Biology Reference
In-Depth Information
More recently, theoretical analyses have suggested that herbivory
might alter the architecture of a plant so as to improve its capacity for
reproductive success, so the interaction between a plant and herbivore
could be mutualistic (Simons and Johnson 1999). In addition, herbivory
concentrated on plants of certain size might also select for compensation
capability, and in some cases, overcompensation. Thus, overcompensation
appears to be possible under various circumstances.
Since the review by Belsky (1986), overcompensation has been
demonstrated in some plants, primarily the biennial herbs scarlet gilia
(
Ipomopsis aggregata
) (see, e.g., Paige 1992) and field gentian (
Gentianella
campestris
) (see, e.g., Lennartsson et al. 1998). In both cases, removal of
plant tissue tended to increase the branching pattern of plant growth,
resulting in increased numbers of flowering branches and flowers. In the
case of field gentian, the timing of plant tissue removal was critical, and
the degree of compensation varied from year to year. These studies
seemed to indicate that complete compensation or overcompensation can
occur under some circumstances, especially for plants that have evolved
under a regime of heavy grazing, but also that overcompensation is not a
common phenomenon.
A third mode of plant defense is indirect defense, the development of
traits that attract enemies of herbivores (Kessler and Baldwin 2002). Indi-
rect defenses include structures such as extrafloral nectaries that attract
predatory arthropods, hollow thorns or stems that provide living sites for
colonies of such predators, and volatile organic compounds that act as
attractants for predators. Plants belonging to at least 66 families produce
extrafloral nectaries, which have been shown in several cases to attract
predatory arthropods that reduce damage by herbivores.Volatile organic
substances are released when plant tissues are damaged by herbivores.
These substances, varied in chemical nature, have been shown to attract
predators and parasitoids, but the degree to which this mechanism
increases plant fitness is still uncertain.
Evolutionary Responses of Herbaceous Plants
to Alien Herbivorous Invertebrates
Few studies of the evolutionary response of plants to herbivorous inver-
tebrates have been attempted.The small spring annual,
Arabidopsis thaliana
,
native to Europe and Asia, was introduced to eastern North America in
the mid-1800s and is now widely distributed. This species has become a
model system for studying many aspects of genetics in relation to physi-
