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Short Introduction
Wolfgang Nentwig
One of the striking features of successful invasive species is that they are a
minority among alien species. Aliens, again, are only a small subgroup of all
species. Consequently, the process of becoming a successful invasive alien
species can be understood as a strong selection sequence. An assemblage of
special characteristics, also called traits, is obviously important to belong to
the spearhead of aliens.
Determining the traits of successful invaders ought to be a very rewarding
task because it would enable us to determine the characteristics of the most
harmful invaders. This would facilitate early diagnosis of potential invaders
and would also offer a promising approach to avoid alien invasive species. It is
obvious that scientists have been focussing on the nature of these traits for a
long time but, do they really exist? At least, we do not get any simple answers
for different groups of invasive alien species which have been investigated in
this respect (Chaps. 7 and 8).
How do invasive species fit into the framework of ecological theories? Is it
true, as some have suggested, that aliens are inherently superior to native
species because they are so successful? And, what is the basis of this superiority?
It has also been suggested that invasives possess novel weapons which
might explain their success. This super-weapon theory may sound strange but
was, among others, responsible for the resurrection of allelopathy in plants
and intensified the discussion of gradual differentiation in chemical defence
among alien populations. If successful aliens principally conquer only empty
niches or develop a loss of defence against natural enemies, as some theories
suggest, this would bear good prospects for our battle. Or are such universal
theoretical explanations not applicable to the reality of a strange mixture of
invasive alien species (Chap. 6)?