Hedin and Ranius 2002). Radio-tracking can also provide detailed information, not
only on individual crossings, but also on the full territory use by animals relative to
the railway location, therefore allowing the impact of the linear infrastructure on the
movements of individuals to be estimated (Clevenger and Sawaya 2010).
Non-invasive genetic sampling (hereinafter “NGS”) methods have been used
mainly to measure the genetic sub-structure of a population bisected by a railway
(see below). However, NGS also enables individual identification based on
microsatellite analysis (Balkenhol and Waits 2009; Clevenger and Sawaya 2010;
Simmons et al. 2010), the so-called “fingerprinting” or “DNA profiling,” which is a
kind of capture-mark-recapture method. For instance, Clevenger and Sawaya
(2010) showed that the passive hair-collection methods based on barbed wire and/or
adhesive strings, followed by microsatellite analyses, was an effective technique for
monitoring wildlife pass use at an individual level for cougars (Puma concolor),
and black and brown bears in Canada. Furthermore, as costs for genetic analyses are
become lower, the sample sizes have increased in recent studies (Simmons et al.
2010). The main limitation of this method is that it is only suitable for large animals
whose remains (hairs, feathers, scats) can be found in the field in sufficient quantity
to extract DNA from them. In addition, fingerprinting requires a relatively high
number of microsatellites. If species-specific microsatellites have not been developed in the target species, they have to be specifically developed, increasing both
time and costs. However, microsatellites already developed for related species can
be tested, as sometimes they amplify the DNA from the target species as well.
Indirect Methods
(1) Census at both sides of a railway. This is the most simplistic approach, either to
assess population densities (e.g., Waterman et al. 2002; Li et al. 2010; Wiącek
et al. 2015), or to calculate diversity indices of community structures (e.g., Qian
et al. 2009). However, it is also the most limited approach to identifying causal
factors or population dynamic-related processes. Failure to control for potentially confounding variables makes that detected patterns cannot be clearly
associated with railway impact.
(2) Genetic-based assessment of functional connectivity. Even more important than
confirming crossing is to assessing the functional connectivity (or the barrier
effect)—that is, to detect whether individuals reproduce on both sides of the
railway. It is worth noting that moderate to low crossing rates may not necessarily imply functional connectivity (Riley et al. 2006). This type of information is logistically difficult to obtain using other than genetic methods
(Clevenger and Sawaya 2010; Simmons et al. 2010).
Because the impact of railways is relatively recent, highly variable markers
such as microsatellites are the most suitable method for estimating demographic
4 Railways as Barriers for Wildlife: Current Knowledge
51
only on individual crossings, but also on the full territory use by animals relative to
the railway location, therefore allowing the impact of the linear infrastructure on the
movements of individuals to be estimated (Clevenger and Sawaya 2010).
Non-invasive genetic sampling (hereinafter “NGS”) methods have been used
mainly to measure the genetic sub-structure of a population bisected by a railway
(see below). However, NGS also enables individual identification based on
microsatellite analysis (Balkenhol and Waits 2009; Clevenger and Sawaya 2010;
Simmons et al. 2010), the so-called “fingerprinting” or “DNA profiling,” which is a
kind of capture-mark-recapture method. For instance, Clevenger and Sawaya
(2010) showed that the passive hair-collection methods based on barbed wire and/or
adhesive strings, followed by microsatellite analyses, was an effective technique for
monitoring wildlife pass use at an individual level for cougars (Puma concolor),
and black and brown bears in Canada. Furthermore, as costs for genetic analyses are
become lower, the sample sizes have increased in recent studies (Simmons et al.
2010). The main limitation of this method is that it is only suitable for large animals
whose remains (hairs, feathers, scats) can be found in the field in sufficient quantity
to extract DNA from them. In addition, fingerprinting requires a relatively high
number of microsatellites. If species-specific microsatellites have not been developed in the target species, they have to be specifically developed, increasing both
time and costs. However, microsatellites already developed for related species can
be tested, as sometimes they amplify the DNA from the target species as well.
Indirect Methods
(1) Census at both sides of a railway. This is the most simplistic approach, either to
assess population densities (e.g., Waterman et al. 2002; Li et al. 2010; Wiącek
et al. 2015), or to calculate diversity indices of community structures (e.g., Qian
et al. 2009). However, it is also the most limited approach to identifying causal
factors or population dynamic-related processes. Failure to control for potentially confounding variables makes that detected patterns cannot be clearly
associated with railway impact.
(2) Genetic-based assessment of functional connectivity. Even more important than
confirming crossing is to assessing the functional connectivity (or the barrier
effect)—that is, to detect whether individuals reproduce on both sides of the
railway. It is worth noting that moderate to low crossing rates may not necessarily imply functional connectivity (Riley et al. 2006). This type of information is logistically difficult to obtain using other than genetic methods
(Clevenger and Sawaya 2010; Simmons et al. 2010).
Because the impact of railways is relatively recent, highly variable markers
such as microsatellites are the most suitable method for estimating demographic
4 Railways as Barriers for Wildlife: Current Knowledge
51