Biology Reference
In-Depth Information
cherry laurel (
Prunus laurocerasus
) comes from the Middle East. North
America has contributed holly-leaved barberry (
Mahonia aquifolium
). Sev-
eral species are native to eastern Asia: Nepal camphor tree (
Cinnamomum
glanduliferum
), silverberry (
Eleagnus pungens
), glossy privet (
Ligustrum
lucidum
), windmill palm (
Tr achycarpus fortunei
), and several species of
cotoneasters (
Cotoneaster
spp.) and honeysuckles (
Lonicera
spp.).This inva-
sion has created a new forest type, which will provide a habitat for many
exotic animals and microorganisms, and a new evolutionary stage for both
native and exotic species.
The developmental trajectory of ecosystems is thus being altered per-
manently by alien invasions coupled with global climate change. The
species composition of many communities has been altered by intercon-
tinental and interoceanic invasions of aliens that have become established
so extensively that their eradication is impossible. Global climate change
has now shifted climatic zones so that species from locations within con-
tinents and oceans are invading new areas (Root and Schneider 2002;
Beaugrand et al. 2002). The ecological and evolutionary adjustments by
both alien and native species mean that maintaining or restoring the orig-
inal community composition of many areas is impossible.
Under the influence of global change, the question of what species are
truly alien becomes nebulous because range expansions within continen-
tal areas by many species involve hundreds of kilometers. Numerous
species are invading new regions, both through human-assisted introduc-
tion and by natural dispersal.These changes, and their evolutionary con-
sequences, pose major issues for environmental management.
Changing Communities and Ecosystems
The numbers of alien species that are established in many ecosystems has
stimulated the origin of a new measure in community structure—xeno-
diversity: the richness of a community or biota in alien species. In many
environments, xenodiversity equals or exceeds the diversity of native
species. Alien species, together with changing distributions and abun-
dances of native species in response to global change, are altering the
structure of all types of communities: terrestrial, freshwater, and marine.
Xenodiversity is evident at the community level, as well. New com-
munity types, created by alien invasions of native assemblages, have
become established over wide areas in North America and many other
parts of the world (Cox 1999). In North America, for example, invasive
trees such as casuarinas (
Casuarina
spp.), Australian paperbark (
Melaleuca
