We also analysed the effects of urbanisation on the frequency of exotic and
invasive plants to test our hypothesis H2 (urbanisation effects should be attenuated
if communities are connected). We found the well-known positive urbanisation
effect on exotic and invasive species, i.e., increased species frequency in highly
urbanised areas (Penone et al. 2012). This result possibly suggests that the
urbanisation effect was not attenuated by railways connectivity and thus railways
did not provide additional functional connectivity for exotic and invasive species in
urban landscapes.
To further confirm these results we performed a genetic analysis on S. inaequidens in the same region (Blanchet et al. 2014). This perennial herb presents a
self-incompatible reproductive system and is pollinated by insects; the effective
fruit production is high, with 75% viable achenes, which are well dispersed by wind
and animals (Lopez-Garcia and Maillet 2005). This species was introduced in the
south of France (Mazamet) in 1936 and then observed in Gare d’Austerlitz (a
Parisian railway station) in 1990 (Henry and Maurin 1999). Consequently, we
hypothesised that if railway verges play a corridor role for S. inaequidens, genetic
diversity of this species should be structured along railways. We collected 450
individuals from 15 populations regularly on 3 different railway lines connecting
the centre of Paris to its periphery. Given that the species was first seen in Parisian
railway stations and assuming that railways act as corridors for S. inaequidens in the
Parisian region, we expected a significant genetic differentiation among the main
departure stations within Paris centre and among the three railway lines. We also
expected significant isolation by distance (i.e., positive correlation between genetic
distance and geographical distance) if the main central stations act as sources for
other populations in the surroundings.
Our results did not confirm these scenarios: despite a clear differentiation among
the three departure stations of the centre of Paris and a genetic cline along one of the
lines, the differentiation among lines was low. Similarly, we did not detect significant isolation by distance among populations within lines, supporting the
absence of population structure along railways (Blanchet et al. 2014). One explanation could lie in multiple sources of introduction of S. inaequidens from other
European regions (e.g. Belgium or Germany). This species has high dispersal
ability and is also present in other urban habitats (e.g. wastelands), which could also
contribute to the high gene flow detected. In sum, the impact of railways as corridors on the genetic structure of S. inaequidens was not confirmed in our study,
especially outside the centre of Paris where the deep interconnection with other
corridors such as motorways in the suburban matrix may have a significant impact
by reducing genetic differentiation among railway lines.
All the results that we put together converge and tend to suggest that in a highly
urbanized and anthropogenic region, railway verges provide functional connectivity
for exotic species but not for invasive species. This is inconsistent with many other
studies on other transport networks (roadsides, highways), and should then been
taken with care (but see Kalwij et al. 2008 for a similar result). Although railway
verges did not appear as clear corridors for invasive species, trains might constitute
an efficient transportation vector for seeds and contribute to their spread outside
270
J.-C. Vandevelde and C. Penone
invasive plants to test our hypothesis H2 (urbanisation effects should be attenuated
if communities are connected). We found the well-known positive urbanisation
effect on exotic and invasive species, i.e., increased species frequency in highly
urbanised areas (Penone et al. 2012). This result possibly suggests that the
urbanisation effect was not attenuated by railways connectivity and thus railways
did not provide additional functional connectivity for exotic and invasive species in
urban landscapes.
To further confirm these results we performed a genetic analysis on S. inaequidens in the same region (Blanchet et al. 2014). This perennial herb presents a
self-incompatible reproductive system and is pollinated by insects; the effective
fruit production is high, with 75% viable achenes, which are well dispersed by wind
and animals (Lopez-Garcia and Maillet 2005). This species was introduced in the
south of France (Mazamet) in 1936 and then observed in Gare d’Austerlitz (a
Parisian railway station) in 1990 (Henry and Maurin 1999). Consequently, we
hypothesised that if railway verges play a corridor role for S. inaequidens, genetic
diversity of this species should be structured along railways. We collected 450
individuals from 15 populations regularly on 3 different railway lines connecting
the centre of Paris to its periphery. Given that the species was first seen in Parisian
railway stations and assuming that railways act as corridors for S. inaequidens in the
Parisian region, we expected a significant genetic differentiation among the main
departure stations within Paris centre and among the three railway lines. We also
expected significant isolation by distance (i.e., positive correlation between genetic
distance and geographical distance) if the main central stations act as sources for
other populations in the surroundings.
Our results did not confirm these scenarios: despite a clear differentiation among
the three departure stations of the centre of Paris and a genetic cline along one of the
lines, the differentiation among lines was low. Similarly, we did not detect significant isolation by distance among populations within lines, supporting the
absence of population structure along railways (Blanchet et al. 2014). One explanation could lie in multiple sources of introduction of S. inaequidens from other
European regions (e.g. Belgium or Germany). This species has high dispersal
ability and is also present in other urban habitats (e.g. wastelands), which could also
contribute to the high gene flow detected. In sum, the impact of railways as corridors on the genetic structure of S. inaequidens was not confirmed in our study,
especially outside the centre of Paris where the deep interconnection with other
corridors such as motorways in the suburban matrix may have a significant impact
by reducing genetic differentiation among railway lines.
All the results that we put together converge and tend to suggest that in a highly
urbanized and anthropogenic region, railway verges provide functional connectivity
for exotic species but not for invasive species. This is inconsistent with many other
studies on other transport networks (roadsides, highways), and should then been
taken with care (but see Kalwij et al. 2008 for a similar result). Although railway
verges did not appear as clear corridors for invasive species, trains might constitute
an efficient transportation vector for seeds and contribute to their spread outside
270
J.-C. Vandevelde and C. Penone