Biology Reference
In-Depth Information
Time Lags in Impacts of Aliens
Many alien species show a substantial time lag—years or decades—
between initial establishment and the appearance of strong ecological
impacts (Crooks and Soulé 1999).Time lags can exist for both ecological
and evolutionary reasons (Kowarik 1995). For example, a time lag can
result simply from the fact that a new alien species requires time to dis-
perse into favorable habitat patches throughout a region and to build up
populations capable of producing abundant seeds or offspring. At some
point, it then becomes capable of a major population explosion and seri-
ous ecological impact. Initial populations may also suffer from low repro-
ductive success because low population density restricts beneficial social
interactions among individuals, the Allee effect (Lewis and Kareiva 1993).
An Allee effect appears, for example, to account for the slow increase in
the eastern population and range of the house finch (
Carpodacus mexi-
canus
) between its introduction on Long Island, New York, in 1940 and
the abrupt increase in its spread in about 1960 (Veit and Lewis 1996). Cli-
matic shifts may also make environmental conditions favorable for inva-
sive spread of a species long after its initial establishment.
On the other hand, an initial population may lack evolutionary adap-
tations that permit explosive population growth. After some time period,
through genetic reorganization within the population, an evolutionary
breakthrough may occur, enabling the alien to become an invasive
species. A switch to earlier flowering, for example, has apparently enabled
a ragwort (
Senecio inaequidens
) to become highly invasive in Europe
(Kowarik 1995). Hybridization between species or between populations
of a species from different source areas may be a major impetus for the
evolutionary emergence of an invasive form after a time lag (Ewel et al.
1999; Ellstrand and Schierenbeck 2000).
Extirpation or Extinction of Native Species
Invasive alien species often cause the local extirpation and sometimes the
complete extinction of native species. In particular, alien species intro-
duced to insular environments such as islands, lakes, or rivers have caused
numerous extirpations and extinctions in short periods of ecological
time. Continental and oceanic areas are now beginning to experience
these effects. Alien species may drive native species to extinction by com-
petition, predation, or disease effects. Many native plants have suffered
