ArticlePDF Available

Movement re-established but not restored: Inferring the effectiveness of road-crossing mitigation for a gliding mammal by monitoring use

Authors:

Abstract and Figures

Wildlife crossing structures are commonly used to mitigate the barrier and mortality impacts of roads on wildlife. For arboreal mammals, canopy bridges, glider poles and vegetated medians are used to provide safe passage across roads. However, the effectiveness of these measures is unknown. We investigate the effect of canopy bridges, glider poles and vegetated medians on squirrel glider movement across a freeway in south-east Australia. We monitored structures directly using motion-triggered cameras and passive integrated transponder (PIT) scanners. Further, post-mitigation radio-tracking was compared to a pre-mitigation study. Squirrel gliders used all structure types to cross the freeway, while the unmitigated freeway remained a barrier to movement. However, movement was not restored to the levels observed at non-freeway sites. Nevertheless, based on the number and frequency of individuals crossing, mitigation is likely to provide some level of functional connectivity. The rate of crossing increased over several years as animals habituated to the structure. We also found that crossing rate can be a misleading indicator of effectiveness if the number of individuals crossing is not identified. Therefore, studies should employ long-term monitoring and identify individuals crossing if inferences about population connectivity are to be made from movement data alone.
Content may be subject to copyright.
Page 1 of 28
Movement re-established but not restored: inferring the effectiveness of crossing
mitigation by monitoring use
Kylie Soanesa,b*, Melissa Carmody Loboa,b, Peter A. Veskb, Michael A. McCarthyb, Joslin L.
Moorea and Rodney van der Reea
a Australian Research Centre for Urban Ecology, Royal Botanic Gardens, Melbourne, VIC
3010, Australia
b School of Botany, University of Melbourne, VIC 3010, Australia
* corresponding author: Phone +61 (03) 8344 0146, Fax +61 (03) 9347 9123, e-mail:
k.soanes@pgrad.unimelb.edu.au (K. Soanes)
Keywords: arboreal mammals, squirrel glider, road mitigation, functional connectivity,
barrier effect, monitoring effort
Page 2 of 28
Abstract
Wildlife crossing structures are commonly used to mitigate the barrier and mortality
impacts of roads on wildlife. For arboreal mammals, canopy bridges, glider poles and
vegetated medians are used to provide safe passage across roads. However, the effectiveness
of these measures is unknown. We investigate the effect of canopy bridges, glider poles and
vegetated medians on squirrel glider movement across a freeway in south-east Australia. We
monitored structures directly using motion-triggered cameras and passive integrated
transponder (PIT) scanners. Further, post-mitigation radio-tracking was compared to a pre-
mitigation study. Squirrel gliders used all structure types to cross the freeway, while the
unmitigated freeway remained a barrier to movement. However, movement was not restored
to the levels observed at non-freeway sites. Nevertheless, based on the number and frequency
of individuals crossing, mitigation is likely to provide some level of functional connectivity.
The rate of crossing increased over several years as animals habituated to the structure, with
less than five crossings detected during the first 12 months of monitoring. We also found that
crossing rate can be a misleading indicator of effectiveness if the number of individuals
crossing is not identified. Therefore, studies should employ long-term monitoring and
identify individuals crossing if inferences about population connectivity are to be made from
movement data alone.
Page 3 of 28
1. Introduction
Roads and traffic threaten the persistence of wildlife populations by fragmenting
habitat, reducing gene flow and increasing mortality rates through roadkill (Bennett 1991;
Fahrig and Rytwinski 2009; Forman and Alexander 1998; Holderegger and Di Giulio 2010).
Wildlife crossing structures aim to mitigate these impacts by providing safe passage for
wildlife across roads, yet in most cases their effectiveness has not been evaluated (Clevenger
2005; Forman et al. 2003; van der Ree et al. 2007; van der Ree et al. 2009). The first step in
evaluating the effectiveness of crossing structures is to determine the frequency of crossing
by target species and the number of individuals that use the structure (van der Ree et al.
2007).
The monitoring method employed and survey duration are critical as they are likely
to affect the number of crossings detected and thus the perceived success of crossing
structures (Hardy et al. 2003; Mateus et al. 2011). Thorough evaluation of the effectiveness
of wildlife crossing structures is essential to ensure that successful measure are widely
adopted, and unsuccessful ones are not repeated. Short-term studies which monitor the use of
structures by wildlife without quantifying impacts of the road prior to mitigation, provide
only a limited assessment of the extent to which wildlife crossing structures can restore, or
maintain, connectivity (Hardy et al. 2003; van der Ree et al. 2007).
Arboreal mammals are highly susceptible to the mortality and barrier impacts created
by large roads as they are often unable, or unwilling, to cross large gaps in tree cover (e.g.
Asari et al. 2010; Goldingay and Taylor 2009; Laurance 1990; van der Ree et al. 2003). Road
agencies increasingly rely on mitigation such as canopy bridges, glider poles or vegetated
medians (retaining tall trees in the road median) to reduce the impacts on arboreal mammals,
particularly on threatened species. However research on the use of these measures by wildlife
Page 4 of 28
is limited to a few studies, including canopy bridges over rainforest roads (Weston et al.
2011), canopy bridges across a major freeway (Goldingay et al. 2012) and glider poles on
landbridges which cross major roads (Goldingay et al. 2011).
Radio-tracking of individuals has demonstrated that the Hume Freeway in south-east
Australia is a barrier to squirrel glider (Petaurus norfolcensis) movement, while vegetated
medians retained during construction facilitated road crossing (van der Ree et al. 2010).
Canopy bridges and glider poles have been built to mitigate the impacts on squirrel glider
movement, although the effectiveness of these measures is unknown. Here, we investigate the
effect of canopy bridges, glider poles and vegetated medians on squirrel glider movement
using remotely-triggered cameras, passive integrated transponder (PIT) scanners and post-
mitigation radio-tracking of individuals. We also explore the effects of survey duration and
monitoring method on detected crossing rates and how these factors influence the perceived
effectiveness of crossing structures.
2. Methods
2.1 Site and study species
We studied a 70 km section of the Hume Freeway between the rural towns of Avenel
(33o 42' S, 148o 176' E) and Benalla (36o 55' S, 145o 98' E) in south-east Australia (Figure 1).
This section was upgraded to a four-lane divided freeway during the 1970-80s with an
average width of 53 m (44 76 m) including a centre median (21 38 m wide). The average
traffic volume is 10 000 vehicles per day (speed limit 110 km/hr) 25 % of which occurs
between 10 pm and 5 am (VicRoads, unpub. data), when native mammal species are most
active. The surrounding landscape is predominantly cleared agricultural land with less than 5
% of the original (pre-European) tree cover remaining (Figure 1). The majority (83%) of
remnant box-gum wood land (Eucalyptus spp.) exists as a network of linear strips along
Page 5 of 28
roadsides and waterways (van der Ree 2002). Where linear strips are bisected by the freeway
mature trees occur 5 20 m from the road edge. During the freeway upgrade, vegetated
medians containing trees 20 30 m tall were retained at some sites reducing the gap in tree
cover across the road to <15 m. Sites without vegetated medians where the treeless gap
exceeds 50 m are referred to as 'unmitigated'. Linear remnants in this region contain a high
density of large, hollow-bearing trees providing critical habitat for the squirrel glider, a small
(~250 g), nocturnal gliding marsupial (family Petauridae) which is threatened in south-east
Australia (van der Ree 2002). A gliding membrane that extends from each wrist to each hind
leg allows individuals to glide from tree to tree. The average glide length is 20 35 m with a
maximum of approximately 70 m, depending on launch height (Goldingay and Taylor 2009;
van der Ree et al. 2003). Squirrel gliders very rarely move along the ground (Fleay 1947).
2.2 Crossing structures
In July 2007, approximately 20 30 years after the highway was upgraded, crossing
structures were installed at five sites where the treeless gap across the road exceeded 50 m:
Longwood (canopy bridge), Violet Town (canopy bridge), Balmattum (glider poles),
Baddaginnie (glider pole) and Warrenbayne (glider pole). Prior to mitigation, radio-tracking
at these sites detected no road crossings by squirrel gliders (van der Ree et al. 2010).
Structures were placed where a linear strip of remnant woodland (usually along a single-lane
rural road) intersected the freeway (Figure 1).
Each canopy bridge is approximately 70 m long and 0.5 m wide, constructed of UV
stabilised marine-grade rope in a flat lattice-work configuration (i.e. analogous to a rope
Page 6 of 28
ladder laid horizontally). The canopy bridges are suspended between two timber poles placed
near roadside habitat trees at a minimum height of 6 m above the road surface (Figure 2a).
