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An instructive example is recent emphasis on the study of transportation vectors
or pathways in alien invasions (vector science) (Ruiz 2003; Mooney et al. 2005;
Kowarik and von der Lippe 2007) to compliment earlier biological research that
emphasized postarrival invasion processes (Richardson and Pysek 2006).
Understanding biotic invasions through the lens of different transport pathways
allows framing of research in a way that is tailored to the institutional context of
particular pathways. Recent research has, for instance, focused on the transport of
alien species through water ballast in marine shipping (Minton et al. 2005) or the
horticultural trade (Reichard and White 2001). The framing of research based on
pathways has improved risk assessments for managers by suggesting new forms of
context dependence. For instance, risk management of Mediterranean fruit fly
introductions to the US, through trade of pink tomatoes from Northern Africa,
combines regulations based on the origin of the product, trade restrictions based on
the season, and preventative measures during production and transport (Hallman
2007). Although vector science has helped to better understand and manage the
spread of alien species, it does not solve the problem of managing impacts in
invaded habitats.
To reduce disconnect between managers and researchers interested in invasive
species impacts, we suggest that greater emphasize should be placed on context-
dependent management and research based on careful consideration of human val-
ues. We identify a typology of four generalizable habitat types that, according to
our experience, represent typical contexts of invasions. We discuss how the traits of
successful invaders and their impacts may differ between these four habitat types,
and how a consideration of these types may therefore help guide invasive species
research and facilitate transfer of management experiences between individual
cases. We illustrate the typology mainly using examples from oceanic islands,
which have long been recognized as model systems for invasion biology (Elton
1958). Although we mostly discuss plant invasions, the proposed typology can also
be applied to other taxonomic groups. We define invasive species as species that
have been introduced and spread outside of their native range, presenting some
problem according to the relevant experts and stakeholders.
5.2 A Framework for Invasive Species Research
To better understand, value and manage impacts of invasive species, we propose a
habitat-classification framework based on four habitat types that are defined by
their differences in type and degree of human habitat modification, and differences
in human valuation (Fig. 5.1, Table 5.1). The first type,
anthropogenic habitat
,
encompasses highly disturbed and anthropogenic areas such as agriculture, planta-
tion forestry, or urban areas. In anthropogenic habitats, cultivated plants have
replaced most natural vegetation, and management emphasizes socioeconomic
gains from production and/or ecosystem services. The second type,
reference habi-
tat
, represents relatively undisturbed habitats dominated by native species.
Reference habitat is typically part of a protected area or a wilderness area, and the
