information already exists and is available in the various on-line databases that
profile problematic invaders, such as the Invasive Species Compendium (http://
sites.cabi.org/isc/), Invasoras (http://invasoras.pt) or the ‘Global Invasive
Species Database’ (http://www.iucngisd.org/).
4. Manage invasive species populations. For some highly problematic invasive
species it may be more effective to focus on management techniques that directly
target their populations. Traditional control techniques, such as the use of prescribed fire or herbicides, as well as mechanical removal may help to alleviate the
competitive effects of some species and limit further spread. However, care
should be taken to minimize impacts on native species. If the non-native species,
particularly plants, have unrestricted dispersal but infrequent propagule arrival,
mapping and removing the individual colonist patches soon after they arise, may
achieve effective control. This is most important, as such patches often pose a
high risk of becoming sources of subsequent dispersal (Moody and Mack 1988).
Conversely, if the invasive species has limited dispersal ability, an effective
control may be achieved by creating spatial discontinuities on vegetation types at
an extent greater than the seed dispersal range, thereby breaking the continuity of
the habitat to the invader (Christen and Matlack 2006).
Currently, railroad companies routinely clear-cut and/or spray with herbicide all
vegetation that grows too close to the tracks. However, although this method
eradicates most of the vegetation (including native species), it also favors the
dominance of species that respond favorably to clear-cutting or resist herbicides.
Actually, in some cases, roadside herbicide treatments are known to reduce the
cover of some non-native species favoring others (Gelbard and Belnap 2003).
Hence, investing exclusively in the direct management of invasive populations
should be accompanied by a careful evaluation of trade-offs and probably the best
option is often a combination of various management possibilities.
Where to begin the control of invasive species is a major question when
managing railway verges. One factor that apparently influences the propagule
pressure is the disturbance level of the transportation infrastructure. For example, it
is known that older roads typically have higher cumulative levels of traffic and
maintenance than younger roads, which might result in an increase in non-native
species occurrence near old roads simply due to higher disturbance and therefore of
propagule pressure. This was found for earthworms (Cameron and Bayne 2009) and
the invasive common reed (Phragmites australis) (Jodoin et al. 2008). Likewise,
plant communities adjacent to roads that receive heavy traffic might be expected to
be invaded more than those adjacent to infrequently used and unpaved roads
(Parendes and Jones 2000; Gelbard and Belnap 2003). Hence, the degree of perturbation of the transportation corridor, namely the traffic level, could be used as an
indicator of the invasion level of verges in railways, and to identify which corridor
should be targeted for management.
During the project phase of new railway corridors, engineers should consider
whether some routes might aid the dispersal of non-native species more than others.
For example, routes expanding railways that are highly colonized by problematic
5 Aliens on the Move: Transportation Networks and Non-native …
75
profile problematic invaders, such as the Invasive Species Compendium (http://
sites.cabi.org/isc/), Invasoras (http://invasoras.pt) or the ‘Global Invasive
Species Database’ (http://www.iucngisd.org/).
4. Manage invasive species populations. For some highly problematic invasive
species it may be more effective to focus on management techniques that directly
target their populations. Traditional control techniques, such as the use of prescribed fire or herbicides, as well as mechanical removal may help to alleviate the
competitive effects of some species and limit further spread. However, care
should be taken to minimize impacts on native species. If the non-native species,
particularly plants, have unrestricted dispersal but infrequent propagule arrival,
mapping and removing the individual colonist patches soon after they arise, may
achieve effective control. This is most important, as such patches often pose a
high risk of becoming sources of subsequent dispersal (Moody and Mack 1988).
Conversely, if the invasive species has limited dispersal ability, an effective
control may be achieved by creating spatial discontinuities on vegetation types at
an extent greater than the seed dispersal range, thereby breaking the continuity of
the habitat to the invader (Christen and Matlack 2006).
Currently, railroad companies routinely clear-cut and/or spray with herbicide all
vegetation that grows too close to the tracks. However, although this method
eradicates most of the vegetation (including native species), it also favors the
dominance of species that respond favorably to clear-cutting or resist herbicides.
Actually, in some cases, roadside herbicide treatments are known to reduce the
cover of some non-native species favoring others (Gelbard and Belnap 2003).
Hence, investing exclusively in the direct management of invasive populations
should be accompanied by a careful evaluation of trade-offs and probably the best
option is often a combination of various management possibilities.
Where to begin the control of invasive species is a major question when
managing railway verges. One factor that apparently influences the propagule
pressure is the disturbance level of the transportation infrastructure. For example, it
is known that older roads typically have higher cumulative levels of traffic and
maintenance than younger roads, which might result in an increase in non-native
species occurrence near old roads simply due to higher disturbance and therefore of
propagule pressure. This was found for earthworms (Cameron and Bayne 2009) and
the invasive common reed (Phragmites australis) (Jodoin et al. 2008). Likewise,
plant communities adjacent to roads that receive heavy traffic might be expected to
be invaded more than those adjacent to infrequently used and unpaved roads
(Parendes and Jones 2000; Gelbard and Belnap 2003). Hence, the degree of perturbation of the transportation corridor, namely the traffic level, could be used as an
indicator of the invasion level of verges in railways, and to identify which corridor
should be targeted for management.
During the project phase of new railway corridors, engineers should consider
whether some routes might aid the dispersal of non-native species more than others.
For example, routes expanding railways that are highly colonized by problematic
5 Aliens on the Move: Transportation Networks and Non-native …
75