Verges as Habitat and Corridors for Non-native Species
Vegetated verges generally border transportation infrastructures, in particular railways and roads. These verges are regularly mowed in order to ensure traffic safety,
resulting in open and well-lighted areas. In places where the transportation corridor
is wider, intense mowing is applied only to the zone closest to traffic, while the
farthest zone is managed less intensively, leading to the creation of a
well-developed vegetation structure. These two zones of management regime
provide habitat not only for native species but also for many non-native organisms.
In Portugal, for example, many kilometers of railway and road verges are dominated by silver wattle (Acacia dealbata), one of the most widespread and damaging
invasive plants in the country (Sheppard et al. 2006; Vicente et al. 2011). This small
tree is native to Australia, and was introduced in Europe in the 1820s. In addition to
its great natural dispersal ability, silver wattle inhibits undergrowth species from
growing, due to allelopathy, i.e. the ability to produce biochemical agents that the
growth, survival, and reproduction of other organisms. Many other examples are
referenced in the literature on the presence and sometimes dominance of non-native
weedy species in transportation verges (Ernst 1998; Parendes and Jones 2000;
Tikka et al. 2001; Gelbard and Belnap 2003; Albrecht et al. 2011; McAvoy et al.
2012; Penone et al. 2012; Suárez-Esteban et al. 2016). Even maritime or wetland
species may spread their populations into inland areas along railway and road
verges, as found in Finland (Suominen 1970), England (Scott and Davison 1982),
or the USA (Wilcox 1989).
By facilitating dispersal, railways and roads may lead to homogeneous communities, sometimes dominated by invasive species. For example, Hansen and
Clevenger (2005) measured the frequency of several non-native plant species along
transects from 0 to 150 m from the edge of railways and highways in grasslands
and forests, as well as at control sites away from corridors. These authors found that
both transportation corridors had a higher frequency of non-native species than the
respective control sites. Also, grasslands had a higher frequency of non-native
species than forested habitats, but this frequency did not differ between the highways and the railways. Other studies have described a decline in the presence of
invasive species as a function of the distance to the transportation corridor (see
Gelbard and Belnap 2003). Interestingly, the penetration of non-native species in
areas adjacent to the transportation corridor is likely to vary according to the
dominant land cover. In the study mentioned above, Hansen and Clevenger (2005)
discovered that the frequency of non-native species in grasslands along railways
and highways was higher than at control sites up to 150 m from the corridor’s edge,
whereas in forested habitats the higher frequency of non-native species was only
evident up to 10 m away from the corridor’s edge. A similar result was found in
forested areas in the Chequamegon National Forest (USA), where invasive species
were most prevalent within 15 m of roads but were uncommon in the interior of the
forest (Watkins et al. 2003). Hence, it appears that the dispersal of non-native
5 Aliens on the Move: Transportation Networks and Non-native …
71
Vegetated verges generally border transportation infrastructures, in particular railways and roads. These verges are regularly mowed in order to ensure traffic safety,
resulting in open and well-lighted areas. In places where the transportation corridor
is wider, intense mowing is applied only to the zone closest to traffic, while the
farthest zone is managed less intensively, leading to the creation of a
well-developed vegetation structure. These two zones of management regime
provide habitat not only for native species but also for many non-native organisms.
In Portugal, for example, many kilometers of railway and road verges are dominated by silver wattle (Acacia dealbata), one of the most widespread and damaging
invasive plants in the country (Sheppard et al. 2006; Vicente et al. 2011). This small
tree is native to Australia, and was introduced in Europe in the 1820s. In addition to
its great natural dispersal ability, silver wattle inhibits undergrowth species from
growing, due to allelopathy, i.e. the ability to produce biochemical agents that the
growth, survival, and reproduction of other organisms. Many other examples are
referenced in the literature on the presence and sometimes dominance of non-native
weedy species in transportation verges (Ernst 1998; Parendes and Jones 2000;
Tikka et al. 2001; Gelbard and Belnap 2003; Albrecht et al. 2011; McAvoy et al.
2012; Penone et al. 2012; Suárez-Esteban et al. 2016). Even maritime or wetland
species may spread their populations into inland areas along railway and road
verges, as found in Finland (Suominen 1970), England (Scott and Davison 1982),
or the USA (Wilcox 1989).
By facilitating dispersal, railways and roads may lead to homogeneous communities, sometimes dominated by invasive species. For example, Hansen and
Clevenger (2005) measured the frequency of several non-native plant species along
transects from 0 to 150 m from the edge of railways and highways in grasslands
and forests, as well as at control sites away from corridors. These authors found that
both transportation corridors had a higher frequency of non-native species than the
respective control sites. Also, grasslands had a higher frequency of non-native
species than forested habitats, but this frequency did not differ between the highways and the railways. Other studies have described a decline in the presence of
invasive species as a function of the distance to the transportation corridor (see
Gelbard and Belnap 2003). Interestingly, the penetration of non-native species in
areas adjacent to the transportation corridor is likely to vary according to the
dominant land cover. In the study mentioned above, Hansen and Clevenger (2005)
discovered that the frequency of non-native species in grasslands along railways
and highways was higher than at control sites up to 150 m from the corridor’s edge,
whereas in forested habitats the higher frequency of non-native species was only
evident up to 10 m away from the corridor’s edge. A similar result was found in
forested areas in the Chequamegon National Forest (USA), where invasive species
were most prevalent within 15 m of roads but were uncommon in the interior of the
forest (Watkins et al. 2003). Hence, it appears that the dispersal of non-native
5 Aliens on the Move: Transportation Networks and Non-native …
71