the area. Hence, reducing the disturbance of the existing communities as much as
possible, may be itself a good management practice.
Propagule pressure is also important. Christen and Matlack (2006) suggested
that undisturbed areas are generally less invaded precisely because they may receive
fewer non-native propagules. Avoiding the introduction of non-native propagules
prevents not only the establishment of invasive species, but also offers much greater
economic benefits than the management of invasive populations after their establishment (Keller et al. 2007). Equally important, management options available
prior to invasion are more numerous and include legislation or quarantine rules
(Keller et al. 2011; Buckley and Catford 2016). In this context, the inspection of
cargo and their containers is of great importance, particularly for those having an
international origin. For example, a recent inspection of international cargo entering
the USA by rail enabled the identification of dozens of undeclared organisms,
among which were invasive insects, noxious weeds and vectors of human diseases
(https://goo.gl/fJ1uMo). Following legal rulings, some of the inspected cargo was
re-exported to its origin in order to prevent the dispersal of the unwanted pests.
In many cases, however, prevention is not possible as the non-native species are
already established in the landscape. In such cases, management actions should aim
at containing or eradicating these species. However, the broad range of invasive
species and the different ways that humans value the colonized ecosystems mean
that few generalizations can be provided for management and policy guidelines. In
other words, appropriate management and policies for invasive species is highly
context dependent (Keller et al. 2011). However, based on the review provided by
Catford et al. (2012), one can consider four not mutually exclusive main approaches
for managing transportation corridor verges when attempting to control or eradicate
invasive species: (1) “Do nothing;” (2) “Control at the introduction level by
managing the propagule supply of non-native and native species;” (3) “Manage
environmental conditions;” and (4) “Manage invasive species populations.”
1. Do nothing. This is an option when invaders are well established and are successional colonists. Several studies have shown that the proportion of invasive
species can decrease with time since disturbance–not only weedy plants
(Bellingham et al. 2005), but also larger plants (Dewine and Cooper 2008).
Hence, active management by removal of invasive species immediately following disturbance may be unnecessary and counterproductive. For example,
tamarisk species (Tamarix ramosissima, T. chinensis, T. gallica and hybrids)
have invaded riparian zones throughout western North America, southern
Africa, Argentina, Hawaii, and Australia, demanding expensive control efforts.
However, as a relatively recent addition to North American plant communities
(the 1920s–1960s was the period of main invasion), the competitive and successional processes are still ongoing. In fact, Dewine and Cooper (2008)
demonstrated that box elder (Acer negundo), a native species found in canyons
throughout western North America, is a superior competitor to tamarisk and is
capable of becoming established under dense tamarisk canopies, overtopping
and eventually killing the tamarisks. Thus, superior shade tolerance appears to
5 Aliens on the Move: Transportation Networks and Non-native …
73
possible, may be itself a good management practice.
Propagule pressure is also important. Christen and Matlack (2006) suggested
that undisturbed areas are generally less invaded precisely because they may receive
fewer non-native propagules. Avoiding the introduction of non-native propagules
prevents not only the establishment of invasive species, but also offers much greater
economic benefits than the management of invasive populations after their establishment (Keller et al. 2007). Equally important, management options available
prior to invasion are more numerous and include legislation or quarantine rules
(Keller et al. 2011; Buckley and Catford 2016). In this context, the inspection of
cargo and their containers is of great importance, particularly for those having an
international origin. For example, a recent inspection of international cargo entering
the USA by rail enabled the identification of dozens of undeclared organisms,
among which were invasive insects, noxious weeds and vectors of human diseases
(https://goo.gl/fJ1uMo). Following legal rulings, some of the inspected cargo was
re-exported to its origin in order to prevent the dispersal of the unwanted pests.
In many cases, however, prevention is not possible as the non-native species are
already established in the landscape. In such cases, management actions should aim
at containing or eradicating these species. However, the broad range of invasive
species and the different ways that humans value the colonized ecosystems mean
that few generalizations can be provided for management and policy guidelines. In
other words, appropriate management and policies for invasive species is highly
context dependent (Keller et al. 2011). However, based on the review provided by
Catford et al. (2012), one can consider four not mutually exclusive main approaches
for managing transportation corridor verges when attempting to control or eradicate
invasive species: (1) “Do nothing;” (2) “Control at the introduction level by
managing the propagule supply of non-native and native species;” (3) “Manage
environmental conditions;” and (4) “Manage invasive species populations.”
1. Do nothing. This is an option when invaders are well established and are successional colonists. Several studies have shown that the proportion of invasive
species can decrease with time since disturbance–not only weedy plants
(Bellingham et al. 2005), but also larger plants (Dewine and Cooper 2008).
Hence, active management by removal of invasive species immediately following disturbance may be unnecessary and counterproductive. For example,
tamarisk species (Tamarix ramosissima, T. chinensis, T. gallica and hybrids)
have invaded riparian zones throughout western North America, southern
Africa, Argentina, Hawaii, and Australia, demanding expensive control efforts.
However, as a relatively recent addition to North American plant communities
(the 1920s–1960s was the period of main invasion), the competitive and successional processes are still ongoing. In fact, Dewine and Cooper (2008)
demonstrated that box elder (Acer negundo), a native species found in canyons
throughout western North America, is a superior competitor to tamarisk and is
capable of becoming established under dense tamarisk canopies, overtopping
and eventually killing the tamarisks. Thus, superior shade tolerance appears to
5 Aliens on the Move: Transportation Networks and Non-native …
73