Biology Reference
In-Depth Information
together with reduced resource allocation needs, contributes to increased
plant fitness. The effectiveness of deterrence, either constitutive or
induced, can be measured as the reproductive performance of defended
plants in the presence of herbivores relative to their performance in the
absence of herbivores.
A second defense mode is development of tolerance—mechanisms that
compensate for damage caused by herbivory (McNaughton 1983). Mech-
anisms of compensation include a high intrinsic growth rate, increase in
photosynthetic production after herbivory, increased branching or tillering
after herbivory, storage of photosynthate in roots or other organs protected
from herbivory, and transfer of stored photosynthate to portions of the
plant damaged by herbivory (Strauss and Agrawal 1999). In some cases,
plants that have been damaged by herbivores experience reduced compe-
tition due to herbivore consumption of surrounding vegetation. Thus,
selection may favor the ability for rapid use of nutrients made available in
a low-competition environment (Westoby 1989). Tolerance, like deter-
rence, requires investments in structural and physiological mechanisms
related to the above processes. For plants emphasizing tolerance, compen-
sation can also be measured as the reduction in reproductive performance
of plants exposed to herbivory relative to those protected from herbivory.
Deterrence and tolerance of herbivory are not mutually exclusive
strategies, however (Mauricio et al. 1997). Many plant species exhibit
genetic variability in traits related both to deterrence and to tolerance.
Thus, how selection will act is likely to depend on the precise nature and
location of herbivore impact—root or shoot, internal or external, tissue
or vascular fluid consumption.The action of natural selection may also be
constrained by external factors such as nutrient availability as it relates to
compensatory growth and seasonal patterns of availability of pollinators
(Stowe et al. 2000).
The degree to which compensation for herbivory on individual plants
is possible has been the subject of considerable controversy. This contro-
versy, still active, has centered on whether or not the interaction between
an herbivore and a plant can be mutualistic, with the harvest of plant tis-
sue both benefiting the herbivore and increasing the growth or evolution-
ary fitness of the plant. The hypothesized result of such mutualism is over-
compensation, in which the plant realizes greater growth or fitness when
grazed or browsed than when protected against herbivory. Many early
studies did not strictly address this question or did not consider all aspects
of plant growth and reproduction. Belsky (1986) and Belsky et al. (1993)
concluded that the evidence for overcompensation in nature was lacking.
