concerns, railways. The first mechanism arises directly from trading activities,
where the traded commodity may itself be the non-native species. In these cases, the
“importation” of the species is deliberate, because some of its attributes are desired
in the area of destination. The reasons behind the importation of species are varied,
and include farming, forestry, livestock, ornamental plants and pets, laboratory
testing or biocontrol. In some situations, the goal is purposely the formation of wild
populations, as is the case of gaming and fishing or biocontrol agents. In other
cases, the imported species are to be stored in enclosed environments, but they often
escape from captivity. For instance, the American mink (Neovison vison) has
invaded many European countries due to accidental escape or deliberate release
from fur farms (Vidal-Figueroa and Delibes 1987; Bonesi and Palazon 2007). It is
worrisome that a great number of problematic invaders is still actively marketed
today, including freshwater macroinvertebrates and fish (Capinha et al. 2013;
Consuegra et al. 2011) and plants (Humair et al. 2015).
Trading activities may also lead to the arrival of non-native species as an
accidental “by-product” of a commodity, such as a commensal, a parasite or a
disease. These species can remain undetected for long periods of time and may
benefit from measures towards the establishment to their hosts in the wild. An
illustrative example of such by-products of a traded commodity is the crayfish
plague (Aphanomyces astaci) in Europe. This fungus-like disease is hosted by
several North American crayfish species that were introduced in European wetlands
in order to boost the wild stocks of this food item. However, unlike for North
American species that co-evolved with the disease, the crayfish plague is deadly to
European species and has already caused numerous extinctions of local populations
(Capinha et al. 2013).
Stowaway species can be associated with trading activities or any other activity
that involves a transportation vehicle. In other words, a non-native stowaway
species is not specific to a particular commodity, and it can be any organism that at
some point is displaced by a vehicle or its load. This occurs more often with species
that are difficult to detect, such as those that are small or stealthy. Known examples
of these stowaway species include land snails attached to trains or their cargo
(Peltanová et al. 2011), plankton in ship’s ballast waters (Hulme et al. 2008), or
seeds in soil attached to automobiles (Hodkinson and Thompson 1997). Centers of
human and commodity transportation and nearby areas (e.g. seaports and railroad
stations and yards) often provide the first records of non-native stowaways (e.g.
Noma et al. 2010) and can host diverse communities of invasive species (Drake and
Lodge 2004).
Finally, transport infrastructures may also act as “corridors” for the natural
dispersal of non-native biodiversity. These infrastructures facilitate the dispersal of
non-native organisms by allowing their movement across physical and environmental barriers (e.g. a mountain range now crossed by a tunnel), or by supplying
suitable habitat for expanding invasive populations. Concerning the latter case, a
few characteristics of the areas managed by transportation companies (e.g. road and
railway verges) are considered beneficial to the establishment of non-native species,
particularly the regular occurrence of disturbance that gives rise to “vacant” niches
5 Aliens on the Move: Transportation Networks and Non-native …
67
where the traded commodity may itself be the non-native species. In these cases, the
“importation” of the species is deliberate, because some of its attributes are desired
in the area of destination. The reasons behind the importation of species are varied,
and include farming, forestry, livestock, ornamental plants and pets, laboratory
testing or biocontrol. In some situations, the goal is purposely the formation of wild
populations, as is the case of gaming and fishing or biocontrol agents. In other
cases, the imported species are to be stored in enclosed environments, but they often
escape from captivity. For instance, the American mink (Neovison vison) has
invaded many European countries due to accidental escape or deliberate release
from fur farms (Vidal-Figueroa and Delibes 1987; Bonesi and Palazon 2007). It is
worrisome that a great number of problematic invaders is still actively marketed
today, including freshwater macroinvertebrates and fish (Capinha et al. 2013;
Consuegra et al. 2011) and plants (Humair et al. 2015).
Trading activities may also lead to the arrival of non-native species as an
accidental “by-product” of a commodity, such as a commensal, a parasite or a
disease. These species can remain undetected for long periods of time and may
benefit from measures towards the establishment to their hosts in the wild. An
illustrative example of such by-products of a traded commodity is the crayfish
plague (Aphanomyces astaci) in Europe. This fungus-like disease is hosted by
several North American crayfish species that were introduced in European wetlands
in order to boost the wild stocks of this food item. However, unlike for North
American species that co-evolved with the disease, the crayfish plague is deadly to
European species and has already caused numerous extinctions of local populations
(Capinha et al. 2013).
Stowaway species can be associated with trading activities or any other activity
that involves a transportation vehicle. In other words, a non-native stowaway
species is not specific to a particular commodity, and it can be any organism that at
some point is displaced by a vehicle or its load. This occurs more often with species
that are difficult to detect, such as those that are small or stealthy. Known examples
of these stowaway species include land snails attached to trains or their cargo
(Peltanová et al. 2011), plankton in ship’s ballast waters (Hulme et al. 2008), or
seeds in soil attached to automobiles (Hodkinson and Thompson 1997). Centers of
human and commodity transportation and nearby areas (e.g. seaports and railroad
stations and yards) often provide the first records of non-native stowaways (e.g.
Noma et al. 2010) and can host diverse communities of invasive species (Drake and
Lodge 2004).
Finally, transport infrastructures may also act as “corridors” for the natural
dispersal of non-native biodiversity. These infrastructures facilitate the dispersal of
non-native organisms by allowing their movement across physical and environmental barriers (e.g. a mountain range now crossed by a tunnel), or by supplying
suitable habitat for expanding invasive populations. Concerning the latter case, a
few characteristics of the areas managed by transportation companies (e.g. road and
railway verges) are considered beneficial to the establishment of non-native species,
particularly the regular occurrence of disturbance that gives rise to “vacant” niches
5 Aliens on the Move: Transportation Networks and Non-native …
67