Single strands of rope extend from the terminal ends of the structure into the adjacent tree
canopy to encourage access by arboreal mammals. Wooden glider poles act as surrogate trees
to reduce the gap in tree cover allowing the road to be crossed in several short glides (Figure
2b). A 2 m long cross-beam fixed 50 cm below the top of the pole provides a suitable launch
site. A single glider pole (12 14 m high, 40 50 cm diameter) was installed in the centre
median at each site (Balmattum, Baddaginnie and Warrenbayne) reducing the maximum
glide distance across the road to less than 35 m. A second pole was required in the road verge
at Balmattum due to the absence of tall, roadside trees.
2.3 Remote monitoring equipment
To detect arboreal mammals using the crossing structures we installed motion-
triggered digital cameras on all canopy bridges and glider poles (Olympus, Faunatech
Austbat, Pty Ltd). Cameras mounted to the supporting pole at each end of the canopy bridge
were triggered by the movement of animals past two active infra-red sensor beams on the
bridge (approximately one and four metres from the camera). At glider poles, cameras were
mounted to a bracket providing a view of the cross-beam, where glides would most likely
take place. Animals triggered the camera as they climbed past a set of sensor beams
circumnavigating the pole below the cross-beam. Once triggered, all cameras recorded a
series of images taken at three second intervals (five images at canopy bridges, nine images at
glider poles) providing a sequence of crossing behaviour. The time and date of each image
was also recorded. All camera systems were powered by a 12 V battery kept continuously
charged by a solar panel. We downloaded images approximately once a fortnight, at which
time we inspected the road and roadside within 100 m of each structure for dead squirrel
gliders that may indicate predation or roadkill.
Page 7 of 28
We monitored the canopy bridges from August 2007 May 2011 and the glider poles
from December 2009 March 2011. Monitoring at the canopy bridges began one month after
the structures were built. Glider poles were not monitored during the first 2.5 years as
cameras suitable for long-term installation had to be designed and custom built. Due to false
triggers (e.g. heavy rain, insects or debris) and equipment failure, data collection was not
continuous throughout this period and the total monitoring effort varied at each structure. Of
1388 possible monitoring nights, cameras at the Longwood and Violet Town canopy bridges
were operational on 56.7 % (787) and 62.9 % (873) of nights respectively. Monitoring effort
at the glider poles was much lower. Of a possible 438 monitoring nights, cameras were
operational on 19.8% (87) of nights at Balmattum, 8.4 % (37) of nights at Baddaginnie, and
5.0 % (22) of nights at Warrenbayne.
To investigate rate of use by individuals we trialled PIT scanning equipment at the
Longwood canopy bridge. A single flatbed antenna connected to a decoder unit was installed
at one end of the bridge (Trovan ANT-612 antenna and LID650 decoder, Microchips
Australia, Pty Ltd). Tagged animals are detected as they pass over the antenna, and the time
and date of crossings are recorded. The system was powered by a 12 V battery connected to a
solar panel. The scanner was operational for 46 nights between November 2010 and April
2011.
2.4 Radio-tracking
Pre-mitigation radio-tracking was conducted along the freeway at unmitigated (n = 3)
and vegetated median (n = 3) sites from December 2005 May 2006 (see van der Ree et al.
2010). The pre-mitigation study also included control sites (n = 2) located over 6 km away
from the freeway, where squirrel gliders had to cross single lane, low traffic-volume roads
(less than 10 m wide, ~100 vehicles per day). Habitat quality and configuration were similar
Page 8 of 28
at all site types. We replicated the pre-mitigation radio-tracking to determine the impact of
mitigation on squirrel glider movement. Post-mitigation radio-tracking was conducted at
vegetated median, canopy bridge, glider pole, unmitigated freeway and control sites (Figure
1, Table 1). All sites that were unmitigated during the 2005/06 survey had crossing structures
installed in 2007, so we included two additional unmitigated sites in this study. None of the
individuals collared in the pre-mitigation study were also collared in the post-mitigation
study. There was no difference in traffic volume pre- and post-mitigation.
Post-mitigation, squirrel gliders were trapped at 11 sites during 2575 trap nights
between November 2010 and March 2011 (using identical methods to van der Ree et al.
2010). The trapping effort varied between each site depending on the time taken to capture
and collar a sufficient number of animals. Wire-mesh cage traps (17 cm x 20 cm x 50cm)
were baited with a mixture of rolled oats, peanut butter and honey, and nailed to tree trunks at
a height of 2 4 m. Trapping transects extended along linear woodland strips intersecting the
freeway or local, low traffic volume roads (control sites). Traps were set on both sides of the
road intersection at approximately 50 m intervals, beginning at the road edge and extending
up to 250 m away.
Resident adults at each site were fitted with single-stage tuned-loop radio-collars (150
MHz; Sirtrack, New Zealand) weighing less than 5% of body weight (Table 1). To reduce the
chance that a collared animal had to cross through opposing home ranges to access the
crossing structure, only animals captured within 250 m of the road were collared. Radio-
tracking was undertaken on foot using a Regal 2000 receiver and a Yagi 3-element antenna
(Titley Electronics, Australia) using the same methods as the pre-mitigation study (van der
Ree et al. 2010). In brief, we collected homing fixes (the actual location of the animal to an
accuracy of ±10 m) as well as directional fixes from the road edge to determine which side of
the road the glider was on.
Page 9 of 28
2.5 Data analysis
2.5.1 Camera monitoring of crossing rates
The crossing rate at canopy bridges and glider poles was calculated by dividing the
number of crossings that occurred by the number of nights that the camera was operational.
Placing a camera at each end of the canopy bridge allowed us to distinguish crossings from
non-crossings by confirming that the animals passed both cameras. When a second camera
was not functioning we inferred crossings based on the animal behaviour and direction of
travel. We could not confirm crossings at glider poles as it was not possible to observe which
side of the road an animal originated from or travelled to. Therefore, we may have
overestimated the number of crossings recorded at glider poles if a large number of animals
glided to the centre pole but returned to their original side without crossing.
We expected the crossing rate to increase over time and approach some asymptote as
animals habituated to the structure. Therefore, we assumed the crossing rate t years after the
crossing structure was installed at site i followed a logistic function of the form:
xi,t = Ki/(1+ exp(−(abt))),
where a and b are coefficients that define how the crossing rate changes over time (b is
constrained to be positive) and Ki is the asymptotic crossing rate, which is site specific. We
assumed a and b were common for all sites because we had insufficient data to estimate these
parameters separately for each site. We felt that K was more likely to differ among sites,
providing a better indication of structure effectiveness and there was no prior information to
suggest how a or b would vary among sites.
The data used to estimate the model parameters were the number of crossings
observed within each of 59 time periods during which the cameras were operating. These
Page 10 of 28
periods varied in length. Let t1 be the time at the start of a period, and t2 be the time at the end
of that period. Then the expected number of crossings within that period is given by the
integral
i,t1,t2 =
2
1,
t
tti dtx
 
beeK btabta
i/)1log()1log( 12
,
We modelled the actual number of crossings at site i in the time interval [t1, t2] as a sample
from a Poisson distribution with parameter λ i,t1,t2.
We used Bayesian inference to fit the model in Open BUGS 3.2.1 (Spiegelhalter et al.
2011). To reflect the lack of prior information we selected vague prior distributions for all
parameters, using a normal distribution with mean 0 and standard deviation 1000 for each of
Ki and a, and a uniform distribution in the interval 0 to 100 for b. We included a prediction
contrast comparing the post-habituation crossing rate of canopy bridge sites to glider pole
sites to investigate the influence of structure type on crossing rate. Code is supplied as
Appendix A. The model was run for 100 000 iterations after discarding a burn in of 50 000 at
which time we were satisfied that the model had reached convergence.
2.5.1 Radio-tracking movements BACI
We conducted post-mitigation radio-tracking between November 2010 and May 2011,
collecting 1335 fixes from 42 squirrel gliders (18 females and 24 males) that were used in
subsequent analysis (Table 1). The pre-mitigation dataset included 1993 fixes from 47
squirrel gliders (23 females, 24 males). A crossing was recorded when two consecutive fixes
(homing or directional) for an individual were obtained on opposite sides of the road (i.e. all
four lanes and median were crossed). In contrast to van der Ree et al. (2010), we did not
Page 11 of 28
include partial crossings (where an individual moved to the centre median and then returned
to side of origin), as we were primarily interested in complete crossings of the road barrier.
The proportion of individuals crossing at each treatment was calculated and compared with
the results from the pre-mitigation study.
