Biology Reference
In-Depth Information
however, with some herbivores accumulating parasitoid complexes as
great as those in their native region in only 10 yr and others not even after
140 yr. Nevertheless, it appears that accumulation of parasitoids had not
reached a plateau in the case of these herbivorous insects by 155 yr after
introduction.
Only a few studies on accumulation of parasites by introduced verte-
brates are available. In North America, the house sparrow ( Passer domesti-
cus ), first introduced in 1852 or 1853, has accumulated about 33 species
of mites, ticks, fleas, and parasitic flies, compared to 69 species known in
Europe (Brown and Wilson 1975). Of these 33, 18 are species not known
from the Old World. In North America, European starlings Sturnus vul-
garis , introduced in 1890, have only 22 species of parasitic worms, whereas
70 have been recorded in Europe (Hair and Forrester 1970). Of the 22
North American species, 8 are not recorded from Europe.Thus, in a cen-
tury to a century and a half, these species have not accumulated numbers
of parasites equal to those they possess in Europe.
Although no comprehensive analysis is available, many insects intro-
duced as biocontrol agents for herbivorous pests are themselves highly
vulnerable to hyperparasitism (McEvoy and Coombs 2000). The fre-
quency with which hyperparasites become associated with newly intro-
duced parasitoids could thus influence the effectiveness of biocontrol
efforts.
Thus, it appears that colonization of newly introduced host species by
native herbivores, predators, and parasites may follow a pattern similar to
the colonization of small, defaunated mangrove islands in Florida, as stud-
ied by Simberloff and Wilson (1969, 1970) and Wilson (1969). Like a
defaunated island, a newly introduced host may rapidly accumulate many
opportunistic species that eventually reach a maximum richness corre-
sponding to a “noninteractive species equilibrium.” Ecological interac-
tions among these species and later colonizers then may lead to the loss
of some of the original colonists, producing an “assortative equilibrium”
that may be less diverse. Still later, in the case of new host species, evolu-
tionary adjustments may lead to a still different equilibrium richness of
species exploiting the host.
The Worst Case Scenario
The interactions of introduced and native species can thus be complex.
Introduced plants acquire a fauna of both native and alien herbivores, and
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