Biology Reference
In-Depth Information
Unfortunately, most studies of evolutionary adaptation have only
begun to examine the full complexity of herbivore-plant interactions.
Few studies have attempted to analyze reciprocal relations of herbivores
and plants in the field situation.Thus, we are able to present only the first
sketch of evolutionary processes involving alien herbivores and the plants
with which they interact. In chapter 12, we shall examine patterns of
adaptation by plants to alien herbivores and disease agents.
Herbivore Specialization for Plant Hosts
Herbivores show varying degrees of specialization for plant taxa. In gen-
eral, vertebrate herbivores are adapted to plants of certain life forms, such
as grasses or woody plant foliage, or to feeding on particular plant prod-
ucts, such as fruits or seeds, rather than to specific plant species. Herbivo-
rous invertebrates tend to be specialized for feeding on particular plant
parts, such as roots or leaves, or plant components, such as vascular fluids
or leaf tissues.They vary enormously, however, in the variety of plant taxa
used. Some are polyphagous, feeding on a wide variety of plant species or
higher taxa. When introduced to new geographical areas, polyphagous
species often begin to utilize a wide range of new plant species. For
example, the wheat stem sawfly (
Cephus cinctus
), now known to have been
introduced to North America from Asia (Ivie 2001), feeds on a wide
range of cultivated, introduced, and native grasses.The principal criterion
for suitable grasses appears to be relatively large stems that are able to
accommodate the developing sawfly larvae.
Many other herbivores, however, are specialized for feeding on plant
taxa that are characterized by the presence or absence of particular sec-
ondary chemicals. Many herbivorous insects tend to specialize on closely
related species of plants belonging to a single family (Strong et al. 1984).
This specialization is apt to limit their tendency to shift immediately to
new plant hosts when they are introduced to new geographic regions.
Monarch butterflies (
Danaus plexippus
), introduced from North America
to locations such as Hawaii and Australia, for example, still depend on
introduced milkweeds (
Asclepias
spp.) as their host plants.
Many herbivorous arthropods exhibit host races or biotypes adapted
to different plant species (Futuyma and Peterson 1985). Some biotypes are
nongenetic, but many have a genetic basis (Diehl and Bush 1984). In some
cases, these forms are the result of polymorphic variation among individ-
uals in a population. In other cases, they constitute geographic races or
races adapted to different hosts and consist of individuals that are capable
