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Short Introduction
Wolfgang Nentwig
It is easy to state that the process of invasion by alien species has been occurring
since time immemorial and has been slowly accelerating for the last few
centuries. Now, in the age of globalization, this process is exploding and aliens
are spreading all over. Again, in the following four chapters we search for patterns
of invasion but no longer at the species level. Are there supra-species
patterns which promote the success of alien species in general, or of the most
successful invaders?
Globalization, i.e. a bundle of simultaneous changes which happen worldwide,
contains alterations which definitely favour alien species. Agriculture,
land management, and land use change are some of the obvious factors facilitating
the occurrence of alien species (Chap. 9). Also, the invasibility of
ecosystems has changed, with heavily disturbed ecosystems being much easier
to invade. Consequently, modern anthropogenic modifications of natural
ecosystems increase their invasibility, and the global restructuring of the
ecosystems of the world for human purposes implicitly favours biological
invasions (Chap. 11).
Pollution is certainly one of the factors promoting the spread of alien
species. As a by-product of our increasing energy consumption and fertilizer
use, nitrogen compounds flood our environment. Eutrophication of waters
and soils has long received considerable attention but it has been largely overlooked
that nitrogen-fixating plants and plants which profit from high nitrogen
levels are among the most successful invaders. So, the question is justified
as to whether nitrogen enrichment in general supports alien invaders (Chap.
10).
Globalization not only affects ecosystem structures and functions but is
also one of the causes of global climate change. This strongly modifies temperature
and humidity conditions, and completely alters the foundations of
existing species assemblages. Since the establishment of alien species
becomes easier, climate change is considered to promote alien plant invasions
(Chap. 12).