Biology Reference
In-Depth Information
native ant diversity and abundance had recovered to pre-fire ant levels.
Numbers of both species and individuals of other arthropod groups were
also higher in fire ant sites in 1999 than in 1987.The fire ant itself was still
abundant, but not as numerous as in 1987. One factor responsible for the
recovery of native ants and other arthropods was postulated to be acqui-
sition of natural enemies. In particular, a microsporidian parasite of fire
ants,
Thelohania solenopsae
,was found at the study site. This parasite has
been shown to reduce fire ant populations in their native region in South
America.
For many strongly invasive species, however, ecological responses alone
are likely to be delayed or inadequate. These may lead to a “boom and
bust” pattern of population behavior by the introduced species, in which
its population grows rapidly to a very high level and then declines as its
food resource is overexploited (Williamson 1996). Herbivores introduced
to predator-free islands often show this pattern, which may cause the
species to fall to a very low population or even become extinct. On St.
Matthew Island, in the Bering Sea, for example, a population of reindeer
(
Rangifer tarandus
) grew from 29 individuals in 1944 to an estimated 6,000
in 1963 (Klein 1968). In the winter of 1963-64, all but 50 animals, all
females, died of starvation, so the population essentially fell to extinction.
Similar patterns of population growth and crash have been observed for
reindeer on several other arctic islands.
Other invasive species may experience ecological responses that are
negligible or even beneficial. Plants such as salt cedar (
Tamarix
spp.) and
Russian olive (
Eleagnus angustifolia
) receive little negative ecological
response and are used to a degree by native animal species such as birds
and mammals. Russian olive is, in fact, actively dispersed by birds that feed
on its fruits. Other interactions between invasive species and natives may
in some instances become mutualistic. In the western United States,
native animals such as the giant kangaroo rat (
Dipodomys ingens
) and the
valley pocket gopher (
Thomomys bottae
) feed heavily on alien annual plants
and, at the same time, create soil disturbances favoring the growth of these
species (Cox 1999).
Ecological relationships thus both influence and are influenced by the
establishment of alien species that reach new areas by dispersal or human
introduction. These interactions are capable of sorting alien species and
creating an initial degree of community organization. For vertebrate ani-
mals, interspecific competition can be a significant sorting factor for
establishment of alien species that reach an area.The role of competition
among alien species can be evaluated, at least roughly, by examining the
