Biology Reference
In-Depth Information
and freshwater molluscs, marine crustaceans, fish, amphibians, reptiles,
birds, and mammals, in both native and invaded regions. On average, 16
species of parasites were recorded from these species in their native
regions, whereas only about seven species were noted in invaded areas.
Of these seven, only about three were parasites from the native region.
The remaining four were new acquisitions in the areas invaded. The
small fraction of parasites from native regions was presumably due to a
low probability of parasites being included with the founding host
group, their accidental loss from the host population after founding, or
lack of suitable alternate hosts in the new region. In addition, the aver-
age prevalence of parasites among individuals of the host was only about
26% as great in the invaded regions as in the area where the hosts were
native. Thus, this set of invasive host species experienced substantially
reduced parasite pressure.
The European green crab (
Carcinus maenas
), introduced to the east and
west coasts of North America, as well as to South Africa and Australia, is
a good example of the extent of release of alien marine species from par-
asitism (Torchin et al. 2001). In their native range in Europe, these crabs
are frequently parasitized by internally parasitic barnacles and isopods that
stunt growth and block reproduction. These so-called “parasitic castra-
tors” are absent in areas to which green crabs have been introduced.The
frequency of other internal parasites of several groups is also less than 10%
in areas of introduction, compared to more than 90% in Europe. Preda-
tion intensity, as measured by the incidence of limb loss, however, is sim-
ilar in areas of introduction to that in Europe. The net effect of reduced
parasitism is that green crabs reach larger sizes in areas to which they have
been introduced, either because of faster growth or longer life span.A sib-
ling species of the European green crab,
Carcinus aestuarii
, which has been
introduced to waters around Japan, is also virtually free of parasites in areas
of introduction.
Similar patterns exist for many invasive insects. In their native region
in southern Brazil, Argentine ants, for example, are parasitized by phorid
flies (
Pseudacteon
spp.), which lay eggs in the bodies of the ants.The pres-
ence of these flies makes the ants cease foraging and return to their
underground nests (Orr and Seike 1998). In areas in North America and
Europe to which Argentine ants have been introduced, they are freed
from this parasitoid.
Among vertebrates, the European house sparrow (
Passer domesticus
)
and the European starling (
Sturnus vulgaris
) have both flourished follow-
ing their introduction to North America. Both species have experienced
