Biology Reference
In-Depth Information
ment. Clearly, substantial variation existed among seedlings from different
parents, indicating that natural selection based on gypsy moth defoliation
might favor rapid development of resistance by red oak.
Mature red oak trees also show substantial differences in constitutive
and induced defenses (see below) to gypsy moth feeding (Rossiter et al.
1988). Furthermore, the fecundity and pupal mass of gypsy moths are
reduced by high levels of these chemical defenses. Inasmuch as severe,
repeated defoliation can kill mature trees, it seems clear that selection for
resistance to gypsy moth herbivory is also occurring in mature red oak
trees.
Evolutionary Responses by Plants to Herbivory
Native plants are thus very capable of responding to selective pressures
introduced by alien herbivores and disease agents (Rausher 2001).This is
perhaps not surprising, since in their native region these agents are often
associated with related plants that show some resistance to their impacts.
General patterns of evolutionary response have in some cases long been
known and in others are only now being recognized.The extent of such
responses, however, is just beginning to be appreciated.
Plants have three major modes of evolutionary response to herbivory
that reduces their reproductive performance (Levin 1976). One response
mode is deterrence: the production of constitutive or induced defenses,
that is, morphological or chemical traits that prevent or reduce damage by
herbivores. Constitutive defenses are those produced by the plant as nor-
mal products of growth and development, whereas induced defenses are
structural or chemical responses of plants to tissue damage by herbivore
feeding.These responses require the allocation of photosynthetic produc-
tion to defenses rather than to growth and reproduction, so a trade-off
between defense and other components of fitness is expected. In one of
the best-studied systems, that of wild parsnip (
Pastinaca sativa
) and the
parsnip webworm (
Depressaria pastinacella
), chemical defenses and seed
production show a negative relationship, indicating that protection against
the webworm has a fitness cost (Berenbaum et al. 1986).
The capacity for both constitutive and induced responses is genetically
based, but induced defenses require the allocation of resources only if
damage occurs. Both constitutive and induced defenses can be modified
by natural selection (Agrawal et al. 2002a; Zangerl 2003). Theoretical
analyses have shown that induced defenses are probably more difficult for
herbivores to overcome (Gardner and Agrawal 2002). This advantage,