We used a logistic regression to model the probability that a squirrel glider crossed
the road as a function of the treatment and survey period (pre- or post-mitigation). There were
insufficient data to fit an effect of sex, or investigate differences between the two crossing
structure types (i.e. canopy bridges and glider poles were pooled). A Bernoulli distribution
with parameter p was drawn to model whether a squirrel glider, i, crossed or not:
logit(pi) = logit(pp) + bt(ti) + bp(pi) + int(ti, pi)
where logit(pp) is the intercept, bt(ti) is the effect of treatment, bp(pi) is the effect of period,
and int(ti, pi) is the interaction between periods for the each treatment. The categorical
variables bt(ti),and bp(pi) were modelled using a reference class, set arbitrarily to zero for
control sites and the pre-mitigation period. We were unable to fit a random effect for site due
to insufficient data. We used Bayesian inference to fit the model (Open BUGS 3.2.1) using
uninformative priors. The prior for parameter pp was a uniform distribution [0,1]. The priors
for parameters bt(ti), bp(pi) and int(ti, pi) (excluding interactions with parameters set to their
reference state, which were set to zero) were normal distributions (mean 0, standard deviation
100). Code is supplied as Appendix B. The model was run for 100 000 iterations after
discarding a burn in of 50 000, at which time we were satisfied the model had reached
convergence.
3. Results
3.1 Camera monitoring of crossing structures
Page 12 of 28
Cameras detected squirrel gliders at all five crossing structures with 1187 crossings
from 1660 functioning camera nights at canopy bridges and 13 crossings from 146
functioning camera nights at glider poles (Figure 2). No signs of predation or mortality were
observed during regular site surveys and no owls or other potential predators were recorded
on or near the structures. Squirrel gliders were first detected crossing the Violet Town canopy
bridge after eight months of monitoring (i.e. nine months after the structure was installed),
while no crossings were detected at the Longwood canopy bridge during the first 13 months
of monitoring. It was not possible to determine the date of first use at the glider poles as
monitoring of these sites did not begin immediately after mitigation.
The statistical model predicted an increase in squirrel glider crossings over time at all
sites before reaching a maximum, or post-habituation, rate (Figure 3). The median post-
habituation crossing rates were highest at the Longwood canopy bridge (2.47, 95% credible
interval 2.27 2.72) and Warrenbayne glider pole (0.35, 95% credible interval 0.16 0.65),
with less than 0.10 crossings per night at all other sites (Figure 3). Crossing rates were
slightly higher at canopy bridges than the glider poles, with the prediction contrast indicating
a median of 1.07 as many crossings per night at canopy bridges, (95% credible interval, 0.93
1.23). However, this was primarily driven by the high crossing rate at the Longwood
canopy bridge.
3.2 Monitoring using PIT scanners
The PIT scanner recorded crossings by three out of the six tagged squirrel gliders
known to be present within 600 m of the Longwood canopy bridge. One adult female
carrying two pouch young, one adult male, and one young male (recently independent) were
detected crossing 63 times over 46 nights. The female and older male were also radio-
collared, and tracking records show that they share a nest site within 200 m of the canopy
Page 13 of 28
bridge. The average crossing rate for each individual ranged from 0.39 0.76 crossings per
night during the six month period.
3.3 Radio-tracking before-after-control-impact (BACI)
The proportion of squirrel gliders crossing a road during the post-mitigation radio-
tracking study was highest at control sites (70%), with less than 50% of individuals crossing
at any mitigated highway site and no squirrel gliders crossing the unmitigated highway
(Table 1). This is reflected in the logistic regression model, which shows that the probability
of squirrel gliders crossing a road was higher at control sites than at any type of freeway site
(Figure 4). Installing crossing structures (canopy bridges and glider poles) along the freeway
increased the probability of crossing to a similar level as vegetated medians, while the
probability of crossing the unmitigated freeway remained very low during both periods
(Figure 4). The uncertainty around the parameter estimates was broad because less than 50
squirrel gliders could be captured and collared during both the pre- and post-mitigation
surveys.
3.4 Crossing detectability of different methods
From November 2010 May 2011 both canopy bridges and two glider poles were
monitored using cameras and radio-tracking, and a PIT scanner was also operational at one
canopy bridge (Table 2). Radio-tracking underestimated the crossing rate at all sites except
for Violet Town, where the crossing rate was very low. Two individuals at the Longwood
canopy bridge were both PIT-tagged and radio-collared and the PIT scanner detected a higher
crossing rate than radio-tracking for both animals (Table 2).
4. Discussion
4.1 Movement re-established but not restored
Page 14 of 28
Canopy bridges and glider poles can re-establish squirrel glider movement across a
major road. We detected squirrel gliders using all five crossing structures installed at
locations where the Hume Freeway was previously a barrier to movement (van der Ree et al.
2010). Uptake of the crossing structures was rapid considering the freeway has potentially
been a barrier for approximately 30 years. Within four years of their installation, the
probability of a squirrel glider crossing the freeway using crossing structures was similar to
that at vegetated medians which have been present since the freeway was upgraded. All
mitigation measures improved crossing by squirrel gliders relative to unmitigated sites, which
remained a barrier to movement.
Despite the increase in freeway crossing, no mitigation strategy restored movement to
the levels observed at control sites. This suggests that the gap in tree cover is not the only
factor influencing road crossing by squirrel gliders. Squirrel gliders at control sites readily
crossed low traffic volume roads with very little nocturnal traffic, while freeway sites have
approximately 2500 vehicles per night, which may create enough noise and light disturbance
to reduce crossing. Noise and traffic volume were found to reduce the use of crossing
structures by other species (Clevenger et al. 2001; Olsson et al. 2008) and it may be that
crossing structures cannot completely mitigate road impacts where the target species is
vulnerable to traffic disturbance.
While movement across the freeway was not fully restored, if mitigation increases
gene flow and reduces roadkill then it is likely to improve the viability of roadside
populations. Previous mark-recapture research found that the survival rate of squirrel glider
populations living adjacent to the Hume Freeway is 60% lower than at control sites,
suggesting that any reduction in roadkill as a result of mitigation would be beneficial (McCall
et al. 2010). Furthermore, a population viability analysis completed for the greater glider
(Petauroides volans) found that even low dispersal rates would prevent the extinction of sub-
Page 15 of 28
populations separated by a road (Taylor and Goldingay 2009). We detected multiple
individuals of both sexes and all ages using crossing structures, therefore it is likely that some
level of functional connectivity is provided (Bissonette and Adair 2008; Clevenger 2005;
Vucetich and Waite 2000). The next step is to determine what proportion of road crossing
resulted in gene flow, and if roadside populations are now viable as a result of mitigation
(Clevenger 2005; Corlatti et al. 2009; Riley et al. 2006; van der Ree et al. 2007).
4.2 Monitoring method influences the detection of crossings
Short sampling windows can lead to inaccurate conclusions about the effectiveness of
mitigation (Clevenger 2005; Gagnon et al. 2011). We found that the crossing rate for squirrel
gliders increased over time as animals habituated to the structures over several years of
monitoring. For example, if we had stopped monitoring the canopy bridges after 12 months
we would have detected only three crossings at one site, despite almost continuous camera
monitoring during that period. Based on that evidence it would be hard to argue that canopy
bridges were an effective form of mitigation. Most species show an adaption to crossing
structures over time, with some taking up to a decade to habituate (Bond and Jones 2008;
Clevenger and Waltho 2003; Gagnon et al. 2011; Olsson et al. 2008; Weston et al. 2011).
Long-term monitoring ensures that animals have had time to habituate to the structure and
increases the chance of detecting infrequent dispersal movements (Bissonette and Adair
2008; Corlatti et al. 2009; Hardy et al. 2003).
Crossing rate is often used as an indicator of crossing structure effectiveness, yet we
found that monitoring crossing rate alone can be misleading. Though the crossing rate for
squirrel gliders at the Longwood canopy bridge was much higher than any other structure, the
PIT scanner revealed crossings were made by only three out of six tagged individuals known
to be present within 600 m of the structure. Squirrel gliders actively defend their territory
Page 16 of 28
from members of neighbouring social groups and there is little overlap of home ranges of
animals within linear strips (van der Ree and Bennett 2003). Radio-tracking and mark-
recapture surveys suggest that the three animals using the canopy bridge belong to the same
social group and incorporate the structure as part of their territory, crossing regularly to
access resources on both sides of the road. While this is a positive outcome for those
individuals, if a structure benefits only a select few it is unlikely to improve connectivity and
survival for the whole population and therefore unlikely to be effective despite a high
observed crossing rate (Corlatti et al. 2009; Riley et al. 2006; Simmons et al. 2010). The
social organisation and territorial behaviour of target species should be considered when
evaluating the effectiveness of crossing structures as it is likely to influence the number of
individuals able to access a structure.
Similarly, low crossing rates do not necessarily mean a structure is ineffective. Many
studies relate crossing rates to the presence of roadside habitat, local population abundance or
crossing structure design (Cain et al. 2003; Clevenger and Waltho 2000; Ng et al. 2004). In
our study there was no difference in these factors that could explain why some structures had
comparatively lower crossing rates. The site with the lowest crossing rate, Violet Town, had
the highest population density (unpub. data) and five individuals regularly located within 50
m of the structure were never detected crossing (despite high camera monitoring effort).
Where roadside habitat already provides adequate resources, individuals are unlikely to
depend on the crossing structure for daily movements. In these cases the structure may only
be used infrequently for dispersal or re-colonisation and can still be effective despite a low
observed crossing rate.
