Biology Reference
In-Depth Information
In the changing world, areas of natural habitat are declining in size and
becoming highly fragmented, so their effective areas may be overstocked
with species, both locally and for the region as a whole. Thus, eventual
declines in number of species are expected due both to the direct effects
of alien species on native species and to biotic relaxation in communities
that have become reduced to small remnants.
New Species, New Biotic Communities,
and New Ecosystems
Alien species exhibit all stages of speciation, from daughter forms that
show full reproductive isolation to those that are only partially isolated
and show very early degrees of divergence. These daughter populations
are contributing to the evolution of biotic communities that differ not
only in their composition of old and new species but also in the ways in
which they interact with their physical environment. Entire ecosystems
have become permanently altered by the incorporation of aliens and by
the evolutionary processes that have been set in motion. In addition, these
processes are being played out in an environment of global change. We
shall consider some of the implications of the evolutionary biology of
alien species in this context in our final chapter.
