Biology Reference
In-Depth Information
resource conditions such as nutrient availability. Use of resources by alien
plants or animals can bring them into competition with natives. Alien
predators, parasites, and disease agents establish complex new relationships
with both natives and other aliens. The aliens themselves also constitute
resources for native species to exploit. The presence of alien species thus
leads to reorganization of the community food web and associated pres-
sures of natural selection.
The establishment of an alien species thus creates a nucleus of accel-
erated evolution within the invaded community (Carroll and Dingle
1996).The evolution of the alien itself is accelerated, as new selective pres-
sures act on it.As the alien population grows, its influence accelerates evo-
lutionary change by native species. Selection may favor traits that reduce
negative impacts of the alien or that enable native species to take advan-
tage of the resources the alien provides. These adjustments by native
species, known collectively as counteradaptations, in time integrate the
alien into an altered biotic community.
Local communities and entire regional biotas invaded by aliens are
thus permanently changed. The ubiquity and abundance of many alien
species virtually precludes their eradication. Some, in addition, establish
mutually beneficial interactions with native species, in some cases making
their eradication undesirable. Even when alien species can be removed, a
community of species that has been changed by evolution remains.
Return of native species to an original evolutionary state is impossible,
and a “ghost of alien influence” will remain. Alien invasions, alone and in
combination with the influence of global climatic change, have already
created new community types in many locations (Walther 2000).
Evolution by Alien Species and Global Change
On regional and continental scales, the effects of alien invasions are now
being compounded by global climatic and habitat change (Mooney and
Hobbs 2000). Global change has many aspects: climatic warming, atmos-
pheric carbon dioxide increase, increased nitrogen deposition, increased
ultraviolet radiation intensity, deforestation, desertification, chemical pol-
lution, habitat disturbance and fragmentation, and loss of biodiversity.
These aspects of global change are now beginning to strain the adapta-
tion of many native species to the conditions to which they have long
been closely adjusted. Most of these changes also make it easier for alien
species to become established and spread. As a result, the frequency and