We found that radio-tracking detected fewer crossings and fewer individuals than
directly monitoring the structure using cameras or PIT scanners. This is not surprising, as it is
rarely possible to collar and continuously monitor all animals likely to encounter the
Page 17 of 28
structure. Furthermore, BACI radio-tracking studies require an intensive field effort and large
sample sizes that may not be feasible when working with rare species. Cameras and PIT
scanners can provide continuous, long-term monitoring of structures, recording the timing
and direction of crossings, the frequency at which they occur, and the identity and
demographic characteristics of the individuals crossing (Ford et al. 2009; Mateus et al. 2011;
Olsson et al. 2008). Combining these techniques with non-invasive genetic sampling could
allow stronger inferences about the effectiveness of crossing structures to be made in the
absence of intensive population monitoring (Clevenger and Sawaya 2010; Simmons et al.
2010).
4.3 Conclusion
This study shows that canopy bridges and glider poles can rapidly re-establish the
movement of squirrel gliders across a road barrier. Based on the number of individuals and
frequency of crossings, it is likely that canopy bridges, glider poles and vegetated medians
provide some level of functional connectivity for squirrel gliders. However, the impact of the
freeway on movement was only partially mitigated relative to non-freeway sites, suggesting
other factors such as traffic disturbance may influence crossing behaviour. Long-term studies
which identify the number of individuals using a structure and their demographic
characteristics are essential when inferring the impacts of mitigation on connectivity in the
absence of population data (e.g. Clevenger and Waltho 2003; Gagnon et al. 2011). Our work
suggests that monitoring periods of at least two years are required to allow squirrel gliders
adequate time to habituate to retrofitted crossing structures. Longer-term research is required
to determine if the current crossing rates at canopy bridges, glider poles and vegetated
medians are enough to restore gene flow and improve survival rates in roadside populations.
Page 18 of 28
Acknowledgements
We thank the Baker Foundation, The Australian Research Centre for Urban Ecology,
the Australian Research Council (grant LP0560443), The Australian Research Council Centre
of Excellence for Environmental Decisions, the Holsworth Wildlife Research Endowment
(ANZ Trustees Foundation), VicRoads and the New South Wales Roads and Maritime
Services for support. Andrea Taylor and Paul Sunnucks made valuable contributions to the
development of this project. Thanks to Doug Black for generously loaning us the PIT
scanning equipment and Ross Meggs for his technical assistance. All animals were trapped
under the University of Melbourne Animal Ethics Committee permit (0810924.3) and the
Department of Sustainability and Environment wildlife research permit (10004763). W.
Sowersby, S. Harvey, E. Hynes, R. Soanes, P. Zambrano and R. Bull provided help with data
collection. Thanks to Tony Clevenger and an anonymous reviewer for their constructive
comments on an earlier version of this manuscript.
Page 19 of 28
REFERENCES
Asari, Y., Johnson, C.N., Parsons, M., Larson, J., 2010. Gap-crossing in fragmented habitats
by mahogany gliders (Petaurus gracilis). Do they cross roads and powerline corridors?
Australian Mammalogy 32, 10-15.
Bennett, A.F., 1991. Roads, roadsides and wildlife conservation: a review, In Nature
Conservation 2: The role of corridors. eds D.A. Saunders, J.H. Hobbs. Surrey Beatty
Chipping Norton, NSW.
Bissonette, J.A., Adair, W., 2008. Restoring habitat permeability to roaded landscapes with
isometrically-scaled wildlife crossings. Biological Conservation 141, 482-488.
Bond, A.R., Jones, D.N., 2008. Temporal trends in use of fauna-friendly underpasses and
overpasses. Wildlife Research 35, 103-112.
Cain, A.T., Tuovila, V.R., Hewitt, D.G., Tewes, M.E., 2003. Effects of a highway and
mitigation projects on bobcats in Southern Texas. Biological Conservation 114, 189-197.
Clevenger, A.P., 2005. Conservation value of wildlife crossings: Measures of performance
and research directions. Gaia-Ecological Perspectives for Science and Society 14, 124-129.
Clevenger, A.P., Chruszcz, B., Gunson, K., 2001. Drainage culverts as habitat linkages and
factors affecting passage by mammals. Journal of Applied Ecology 38, 1340-1349.
Clevenger, A.P., Sawaya, M.A., 2010. Piloting a non-invasive genetic sampling method for
evaluating population-level benefits of wildlife crossing structures. Ecology and Society 15.
Clevenger, A.P., Waltho, N., 2000. Factors influencing the effectiveness of wildlife
underpasses in Banff National Park, Alberta, Canada. Conservation Biology 14, 47-56.
Clevenger, A.P., Waltho, N., 2003. Long-term, year-round monitoring of wildlife crossing
structures and the importance of temporal and spatial variability in performance studies, I In
2003 International Conference on Ecology and Transportation. eds C.L. Irwin, P. Garrett,
K.P. McDermott, pp. 239-302, North Carolina State University, Raleigh, NC.
Corlatti, L., Hacklander, K., Frey-Roos, F., 2009. Ability of wildlife overpasses to provide
connectivity and prevent genetic isolation. Conservation Biology 23, 548-556.
DSE (2011) The State of Victoria, Department of Sustainability and Environment 'Road
Network 1:25,000 - Vicmap Transport (TR-ROAD/)'.
Fahrig, L., Rytwinski, T., 2009. Effects of roads on animal abundance: an empirical review
and synthesis. Ecology and Society 14.
Fleay, D., 1947. Gliders of the gum trees. Bread and Cheese Club, Melbourne.
Ford, A.T., Clevenger, A.P., Bennett, A., 2009. Comparison of methods of monitoring
wildlife crossing-structures on highways. Journal of Wildlife Management 73, 1213-1222.
Page 20 of 28
Forman, R.T.T., Alexander, L.E., 1998. Roads and their major ecological effects. Annual
Review of Ecology and Systematics 29, 207-231.
Forman, R.T.T., Sperling, D., Bissonette, J.A., Clevenger, A.P., Cutshall, C.D., Dale, V.H.,
Fahrig, L., France, R., Goldman, C.R., Heanue, K., Jones, J.A., Swanson, F.J., Turrentine, T.,
Winter, T.C., 2003. Road Ecology: Science and Solutions. Island Press, Washington.
Gagnon, J.W., Dodd, N.L., Ogren, K.S., Schweinsburg, R.E., 2011. Factors associated with
use of wildlife underpasses and importance of long-term monitoring. Journal of Wildlife
Management 75, 1477-1487.
Goldingay, R.L., Rohweder, D., Taylor, B.D., 2012. Will arboreal mammals use rope-bridges
across a highway in eastern Australia? Australian Mammalogy,
http://dx.doi.org/10.1071/AM12006.
Goldingay, R.L., Taylor, B.D., 2009. Gliding performance and its relevance to gap crossing
by the squirrel glider (Petaurus norfolcensis). Australian Journal of Zoology 57, 99-104.
Goldingay, R.L., Taylor, B.D., Ball, T., 2011. Wooden poles can provide habitat connectivity
for a gliding mammal. Australian Mammalogy 33, 36-43.
Hardy, A., Clevenger, A.P., Huijser, M., Neale, G., 2003. An overview of methods and
approaches for evaluating the effectiveness of wildlife crossing structures: emphasizing the
science in applied science, In 2003 International Conference on Ecology and Transportation.
eds C.L. Irwin, P. Garrett, K.P. McDermott, pp. 319-330, North Carolina State University,
Raleigh, NC.
Holderegger, R., Di Giulio, M., 2010. The genetic effects of roads: A review of empirical
evidence. Basic and Applied Ecology 11, 522-531.
Laurance, W.F., 1990. Comparative responses of five arboreal marsupials to tropical forest
fragmentation. Journal of Mammalogy 71, 641-653.
Mateus, A.R.A., Grilo, C., Santos-Reis, M., 2011. Surveying drainage culvert use by
carnivores: sampling design and cost-benefit analyzes of track-pads vs. video-surveillance
methods. Environmental Monitoring and Assessment 181, 101-109.
McCall, S.C., McCarthy, M.A., van der Ree, R., Harper, M.J., Cesarini, S., Soanes, K., 2010.
Evidence that a highway reduces apparent survival rates of squirrel gliders. Ecology and
Society 15.
Ng, S.J., Dole, J.W., Sauvajot, R.M., Riley, S.P.D., Valone, T.J., 2004. Use of highway
undercrossings by wildlife in southern California. Biological Conservation 115, 499-507.
Olsson, M.P.O., Widen, P., Larkin, J.L., 2008. Effectiveness of a highway overpass to
promote landscape connectivity and movement of moose and roe deer in Sweden. Landscape
and Urban Planning 85, 133-139.
Riley, S.P.D., Pollinger, J.P., Sauvajot, R.M., York, E.C., Bromley, C., Fuller, T.K., Wayne,
R.K., 2006. A southern California freeway is a physical and social barrier to gene flow in
carnivores. Molecular Ecology 15, 1733-1741.
