within countries. There are a wide range of pathways through which alien taxa are
either intentionally or accidentally introduced (Hulme et al. 2008). Alien taxa are
intentionally transported and introduced for many uses, including for agriculture,
horticulture, angling, medicinal purposes and as pets. But organisms are also often
accidentally introduced when their hosts (such as plants or animals, or parts thereof
such as wood or fruit) are intentionally transported between regions, or when they
‘hitchhike’ on or in transport vessels (including ships and aeroplanes).
Because some alien taxa become invasive and have negative environmental or
socio-economic impacts where introduced, it is vital that these taxa are managed
(Pimentel et al. 2001; Blackburn et al. 2011; Simberloff et al. 2013). Often the most
efficient and cost-effective way to manage biological invasions is to prevent the
introduction of taxa that are likely to cause harm (Leung et al. 2002; Puth and Post
2005; Simberloff et al. 2013). Most efforts to achieve this only focus on a few taxa
that have a history of invasion elsewhere (Early et al. 2016; Grosholz 2018).
Unfortunately, this strategy is ineffective in preventing the introduction of taxa
with no invasion history or those that are accidentally introduced (Hulme 2006;
Seebens et al. 2017, 2018; Grosholz 2018). Strategies that aim to identify and
prioritise important pathways of introduction are more appropriate in these instances
(Hulme 2006). These strategies aim to prevent invasions by reducing the propagule
pressure [the number of individuals introduced or number of introduction events for
a specific taxon (Lockwood et al. 2005)] and colonisation pressure [the number of
species introduced (Lockwood et al. 2009)] associated with priority pathways, and
these efforts have been shown in some instances to be highly effective (Bailey et al.
2011; Sikes et al. 2018). For example, enacted policies that require foreign vessels
entering the Laurentian Great Lakes to exchange and/or flush their ballast tanks with
mid-ocean saltwater have markedly reduced the risk of introductions mediated by
the release of ballast water by ships (Bailey et al. 2011).
The importance of managing the pathways of introduction is widely recognised,
and has been included in the Aichi Biodiversity targets set by the Convention on
Biological Diversity (CBD). In order to meet Aichi Biodiversity Target 9, the
countries that are party to the CBD, including South Africa, must identify and
prioritise their pathways of introduction, and manage those pathways to prevent
the introduction of invasive taxa (UNEP 2011). It is, therefore, not surprising that the
pathways of introduction have been studied in many regions [e.g. Czech Republic
(Pyšek et al. 2011) and China (Xu et al. 2006)] and at various spatial scales [e.g.
global (Kraus 2007; Hulme et al. 2008), continental (Katsanevakis et al. 2013) and
sub-continental (Zieritz et al. 2017)]. These studies have demonstrated that the
pathways that are important for the introduction of alien taxa vary across regions
and spatial scales, but also across taxonomic groups, environments and over time
(Hulme et al. 2008; Pyšek et al. 2011; Katsanevakis et al. 2013; Essl et al. 2015;
Faulkner et al. 2016; Zieritz et al. 2017). The pathways also vary in the degree to
which they are associated with taxa that become invasive and/or have negative
impacts (Wilson et al. 2009; Pyšek et al. 2011; Faulkner et al. 2016; Pergl et al.
2017), and in their relative importance in facilitating initial introduction and subsequent dispersal (Padayachee et al. 2017). Species traits, the environment and trends
in socio-economic factors (like the volume and type of goods imported, economics,
12 Pathways of Introduction and Dispersal
315
either intentionally or accidentally introduced (Hulme et al. 2008). Alien taxa are
intentionally transported and introduced for many uses, including for agriculture,
horticulture, angling, medicinal purposes and as pets. But organisms are also often
accidentally introduced when their hosts (such as plants or animals, or parts thereof
such as wood or fruit) are intentionally transported between regions, or when they
‘hitchhike’ on or in transport vessels (including ships and aeroplanes).
Because some alien taxa become invasive and have negative environmental or
socio-economic impacts where introduced, it is vital that these taxa are managed
(Pimentel et al. 2001; Blackburn et al. 2011; Simberloff et al. 2013). Often the most
efficient and cost-effective way to manage biological invasions is to prevent the
introduction of taxa that are likely to cause harm (Leung et al. 2002; Puth and Post
2005; Simberloff et al. 2013). Most efforts to achieve this only focus on a few taxa
that have a history of invasion elsewhere (Early et al. 2016; Grosholz 2018).
Unfortunately, this strategy is ineffective in preventing the introduction of taxa
with no invasion history or those that are accidentally introduced (Hulme 2006;
Seebens et al. 2017, 2018; Grosholz 2018). Strategies that aim to identify and
prioritise important pathways of introduction are more appropriate in these instances
(Hulme 2006). These strategies aim to prevent invasions by reducing the propagule
pressure [the number of individuals introduced or number of introduction events for
a specific taxon (Lockwood et al. 2005)] and colonisation pressure [the number of
species introduced (Lockwood et al. 2009)] associated with priority pathways, and
these efforts have been shown in some instances to be highly effective (Bailey et al.
2011; Sikes et al. 2018). For example, enacted policies that require foreign vessels
entering the Laurentian Great Lakes to exchange and/or flush their ballast tanks with
mid-ocean saltwater have markedly reduced the risk of introductions mediated by
the release of ballast water by ships (Bailey et al. 2011).
The importance of managing the pathways of introduction is widely recognised,
and has been included in the Aichi Biodiversity targets set by the Convention on
Biological Diversity (CBD). In order to meet Aichi Biodiversity Target 9, the
countries that are party to the CBD, including South Africa, must identify and
prioritise their pathways of introduction, and manage those pathways to prevent
the introduction of invasive taxa (UNEP 2011). It is, therefore, not surprising that the
pathways of introduction have been studied in many regions [e.g. Czech Republic
(Pyšek et al. 2011) and China (Xu et al. 2006)] and at various spatial scales [e.g.
global (Kraus 2007; Hulme et al. 2008), continental (Katsanevakis et al. 2013) and
sub-continental (Zieritz et al. 2017)]. These studies have demonstrated that the
pathways that are important for the introduction of alien taxa vary across regions
and spatial scales, but also across taxonomic groups, environments and over time
(Hulme et al. 2008; Pyšek et al. 2011; Katsanevakis et al. 2013; Essl et al. 2015;
Faulkner et al. 2016; Zieritz et al. 2017). The pathways also vary in the degree to
which they are associated with taxa that become invasive and/or have negative
impacts (Wilson et al. 2009; Pyšek et al. 2011; Faulkner et al. 2016; Pergl et al.
2017), and in their relative importance in facilitating initial introduction and subsequent dispersal (Padayachee et al. 2017). Species traits, the environment and trends
in socio-economic factors (like the volume and type of goods imported, economics,
12 Pathways of Introduction and Dispersal
315