regarding the movement of species in ballast waters (Drake and Lodge 2004;
Seebens et al. 2016). We speculate that overland transportation of biological
stowaways is likely to be less frequent than those by aquatic vehicles; however, we
also suspect that the lack of studies for the terrestrial counterpart does not reflect the
true contribution of this process for non-native species dispersal. Below we describe
a few documented cases where terrestrial transportation had a relevant role on the
expansion of biological invaders, with special emphasis on trains.
The spotted knapweed (Centaurea stoebe) is an example of a non-native species
dispersed by trains. This plant, native to south east and central Europe, arrived in
North America in the late 1800s and by the year 2000 it was already found in most
contiguous American states (Sheley et al. 1998). The mechanisms that enabled such
a fast dispersal were unclear for a long time, but a recent reconstruction of the
patterns of invasion of this plant showed a close agreement between the extent of
colonized areas and its velocity dispersal on one side, and the spatial coverage and
development of the railway network in the USA on the other (Broennimann et al.
2014). For instance, the wave of invasion was much faster and wider in the eastern
states, where a denser and older network exists. In favor of the important role of
stowaway transportation of this species in trains, particularly for some long-distance
dispersal events, is the plants’ known ability to become attached to the undercarriages of vehicles (Sheley et al. 1998).
Trains are known to have been a vehicle of dispersal also for ragwort plant
species. One of such cases concerns the South African ragwort (Senecio inaequidens), a species introduced in Europe from South Africa in the first half of the
twentieth century, and that is now found from Norway in the north to Italy in the
south, and from Bulgaria in the east to Spain in the west (Heger and Böhmer 2006).
The achene-type fruit of this species is able to stick to trains in movement or to their
transported commodities, a characteristic that contributed to the rapid dispersal of
the species along railways systems (Heger and Böhmer 2005). Another plant of this
group known to “hitchhike” on trains is the Oxford ragwort (Senecio squalidus), a
hybrid that escaped cultivation from the Oxford Botanic Garden. At the end of the
eighteenth century it was established in some parts of Oxford (Harris 2002; Heger
and Böhmer 2005), and it currently invades most of Britain. George Druce, an
English botanist, described the dispersal of this plant as follows (1927b, p. 241, in
Harris 2002): “ ... the vortex of air following the express train carries the fruits in its
wake. I have seen them enter a railway-carriage window near Oxford and remain
suspended in the air in the compartment until they found an exit at Tilehurst [about
40 km from Oxford]”.
A few records of animal species being transported as stowaways in trains can
also be found in the scientific literature and the media. Perhaps the most recurrent
cases refer to urban pest species, such as rats and mice (Li et al. 2007), but there are
also references to ants (Elton 1958), beetles (White 1973), spiders (Nentwig and
Kobelt 2010) and even armadillos (Hofmann 2009). However, in these cases the
contribution of train-mediated transportation to the overall process of dispersal
remains poorly studied.
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F. Ascensão and C. Capinha
Seebens et al. 2016). We speculate that overland transportation of biological
stowaways is likely to be less frequent than those by aquatic vehicles; however, we
also suspect that the lack of studies for the terrestrial counterpart does not reflect the
true contribution of this process for non-native species dispersal. Below we describe
a few documented cases where terrestrial transportation had a relevant role on the
expansion of biological invaders, with special emphasis on trains.
The spotted knapweed (Centaurea stoebe) is an example of a non-native species
dispersed by trains. This plant, native to south east and central Europe, arrived in
North America in the late 1800s and by the year 2000 it was already found in most
contiguous American states (Sheley et al. 1998). The mechanisms that enabled such
a fast dispersal were unclear for a long time, but a recent reconstruction of the
patterns of invasion of this plant showed a close agreement between the extent of
colonized areas and its velocity dispersal on one side, and the spatial coverage and
development of the railway network in the USA on the other (Broennimann et al.
2014). For instance, the wave of invasion was much faster and wider in the eastern
states, where a denser and older network exists. In favor of the important role of
stowaway transportation of this species in trains, particularly for some long-distance
dispersal events, is the plants’ known ability to become attached to the undercarriages of vehicles (Sheley et al. 1998).
Trains are known to have been a vehicle of dispersal also for ragwort plant
species. One of such cases concerns the South African ragwort (Senecio inaequidens), a species introduced in Europe from South Africa in the first half of the
twentieth century, and that is now found from Norway in the north to Italy in the
south, and from Bulgaria in the east to Spain in the west (Heger and Böhmer 2006).
The achene-type fruit of this species is able to stick to trains in movement or to their
transported commodities, a characteristic that contributed to the rapid dispersal of
the species along railways systems (Heger and Böhmer 2005). Another plant of this
group known to “hitchhike” on trains is the Oxford ragwort (Senecio squalidus), a
hybrid that escaped cultivation from the Oxford Botanic Garden. At the end of the
eighteenth century it was established in some parts of Oxford (Harris 2002; Heger
and Böhmer 2005), and it currently invades most of Britain. George Druce, an
English botanist, described the dispersal of this plant as follows (1927b, p. 241, in
Harris 2002): “ ... the vortex of air following the express train carries the fruits in its
wake. I have seen them enter a railway-carriage window near Oxford and remain
suspended in the air in the compartment until they found an exit at Tilehurst [about
40 km from Oxford]”.
A few records of animal species being transported as stowaways in trains can
also be found in the scientific literature and the media. Perhaps the most recurrent
cases refer to urban pest species, such as rats and mice (Li et al. 2007), but there are
also references to ants (Elton 1958), beetles (White 1973), spiders (Nentwig and
Kobelt 2010) and even armadillos (Hofmann 2009). However, in these cases the
contribution of train-mediated transportation to the overall process of dispersal
remains poorly studied.
70
F. Ascensão and C. Capinha