Page 21 of 28
Simmons, J.M., Sunnucks, P., Taylor, A.C., van der Ree, R., 2010. Beyond roadkill,
radiotracking, recapture and F-ST-a review of some genetic methods to improve
understanding of the influence of roads on wildlife. Ecology and Society 15.
Spiegelhalter, D., Thomas, A., Best, N., Lunn, D., 2011. Open BUGS User Manual, Version
3.2.1. MRC Biostatistics Unit, Cambridge, UK.
Taylor, B.D., Goldingay, R.L., 2009. Can road-crossing structures improve population
viability of an urban gliding mammal? Ecology and Society 14.
van der Ree, R., 2002. The population ecology of the squirrel glider (Petaurus norfolcensis)
within a network of remnant linear habitats. Wildlife Research 29, 329-340.
van der Ree, R., Bennett, A.E., 2003. Home range of the squirrel glider (Petaurus
norfolcensis) in a network of remnant linear habitats. Journal of Zoology 259, 327-336.
van der Ree, R., Bennett, A.F., Gilmore, D.C., 2003. Gap-crossing by gliding marsupials:
thresholds for use of isolated woodland patches in an agricultural landscape. Biological
Conservation 115, 241-249
van der Ree, R., Cesarini, S., Sunnucks, P., Moore, J.L., Taylor, A., 2010. Large gaps in
canopy reduce road crossing by a gliding mammal. Ecology and Society 15.
van der Ree, R., Gulle, N., Holland, K., van der grift, E., Mata, C., Suarez, F., 2007.
Overcoming the barrier effect of roads - how effective are mitigation strategies?, In
International Conference on Ecology and Transportation. eds C.L. Irwin, D. Nelson, K.P.
McDermott, pp. 423-431, Centre of Transportation and The Environment, North Carolina
State University, Raleigh, North Carolina and Little Rock, Akansas, USA.
van der Ree, R., Heinze, D., McCarthy, M., Mansergh, I., 2009. Wildlife tunnel enhances
population viability. Ecology and Society 14.
Vucetich, J.A., Waite, T.A., 2000. Is one migrant per generation sufficient for the genetic
management of fluctuating populations? Animal Conservation 3, 261-266.
Weston, N., Goosem, M., Marsh, H., Cohen, M., Wilson, R., 2011. Using canopy bridges to
link habitat for arboreal mammals: successful trials in the Wet Tropics of Queensland.
Australian Mammology 33, 93-105.
Page 22 of 28
Figure captions
Figure 1. Map of the study area surrounding the Hume Freeway in south-east Australia
showing the location of crossing structures and pre- and post-mitigation radio-tracking sites
(DSE 2011).
Figure 2. Canopy bridge (a,c) and glider pole (b,d) installed along the Hume Freeway in
south-east Australia.
Figure 3. The predicted change in the median crossing rate (number of crossings per night)
of squirrel gliders over time since installation of the Longwood canopy bridge (diamonds)
and Warrenbayne glider pole (squares) with 95% credible intervals indicated by the dotted
line. The post-habituation median crossing rate at the Violet Town canopy bridge (cross) and
the Balmattum and Baddaginnie glider poles (triangle and circle, respectively) is also shown
(error bars indicate 95% credible intervals).
Figure 4. The mean predicted probability of radio-collared squirrel gliders crossing the road
at each treatment type pre-mitigation (closed circles) and post-mitigation (open circles).
Canopy bridges and glider poles were grouped as 'crossing structure'. It was assumed that the
likelihood of individuals crossing at a crossing structure prior to mitigation was the same as
at unmitigated sites. Error bars are 95% credible intervals.
Page 23 of 28
Figure 1
Page 24 of 28
Figure 2
Page 25 of 28
Table 1. Summary of post-mitigation radio-tracking effort of 42 squirrel gliders along the
Hume Freeway and control sites in south-east Australia.
Treatment and sex
No. of
individuals
tracked
Mean no. of
tracking
nightsa
Mean no.
of fixesa
No. of
individuals
crossing
Control (n = 3)
Male
5
23.8 ± 2.6
31.0 ± 2.0
4
Female
5
19.2 ± 2.8
27.8 ± 2.9
3
Freeway, vegetated median
(n = 2)
Male
3
28.3 ± 0.3
30.7 ± 0.7
1
Female
3
28.3 ± 0.9
30.3 ± 0.9
1
Unmitigated (n = 2)
Male
8
29.1 ± 2.2
34.9 ± 1.7
0
Female
4
16.3 ± 0.5
30.3 ± 1.5
0
Canopy bridge (n = 2)
Male
3
29.0 ± 2.1
34.0 ± 2.6
1
Female
4
19.8 ± 3.4
31.8 ± 1.3
2
Glider pole (n = 2)
Male
5
28.2 ± 2.8
33.8 ± 1.7
1
Female
2
13.5 ± 2.5
30.0 ± 4.0
0
a Where values are averages, ± 1 SE is shown.
Page 26 of 28
Figure 3
0
250
500
750
1000
1250
1500
Predicted median crossing rate
Number of days since crossing structure installed
Page 27 of 28
Figure 4
0
0.2
0.4
0.6
0.8
1
Predicted probability of road crossing
Control
Vegetated
median Crossing
structures Unmitigated
Freeway
Page 28 of 28
Table 2. Comparison of crossing rate of squirrel gliders detected by motion-triggered camera,
PIT scanner and radio-tracking between November 2010 and May 2011.
Structure
type
Camera
Radio-tracking
PIT scanner
Longwood
Canopy bridge
2.66
0.40
1.37
Female
-
0.31
0.76
Male
-
0.32
0.39
Violet Town
Canopy bridge
0.02
0.021
-
Warrenbayne
Glider pole
0.38
0.23
-
Balmattum
Glider pole
0.00
0.00
-
... Despite recent arboreal bridge studies from Peru (Gregory et al., 2017) and China (Chan et al., 2020), most arboreal wildlife bridge studies are from Australia, with multiple projects trialling a variety of designs throughout different regions (Abson, and Lawrence, 2003;Weston, 2003;Taylor and Goldingay, 2009). Collectively, over 10 arboreal bridge designs and variants have been trialled and tested including single ropes, rope tunnels (with and without square cross-sections), rope ladders, rope bridges with glider pole intervals, rope and mesh combination bridges, and woven rope bridges (Goosem et al., 2005;Taylor and Goldingay, 2009;Soanes and van der Ree, 2010;Soanes et al., 2013;Soanes et al., 2015;Goldingay and Taylor, 2017). This has allowed for effective designs and techniques across different regions and various species in Australia to be increasingly well-defined and have influence on projects in other countries, such as South Africa (Linden et al., 2020), the United Kingdom (White and Hughes, 2019), and Japan (Minato et al., 2012). ...
... Although more costly, the extended durability of ladder might pay off for these more sensitive species that require longer acclimatization. Yet, efficient (cheap and rapid) bridge designs such as the single-rope and double-rope could be used to identify high traffic wildlife pathways, followed by the installation of more robust permanent infrastructure in these prior identified hotspots (Soanes et al., 2013). ...
Article
Full-text available
Linear infrastructures, especially roads, affect the integrity of natural habitats worldwide. Roads act as a barrier to animal movement, cause mortality, decrease gene flow and increase the probability of local extinctions, particularly for arboreal species. Arboreal wildlife bridges increase connectivity of fragmented forests by allowing wildlife to safely traverse roads. However, the majority of studies about such infrastructure are from Australia, while information on lowland tropical rainforest systems in Meso and South America remains sparse. To better facilitate potential movement between forest areas for the arboreal wildlife community of Costa Rica’s Osa Peninsula, we installed and monitored the early use of 12 arboreal wildlife bridges of three different designs (single rope, double rope, and ladder bridges). We show that during the first 6 months of monitoring via camera traps, 7 of the 12 bridges were used, and all bridge designs experienced wildlife activity (mammals crossing and birds perching). A total of 5 mammal species crossing and 3 bird species perching were observed. In addition to preliminary results of wildlife usage, we also provide technical information on the bridge site selection process, bridge construction steps, installation time, and overall associated costs of each design. Finally, we highlight aspects to be tested in the future, including additional bridge designs, monitoring approaches, and the use of wildlife attractants.
... Although the project has been initially planned to mitigate the roadkill of the black lion tamarin on the GRI253 Road, the use of canopy bridges by a total of 10 species showed that the canopy bridges are functional for a variety of arboreal species, including primates. The key factor is to identify the most appropriate design for the species or target group (Valladares-Padua et al. 1995;Goosem et al. 2005;Weston et al. 2011;Goldingay et al. 2013;Soanes et al. 2013;Teixeira et al. 2013). In our study, although the rope bridge was used by some species, the wood pole bridge seemed to be the most suitable for most species, including for the target species of this study, the black lion tamarin. ...
