration in food plant quality following feeding damage from
increasing numbers of herbivores. Species and populations of host trees must respond in a
consistent manner to herbivore damage for the fluctuations of different populations of insects to
remain in synchrony within a region.
1. Fecundity and survival of herbivores will be related to the history of attack on trees.
2. If the response of trees is density dependent, fecundity and survival of herbivores will decline
with increasing density (level of attack) and deterioration in food quality.
3. Decreasing fecundity and survival of herbivores following damage to host plants will be
translated into a decline in the population density.
4. Cropping of herbivore density to reduce damage will prolong the outbreak phase of the
population.
5. Introduction of herbivores to suitable foodplants in sites with no previous herbivore damage
will lead to an outbreak out of synchrony with natural populations.