(3) adding drift fences, (4) avoiding culverts with steep slopes and large hydraulic
jumps or steps; and (5) placing culverts in dry habitats (i.e., dry drainage culverts)
and not only along streams. Flooded or very steep culverts may contribute little as
wildlife crossing structures. Modifying culverts for wildlife crossing can represent
one of the most economical measures to mitigate mortality on railways (Clevenger
and Waltho 1999).
Amphibian tunnels are sometimes used in roads to facilitate crossing in areas
where amphibians concentrate their movements (Glista et al. 2009), and they can be
easily adapted to railways. Similarly, some tunnels can be designed for reptile
crossing—reptile tunnels. Turtles and other small animals may often get caught
between the rails, and a simple measure to prevent their becoming trapped can be
the excavation of the rock ballast between pairs of railway sleepers, thus allowing
animals to cross below the tracks (Dorsey et al. 2015; Pelletier et al. 2006).
Wildlife underpasses facilitate animal movement under linear infrastructures,
and are generally located where railways cross watercourses and roads. However,
underpasses can be specifically designed to be used by animals (Glista et al. 2009;
Jackson and Griffin 2000). Therefore, underpasses show considerable differences in
size, and provide variable crossing facilities. Underpasses can be very large (i.e.,
viaducts, expanded bridges), where railways cross large watercourses and extensive
valleys, in which it is assumed that passage for wildlife is limited, or they can be
relatively small when, for example, they are only meant to allow the access of local
vehicles between agricultural fields. Small underpasses may provide limited use for
wildlife crossing if they are associated with roads with considerable traffic, or when
they are completely flooded by the watercourse.
Wildlife overpasses, also called “ecoducts,” are structures mainly designed for
large animal crossings, such as ungulates and large carnivores, and have often been
used as a mitigation measure in large highways (Jackson and Griffin 2000).
Overpasses with strips of natural vegetation are referred to as “green bridges,” while
the term “landscape connectors” is used for very wide overpasses designed to
maintain landscape connectivity (Forman et al. 2003). Overpasses are often less
confining than underpasses, facilitating the movement of a greater number of
species, and they maintain ambient conditions more easily throughout the year
(Glista et al. 2009; Jackson and Griffin 2000). The main drawback is their high cost
of construction.
Crossing structures designed specifically for wildlife use should take into
account the following characteristics to improve their effectiveness (Glista et al.
2009). However, as different species will favor different structure design, the best
options must be planned for each particular case:
1. Location. The place where crossing structures are implemented is probably the
single, most important factor for their effectiveness; thus, structures should be
implemented where animal movements are more likely (Ando 2003; Jackson
and Griffin 2000; Rodríguez et al. 1996; Yanes et al. 1995).
2. Dimensions. There is no reference size for passage structures, depending greatly
on the target species. However, passages should have a relatively large diameter
32
F. Carvalho et al.
jumps or steps; and (5) placing culverts in dry habitats (i.e., dry drainage culverts)
and not only along streams. Flooded or very steep culverts may contribute little as
wildlife crossing structures. Modifying culverts for wildlife crossing can represent
one of the most economical measures to mitigate mortality on railways (Clevenger
and Waltho 1999).
Amphibian tunnels are sometimes used in roads to facilitate crossing in areas
where amphibians concentrate their movements (Glista et al. 2009), and they can be
easily adapted to railways. Similarly, some tunnels can be designed for reptile
crossing—reptile tunnels. Turtles and other small animals may often get caught
between the rails, and a simple measure to prevent their becoming trapped can be
the excavation of the rock ballast between pairs of railway sleepers, thus allowing
animals to cross below the tracks (Dorsey et al. 2015; Pelletier et al. 2006).
Wildlife underpasses facilitate animal movement under linear infrastructures,
and are generally located where railways cross watercourses and roads. However,
underpasses can be specifically designed to be used by animals (Glista et al. 2009;
Jackson and Griffin 2000). Therefore, underpasses show considerable differences in
size, and provide variable crossing facilities. Underpasses can be very large (i.e.,
viaducts, expanded bridges), where railways cross large watercourses and extensive
valleys, in which it is assumed that passage for wildlife is limited, or they can be
relatively small when, for example, they are only meant to allow the access of local
vehicles between agricultural fields. Small underpasses may provide limited use for
wildlife crossing if they are associated with roads with considerable traffic, or when
they are completely flooded by the watercourse.
Wildlife overpasses, also called “ecoducts,” are structures mainly designed for
large animal crossings, such as ungulates and large carnivores, and have often been
used as a mitigation measure in large highways (Jackson and Griffin 2000).
Overpasses with strips of natural vegetation are referred to as “green bridges,” while
the term “landscape connectors” is used for very wide overpasses designed to
maintain landscape connectivity (Forman et al. 2003). Overpasses are often less
confining than underpasses, facilitating the movement of a greater number of
species, and they maintain ambient conditions more easily throughout the year
(Glista et al. 2009; Jackson and Griffin 2000). The main drawback is their high cost
of construction.
Crossing structures designed specifically for wildlife use should take into
account the following characteristics to improve their effectiveness (Glista et al.
2009). However, as different species will favor different structure design, the best
options must be planned for each particular case:
1. Location. The place where crossing structures are implemented is probably the
single, most important factor for their effectiveness; thus, structures should be
implemented where animal movements are more likely (Ando 2003; Jackson
and Griffin 2000; Rodríguez et al. 1996; Yanes et al. 1995).
2. Dimensions. There is no reference size for passage structures, depending greatly
on the target species. However, passages should have a relatively large diameter
32
F. Carvalho et al.