... Most non-primate species in our study, except two species, had a short habituation period for the use of bridges (less than or equal to 12 months), as well as observed in studies in Australia with arboreal species: 5 months for the giant white-tailed rat (Uromys caudimaculatus Krefft, 1867), 9 months for the Herbert River ringtail possum (Pseudochirulus herbertensis Collett, 1884) (Weston et al. 2011), and 8 months for the squirrel glider (Petaurus norfolcensis Kerr, 1792) (Soanes et al. 2013). Indeed, some species may have a much quicker habituation for the structures, such as the Brazilian squirrel in our study (14 days) and the endangered western ringtail possums (Pseudocheirus occidentalis Thomas, 1888) in other Australian study (36 days) (Yokochi and Bencini 2015). ...
Article
Full-text available
Canopy bridges are crossing structures specific to mitigate the impact of roads on arboreal animals. Long-term monitoring of such infrastructures together with the analysis of design preferences has never been done in South America. To avoid the roadkills of a threatened primate species, the black lion tamarin (Leontopithecus chrysopygus), in Guareí, São Paulo, Brazil, we installed two designs of canopy bridges: a wood pole bridge and a rope bridge. We aimed to (1) evaluate the functionality (number of species and events) of both designs, (2) test the design preference of each species, and (3) determine if there were seasonal differences in the use of canopy bridges. We monitored the canopy bridges continuously since their installation with camera traps during 3 years. We recorded nine mammal and one lizard species crossing on the canopy bridges as well as 13 bird species using them as perches. Overall, the probability of crossing was higher on the wood pole bridge and the number of crossings, considering both designs, was higher during the dry season. One lizard and seven mammal species used the wood pole bridge, including the black lion tamarin, and six mammal species used the rope bridge. Four out of five species tested, including the black lion tamarin, preferred the wood pole bridge. While replications of this experimental design are necessary to obtain a more robust evaluation of the effectiveness of these canopy bridges, our study suggests that wood pole bridges might be an effective tool to reduce roadkills of the endangered black lion tamarin and possibly other arboreal species.
... L'augmentation de l'utilisation des structures avec le temps, ici multipliée par 8 en près de deux ans, est un phénomène souvent observé dans les suivis de passages à faune nouvellement installés (Clevenger & Waltho, 2003 ;Goosem et al., 2005 ;Bellis, 2008 ;Soanes et al., 2013). De multiples processus imbriqués peuvent en fait expliquer l'augmentation du nombre de passages observée au cours du suivi. ...
Article
Full-text available
Use by small and medium mammals of wildlife crossing structures on two motorways in south-western France. Results of the first two years of camera-trap surveys.— Since 2012, camera-trap surveys have been carried out to study the use by wildlife of 9 newly-constructed underpasses on two motorways in south-western France (8 modified culverts and 1 underpass for small animals). A total of 5338 successful crossings of 16 small, medium and large mammal species were detected over a 9 to 24 month period (the duration varied with each site). The results comprise the majority of roadkill species, but micro-mammals were excluded from the survey. The mean observed daily crossing rate of 1.20 ± 0.70 SD per structure is comparable to other studies carried out in Western Europe at similar sites. Of the detected crossings, 91 % were made by 5 species: Eurasian badger Meles meles (45 %), Red fox Vulpes vulpes (19 %), Common rabbit Oryctolagus cuniculus (9 %), Common genet Genetta genetta (9 %) and Stone marten Martes foina (9 %). For these abundant species both the duration of the survey (number of days) and the season were shown to have a significant influence on the number of crossing events. Monitoring began shortly after the installation of each crossing structure. Over a 2-year period, an almost eight-fold increase in use was observed as wild fauna found the crossing structures and incorporated them into their movement patterns through adaptation periods and learning processes. Throughout the survey a decrease in refusal rate by Red fox and Badger was observed, supporting the 'habituation' hypothesis. Usage was significantly different throughout the seasons for Badger, Red fox, Rabbit, and Stone marten. Each of these seasonal patterns could be correlated with species activity throughout the year. The initial survey results are encouraging and suggest that continued long-term monitoring is important to assess the effectiveness of crossing structures after the period of adaptation. To this end, our camera-trap surveys must be accompanied by appropriate additional sampling to measure the individual and population benefits for wildlife of these underpasses. (PDF) Fréquentation de buses dédiées aux passages de la petite et moyenne faune sous deux autoroutes de l’ouest de la France. Bilan des deux premières années de suivis par pièges photographiques. Available from: https://www.researchgate.net/publication/357926484_Frequentation_de_buses_dediees_aux_passages_de_la_petite_et_moyenne_faune_sous_deux_autoroutes_de_l%27ouest_de_la_France_Bilan_des_deux_premieres_annees_de_suivis_par_pieges_photographiques?origin=mail&uploadChannel=re390&reqAcc=Remi-Duguet&useStoredCopy=0 [accessed Jan 26 2022].
... Interestingly, this response is highly idiosyncratic. Species may show less movement if they are reluctant to cross patch boundaries (Kuefler et al. 2010), such as infrastructures blocking movement, which can lead to isolation (Soanes et al. 2013;Lister et al. 2015). In the other extreme, species may show increased movement if they must forage across many distant patches for resources that are left after habitat loss (Andreassen and Ims 1998;Saunders 1980). ...
Article
Full-text available
Context Land-use change is one of the main threats to biodiversity on the global scale. Legacy effects of historical land-use changes may affect population dynamics of long-lived species, but they are difficult to evaluate through observational studies alone. We present here an interdisciplinary modelling approach as an alternative to address this problem in landscape ecology. Objectives Assess effects of agricultural abandonment and anthropisation on the population dynamics of long-lived species. Specifically, we evaluated: (a) how changes in movement patterns caused by land-use change might impact population dynamics; (b) time-lag responses of demographic variables in relation to land-use changes. Methods We applied an individual-based and spatial-explicit simulation model of the spur-tighed tortoise ( Testudo graeca ), an endangered species, to sequences of real-world landscape changes representing agricultural abandonment and anthropisation at the local scale. We analysed different demographic variables and compared an “impact scenario” (i.e., historical land-use changes) with a “control scenario” (no land-use changes). Results While agricultural abandonment did not lead to relevant changes in demographic variables, anthropisation negatively affected the reproductive rate, population density and the extinction probability with time-lag responses of 20, 30 and 130 years, respectively, and caused an extinction debt of 22%. Conclusions We provide an understanding of how changes in animal movement driven by land-use changes can translate into lagged impacts on demography and, ultimately, on population viability. Implementation of proactive mitigation management are needed to promote landscape connectivity, especially for long-lived species for which first signatures of an extinction debt may arise only after decades.
... Alternatives to speed-limit reductions may facilitate animals to avoid the use of the road network or prevent use altogether. For arboreal species such as possums, crossing structures like rope bridges placed in areas where vegetation structure is high on both sides of the road may reduce roadkill rates (Soanes et al., 2013). Although it is thought that these structures may assist in restoring functional connectivity (Soanes et al., 2018) there remains a need to determine the most effective configuration of these structures (Rytwinski et al., 2015). ...
Article
Full-text available
Understanding when and where roadkill is most likely to occur is vital to reducing wildlife-vehicle collisions. However, little is known about how roadkill rates change through time and whether or not the key influences on roadkill also change. Understanding changes in roadkill will facilitate the best implementation of mitigation measures. We aimed to determine how roadkill rates have changed between two distinct time periods and assess whether the spatial and temporal drivers of roadkill rates may have changed: with a view to informing taxon-specific mitigation strategies. We assess the spatial and temporal factors that influence road mortalities in two periods (1998-99 and 2014) at the same site for multiple taxa. Bi-weekly surveys were undertaken from February to May 1998 and 1999 and again from February to June 2014. In total 2 479 individual roadkill were recorded throughout the surveys, with 1.59 roadkill per km per month in the 1990s, increasing to 2.39 per km per month in 2014. Roadkill rates increased primarily with road speed limit with mortalities peaking at moderate (60 – 80 km/h) speeds, however, the structural complexity of roadside vegetation and traffic volume influenced roadkill rates for some taxa but not others. We show that roadkill rates have changed through time with shifts in both the temporal and spatial influences on these roadkill rates. These changes are likely associated with changes in the abundance of taxa and increased vehicle traffic. The spatial and temporal drivers of roadkill rates were found to be taxon specific, and although mitigation measures exist, assessment of their efficacy remains a priority.
... Regarding habitat modification, our results support calls for avoiding the transformation or degradation of sea-and landscapes that currently remain relatively unmodified. Historical landscape modifications will be more difficult to address and may rely on improving connectivity or resource availability [32][33][34] . Where habitat modification is unavoidable, we recommend the integration of movement ecology principles into landscape design and management to facilitate animal movement, resource acquisition, reproduction and dispersal 35 . ...
