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Short Introduction
Wolfgang Nentwig
The following four chapters present the main pathways for alien species
invasions, i.e. they concern the routes by which aliens leave their habitat of
origin, how they are distributed, and the ways by which they enter their new,
invaded habitat. The subdivision into animals, plants, ships and waterways is
admittedly arbitrary but offers useful possibilities to detect parallels and differences
between various invasion pathways. Chapters 2 and 3 deal primarily,
but not exclusively, with terrestrial species whereas Chaps. 4 and 5 concern
the aquatic environment, including both animals and plants (mostly
algae).
The drive behind species distributing across biogeographical barriers into
new habitats where they assume their role as aliens is based on human activities,
by direct migration, by transport of goods or by facilitating dispersal
through the elimination of existing barriers. This process probably began
already when early man started to conquer the world but it has intensified
dramatically over the last few centuries, culminating in the modern phase of
globalization.
Many species have been spread deliberately but, for the majority of
invaders, spread has occurred accidentally. Parallels may be conspicuous
between animals and plants when alien species reach their new habitat as contaminants,
hitchhikers and stowaways or when they are transported by
humans as ornamentals and pets.
Pathway analysis is a first and important step of curtailing the spread of
alien species. Only when the corridors through which species become aliens
are known can effective countermeasures be taken. This is most obvious in
the examples given in Chaps. 4 and 5 where ballast water, hull fouling, waterways
and man-made ocean channels are easily recognizable pathways. By
means of international conventions, it is intended that their harmful effect be
eliminated or at least reduced.