Biology Reference
In-Depth Information
alien, as well as its ecological impact, may decline substantially. In many
cases, native species also make ecological and evolutionary adjustments
that shield them from the impacts of aliens. In addition, the population
outbreak of the alien creates massive ecological and evolutionary oppor-
tunities for exploitation by members of the native community. Ecological
responses may be rapid, as members of the native community learn to
exploit the new community member as a food source and to avoid its
direct detrimental influences. Later, evolutionary adjustments improve the
ability of members of the native community to use the alien as a resource
or to avoid the negative impacts of the alien.
Thus, over the long-term, evolutionary adjustments occur by mem-
bers of the native community, and alien species may become integrated
into the biotic assemblage.The sum of these responses or counteradapta-
tions leads toward the reestablishment of a stable community. Stability, in
this sense, implies that the alien has been accommodated at the level of
dynamics of the landscape at large.That is, its regional population has sta-
bilized, and its action in extirpation and extinction has ceased.
Not to be overlooked is the potential for pest species, native or alien,
to evolve in response to selective pressures by deliberately introduced bio-
logical control agents. Although few such responses have been docu-
mented, most introduced biocontrol agents have been active only for a
few years or decades. As we shall see, enough evidence has accumulated
of this risk to require very detailed screening of prospective biocontrol
agents and the inclusion of procedures to assess their evolutionary poten-
tial (Simberloff and Stiling 1996).
Alien Invasions and Speciation
In the short term, many biologists point out that the worldwide dispersal
of alien species, combined with their role in extinction of species they
encounter, is tending to homogenize the world's biota, creating, in a sense,
a “New Pangaea” (Rosenzweig 2001). An often overlooked evolutionary
result of the spread of alien species, however, is the establishment of new,
independent evolutionary populations in different geographical areas.
Over the long term, divergence and speciation of these populations will
to a degree offset the extinctions that occur in the shorter term.
As we shall see, patterns of evolution of alien species and the native
species with which they interact are rapid, diverse, and often startling.
New species are arising before our eyes by hybridization and divergent
evolution.The patterns of rapid evolution seen in species expanding their