Article
Full-text available
Disturbance and habitat modification by humans can alter animal movement, leading to negative impacts on fitness, survival and population viability. However, the ubiquity and nature of these impacts across diverse taxa has not been quantified. We compiled 208 studies on 167 species from terrestrial and aquatic ecosystems across the globe to assess how human disturbance influences animal movement. We show that disturbance by humans has widespread impacts on the movements of birds, mammals, reptiles, amphibians, fish and arthropods. More than two-thirds of 719 cases represented a change in movement of 20% or more, with increases in movement averaging 70% and decreases −37%. Disturbance from human activities, such as recreation and hunting, had stronger impacts on animal movement than habitat modification, such as logging and agriculture. Our results point to a global restructuring of animal movement and emphasize the need to reduce the negative impacts of humans on animal movement. A meta-analysis pinpoints the severity with which human disturbances, ranging from hunting to habitat modification, affect the movements and home ranges of terrestrial and aquatic animals around the globe.
... For animals with special requirements (i.e. arboreal or flying mammals), rope bridges, glider poles and bat gantries are built (Berthinussen and Altringham 2012; Soanes et al. 2013). Nonetheless, despite the popularity of WCS all over the world, and critical financial investments in their planning and building, we are still far from understanding factors affecting their use by wild animals, especially those for large mammals (van der Ree and van der Grift 2015a, b). ...
Article
Full-text available
Wildlife crossing structures (WCSs) enhance connectivity between habitats of wild animals fragmented by fenced motorways, but factors affecting their use by targeted species remain understudied, particularly in areas recently recolonized by large carnivores. We investigated the use of WCS-6 overpasses (width 30-45m), 5 large underpasses (width 33-114 m) and 4 small underpasses (width 15-19 m)-located along the A4 motorway in the Lower Silesian Forest (western Poland), a large forest tract recently recolonised by wolves (Canis lupus). Identifying and counting tracks of mammals left on sand-beds as well as individuals recorded by camera traps were used to determine species diversity, number and activity patterns of mammals on WCS, and to reveal seasonal and temporal changes of WCS use over 3 years of study (2010-2013). WCSs were mostly used by wild species (51.5%), followed by humans (34.8%), livestock and pets (13.7%). Among wild species, ungulates were the most common (77.4% of crossings), while lagomorphs and carnivores were recorded less often (15% and 7.6% of crossings, respectively). The number of species and crossings of wild mammals, especially wild ungulates and wolves, was substantially higher on overpasses (mean effective number of species (Hill numbers): 0 D = 7.8, 1 D = 4.1 and 2 D = 3.3) than on underpasses (0 D = 6.3, 1 D = 2.9 and 2 D = 2.3) and was not affected by distance between WCS and human settlements or WCS width. There was a higher diversity of wild species and more crossings under large extended bridges than on smaller underpasses. The number of species and number of crossings of wild mammals, domestic animals and people increased from 2010 to 2013. There was a significant difference in activity patterns, with almost all wild species being nocturnal, in contrast to people and dogs. There was no relationship between crossing time and rates of wild carnivores and potential prey. We conclude that overpasses, even with steep entrance slopes (25-26.5%) or integrated with moderately used gravel roads, maintain movement of wild terrestrial mammals much better than underpasses, and the presence of wolves does not hamper the movement of other wild species. As there are significant temporal changes in use of WCS by mammals, we recommend monitoring WCS in all seasons for at least 3 years as a minimum standard for the post-investment assessment of WCS utilization by animals.
Article
Roads that dissect natural habitats present risks to wildlife, creating gaps or barriers which animals have to traverse in order to move within and between their habitats. Restoring habitat connectivity can be achieved naturally by planting trees and vines to reconnect forest gaps, or artificially by creating culverts for small ground vertebrates, building overpasses for large terrestrial animals, or installing canopy bridges for arboreal fauna. The 3-km Old Upper Thomson Road borders the eastern side of the Central Catchment Nature Reserve, the largest nature reserve in Singapore, and isolates it from neighbouring forest patches. To facilitate safe crossing for tree-dwelling animals such as the critically endangered Raffles’ banded langurs ( Presbytis femoralis ) along Old Upper Thomson Road, two rope bridges were installed. We monitored the use of these rope bridges by vertebrates from April 2020 to August 2021 through surveillance cameras attached on one end of each bridge. A total of 64 118 videos were processed, with 6218 (9.70%) containing vertebrates. Seven species, including three primates, two squirrels and two reptiles, utilised the bridges to travel between the forests. In particular, Raffles’ banded langurs made a total of 293 successful crossings. We have shown that these rope bridges are useful for arboreal species and can complement national efforts to restore connectivity in fragmented habitats.
Article
Full-text available
Traffic accidents involving animals occur every year. Roadkill is a serious problem faced by the whole world, including Indonesia. Therefore, it is necessary to modify road accessories to prevent accidents, both from animals and road users. Prevention can be done in several ways, such as by installing fences or creating crossing paths, for animals. The fence can be used as a barrier between the driving lane and the animal path, where they can carry out activities such as playing without disturbing road users. Meanwhile, the making of crossing paths can be used by animals as access for animal migration. This study would like to propose a design for implementing cross-fencing mitigation at Gladak Perak Bridge at Lumajang, Indonesia. This location is an accident-prone area due to the sudden crossing of monkeys, which has been a myth in the community. Through the implementation of the installation of road dividers, it is hoped that the road design at the research site becomes wildlife friendly road and the management of traffic also meet the Indonesian design standards for inter-city roads without reducing tourism potential.
Article
Full-text available
Wildlife passages are structures built across roads to facilitate wildlife movement and prevent wildlife collisions with vehicles. The efficacy of these structures could be reduced if they funnel prey into confined spaces at predictable locations that are exploited by predators. We tested the so-called prey-trap hypothesis using remote cameras in 17 wildlife passages in Quebec, Canada from 2012 to 2015 by measuring the temporal occurrence of nine small and medium-sized mammal taxa (< 30 kg) that we classified as predators and prey. We predicted that the occurrence of a prey-trap would be evidenced by greater frequencies and shorter latencies of sequences in which predators followed prey, relative to prey–prey sequences. Our results did not support the prey-trap hypothesis; observed prey–predator sequences showed no difference or were less frequent than expected, even when prey were unusually abundant or rare or at sites with higher proportions of predators. Prey–predator latencies were also 1.7 times longer than prey–prey sequences. These results reveal temporal clustering of prey that may dilute predation risk inside wildlife passages. We encourage continued use of wildlife passages as mitigation tools.
Article
Full-text available
We attempted a complete review of the empirical literature on effects of roads and traffic on animal abundance and distribution. We found 79 studies, with results for 131 species and 30 species groups. Overall, the number of documented negative effects of roads on animal abundance outnumbered the number of positive effects by a factor of 5; 114 responses were negative, 22 were positive, and 56 showed no effect. Amphibians and reptiles tended to show negative effects. Birds showed mainly negative or no effects, with a few positive effects for some small birds and for vultures. Small mammals generally showed either positive effects or no effect, mid-sized mammals showed either negative effects or no effect, and large mammals showed predominantly negative effects. We synthesized this information, along with information on species attributes, to develop a set of predictions of the conditions that lead to negative or positive effects or no effect of roads on animal abundance. Four species types are predicted to respond negatively to roads: (i) species that are attracted to roads and are unable to avoid individual cars; (ii) species with large movement ranges, low reproductive rates, and low natural densities; and (iii and iv) small animals whose populations are not limited by road-affected predators and either (a) avoid habitat near roads due to traffic disturbance or (b) show no avoidance of roads or traffic disturbance and are unable to avoid oncoming cars. Two species types are predicted to respond positively to roads: (i) species that are attracted to roads for an important resource (e.g., food) and are able to avoid oncoming cars, and (ii) species that do not avoid traffic disturbance but do avoid roads, and whose main predators show negative population-level responses to roads. Other conditions lead to weak or non-existent effects of roads and traffic on animal abundance. We identify areas where further research is needed, but we also argue that the evidence for population- level effects of roads and traffic is already strong enough to merit routine consideration of mitigation of these effects in all road construction and maintenance projects.
Article
Full-text available
Roads and traffic reduce landscape connectivity and increase rates of mortality for many species of wildlife. Species that glide from tree to tree may be strongly affected by roads and traffic if the size of the gap between trees exceeds their gliding capability. Not only are wide roads likely to reduce crossing rates, but mortality may also be increased if gliders that do cross have poor landing opportunities. The road-crossing behavior of 47 squirrel gliders (Petaurus norfolcensis) was investigated in southeast Australia using radio-tracking. The proportion of gliders crossing one or both roadways of a freeway where trees were present or absent from the center median was compared to that at single-lane country roads (control). The proportion of gliders crossing the road at control sites (77%) was similar to the proportion that crossed one or both roadways at the freeway with trees in the median (67%), whereas only a single male (6%) crossed the freeway where trees were absent from the median. The frequency of crossing for each individual was also similar at control sites and freeway sites with trees in the median. The almost complete lack of crossing at sites where trees were absent from the median was attributed to the wider gap in canopy (50 - 64 m vs. 5 - 13 m at sites with trees in the median). This suggests that traffic volume, up to 5,000 vehicles per day on each roadway, and the other characteristics of the freeway we studied are not in themselves complete deterrents to road crossing by squirrel gliders. This study demonstrates that retaining and facilitating the growth of tall trees in the center median of two-way roads may mitigate the barrier effect of roads on gliders, thus contributing positively to mobility and potentially to connectivity. This information will be essential for the assessment of road impacts on gliding species using population viability models.
