Biology Reference
In-Depth Information
influenced by characteristics of the native community and its environ-
ment (Kogan 1991). At first, this accumulation reflects two relationships:
the similarity of the alien species to native host species and the lack of
evolved defenses of aliens to natives that begin to exploit them. In partic-
ular, the presence of plants closely related to a newly arrived alien is a
strong determinant of the speed and diversity of an alien plant's coloniza-
tion by herbivores (Connor et al. 1980). Over time, however, additional
natives may evolve to use the aliens, while the aliens evolve ways to repel
their exploiters.
Accumulation of Herbivorous Arthropods
by Crop Plants
Crop plants introduced to new regions are often quick to accumulate
herbivorous arthropods from the native fauna. Three major factors seem
to determine the speed and magnitude of recruitment of herbivores by
introduced crop plants (Kogan 1991). First, the diversity of generalist her-
bivores and specialist herbivores associated with related plants or plants
with similar patterns of secondary chemical defenses defines the potential
reservoir of herbivores that might be recruited. Second, the degree to
which the phenology of the crop corresponds to the seasonal life history
patterns of herbivores influences those that are most likely to begin to use
the new plant host. Third, the strength of herbivore defenses of the crop
plant influences the success of herbivores that attempt to use the plant. In
some cases, crop plant species have been bred to reduce the content of
defensive chemicals, which can be distasteful to humans. Of course, all of
these relationships are subject to evolutionary modification.
Several extensive studies have been done of the accumulation of her-
bivorous arthropods by introduced agricultural crops. Some of these have
focused on specific crops, others on crops in specific regions.
Some of the most interesting studies have compared the diversity of
herbivorous arthropods on a single crop in different world regions. One
of the most comprehensive studies is for sugar cane (
Saccharum officinarum
)
by Strong et al. (1977).These authors compiled data for 75 world regions
to which sugar cane had been introduced, and for which the number of
arthropod pests, the area of cane in cultivation, and often the date of
introduction were available. Some 1,645 arthropod pests were identified.
More than 58% of these were recorded from a single location and another
9.5% from only two locations. Thus, at least two-thirds, and probably
much more, of the pests were recruited from the local native faunas of the