Article
Full-text available
We attempted a complete review of the empirical literature on effects of roads and traffic on animal abundance and distribution. We found 79 studies, with results for 131 species and 30 species groups. Overall, the number of documented negative effects of roads on animal abundance outnumbered the number of positive effects by a factor of 5; 114 responses were negative, 22 were positive, and 56 showed no effect. Amphibians and reptiles tended to show negative effects. Birds showed mainly negative or no effects, with a few positive effects for some small birds and for vultures. Small mammals generally showed either positive effects or no effect, mid-sized mammals showed either negative effects or no effect, and large mammals showed predominantly negative effects. We synthesized this information, along with information on species attributes, to develop a set of predictions of the conditions that lead to negative or positive effects or no effect of roads on animal abundance. Four species types are predicted to respond negatively to roads: (i) species that are attracted to roads and are unable to avoid individual cars; (ii) species with large movement ranges, low reproductive rates, and low natural densities; and (iii and iv) small animals whose populations are not limited by road-affected predators and either (a) avoid habitat near roads due to traffic disturbance or (b) show no avoidance of roads or traffic disturbance and are unable to avoid oncoming cars. Two species types are predicted to respond positively to roads: (i) species that are attracted to roads for an important resource (e.g., food) and are able to avoid oncoming cars, and (ii) species that do not avoid traffic disturbance but do avoid roads, and whose main predators show negative population-level responses to roads. Other conditions lead to weak or non-existent effects of roads and traffic on animal abundance. We identify areas where further research is needed, but we also argue that the evidence for population-level effects of roads and traffic is already strong enough to merit routine consideration of mitigation of these effects in all road construction and maintenance projects.
Article
Intuitively, wildlife crossing structures should enhance the viability of wildlife populations. Previous research has demonstrated that a broad range of species will use crossing structures, however, questions remain as to whether these measures actually provide benefits to populations. To assess this, studies will need to determine the number of individuals using crossings, their sex, and their genetic relationships. Obtaining empirical data demonstrating population-level benefits for some species can be problematic and challenging at best. Molecular techniques now make it possible to identify species, individuals, their sex, and their genetic relatedness from hair samples collected through non-invasive genetic sampling (NGS). We describe efforts to pilot a method to assess potential population-level benefits of wildlife crossing structures. We tested the feasibility of a prototype NGS system designed to sample hair from black bears (Ursus americanus) and grizzly bears (U. arctos) at two wildlife underpasses. The piloted hair-sampling method did not deter animal use of the trial underpasses and was effective at sampling hair from more than 90% of the bear crossing events at the underpasses. Hair samples were also obtained from non-target carnivore species, including three out of five (60%) cougar (Puma concolor) crossing events. Individual identification analysis revealed that three female and two male grizzly bears used one wildlife underpass, whereas two female and three male black bears were identified as using the other underpass. Of the 36 hair samples from bears analyzed, five failed, resulting in an 87% extraction success rate, and six more were only identified to species. Overall, 70% of the hair samples from bears collected in the field had sufficient DNA for extraction purposes. Preliminary data from our NGS suggest the technique can be a reliable method to assess the population-level benefits of Banff wildlife crossings. Furthermore, NGS can be an important tool for the conservation value of wildlife crossings for other taxa, and we urge others to carry out evaluations of this emerging methodology.
Article
Tree-dwelling mammals are potentially highly vulnerable to discontinuities in habitat created by roads. We used population modeling to assess the viability of a metapopulation of Australia's largest gliding marsupial, the greater glider (Petauroides volans), occurring in forest remnants in the fastesturbanizing region of Australia, where habitat is dissected by major roads. Crossing structures for arboreal mammals (consisting of a land bridge with wooden poles for gliding and adjacent rope canopy bridges) have been installed over an arterial road that separates two of these remnants (one large, one small). It is currently unknown whether this species will use the crossing structures, but available tree height and spacing do not allow a glide crossing, and fences with metal flashing prevent access to the road by terrestrial and arboreal mammals. Our modeling reveals that even a relatively low rate of dispersal facilitated by these structures would substantially reduce the probability of extinction of the smaller subpopulation. This rate of dispersal is plausible given the small distance involved (about 55 m). The inclusion of wildfire as a catastrophe in our model suggests that these two remnants may encounter an undesirable level of extinction risk. This can be reduced to an acceptable level by including inter-patch movement via dispersal among other forest remnants. However, this requires connection to a very large remnant 8 km away, through a set of remnants that straddle two motorways. These motorways create discontinuities in forest cover that are beyond the gliding ability of this species. Crossing structures will be required to enable inter-patch movement. A priority for future research should be whether the greater glider will use road-crossing structures. Loss of habitat and habitat connections is continuing in this landscape and is likely to have dire consequences for wildlife if land managers are unable to retain appropriate habitat cover with corridors and install effective wildlife road-crossing structures where large roads intersect wildlife habitat.
Article
Road reserves provide habitat for wildlife. Roadside vegetation has greatest value as a wildlife habitat when it comprises remnant or regenerated strips of indigenous vegetation. Road, roadside habitats and the aerial space above roads can facilitate the movement of animals along the direction of the road reserve. Road reserves can act as a filter or barrier to the movements of wildlife through the landscape, thus dividing and isolating populations to varying extents. Roads are a source of mortality for wildlife. For some species, particularly those that are large, rare, or are regularly brought into contact with busy roads, road-kills can have a significant effect on conservation status. Road systems are a source of biotic and abiotic effects on the surrounding landscape. -from Author
Article
Abstract A huge road network with vehicles ramifies across the land, representing a surprising frontier of ecology. Species-rich roadsides are conduits for few species. Roadkills are a premier mortality source, yet except for local spots, rates rarely limit population size. Road avoidance, especially due to traffic noise, has a greater ecological impact. The still-more-important barrier effect subdivides populations, with demographic and probably genetic consequences. Road networks crossing landscapes cause local hydrologic and erosion effects, whereas stream networks and distant valleys receive major peak-flow and sediment impacts. Chemical effects mainly occur near roads. Road networks interrupt horizontal ecological flows, alter landscape spatial pattern, and therefore inhibit important interior species. Thus, road density and network structure are informative landscape ecology assays. Australia has huge road-reserve networks of native vegetation, whereas the Dutch have tunnels and overpasses perforating road barriers to enhance ecological flows. Based on road-effect zones, an estimated 15–20% of the United States is ecologically impacted by roads.
Article
Artificial structures designed to promote road-crossing by arboreal mammals are increasingly being installed in Australia but there is a limited understanding of their usefulness. We studied five 50-70-m-long rope-bridges (encompassing three designs) erected across the Pacific Highway, a major freeway in eastern Australia. Native arboreal mammals showed a willingness to explore these structures, being detected by camera traps on four rope-bridges. The vulnerable squirrel glider (Petaurus norfolcensis) crossed on one rope-bridge at least once every 4.5 weeks over a 32-week period. The feathertail glider (Acrobates pygmaeus), common ringtail possum (Pseudocheirus peregrinus) and the common brushtail possum (Trichosurus vulpecula) were detected on one of two rope-bridges that extended under the freeway at creek crossings. The feathertail glider was detected on all three rope-bridge designs. Our results suggest that rope-bridges have the potential to restore habitat connectivity disrupted by roads for some arboreal mammals. Further research is needed to refine the design and placement of rope-bridges as well as to determine whether these structures promote gene flow.
Article
Gliding mammals may be susceptible to habitat fragmentation due to increased vulnerability to predators and road mortality if forced to cross roads and other canopy gaps on the ground. We document three trials where 6-12-m-high wooden poles, also known as glide poles, were installed to provide a link for gliding mammals across 50-75-m-wide canopy gaps, over open pasture or over roads. We used hair-traps over periods of 10-42 months to determine whether squirrel gliders (Petaurus norfolcensis) used the poles. Squirrel glider hair was detected on at least one pole during 69-100% of sampling sessions. At two road locations where poles were installed on wildlife land-bridges, hair was detected on poles in the middle of the bridge in 7-18 sessions, suggesting that complete crossings may have occurred. At one road location a camera-trap recorded a squirrel glider ascending a middle pole on five of 20 nights. Repeated use of the wooden poles by squirrel gliders at three locations suggests that tall wooden poles can restore habitat connectivity for a gliding mammal. We recommend further trials to extend our knowledge of the usefulness of this management tool for a range of gliding mammal species.