Download Managing arboreal mammals during road

Scientific Article
Managing arboreal mammals during road construction:
a field study of the western ringtail possum
(Pseudocheirus occidentalis), brushtail possum
(Trichosurus vulpecula) and brush-tailed phascogale
(Phascogale tapoatafa)
Jessica Hunter Supervisors:
Associate Prof. Roberta Bencini
Dr. Brian Chambers
This thesis is submitted in partial fulfilment of the requirements for a Bachelor of Science
(Conservation Biology), October, 2012
Version modified by Roberta Bencini for submission to Main Roads Western Australia
Research Thesis SCIE4501-4504
Faculty of Natural and Agricultural Sciences
The University of Western Australia
Austral Ecology is the chosen formatting style
Contents Scientific Article ...................................................................................................................................................1
Managing arboreal mammals during road construction: a field study of the western ringtail possum
(Pseudocheirus occidentalis), brushtail possum (Trichosurus vulpecula) and brush-tailed phascogale
(Phascogale tapoatafa) .........................................................................................................................................1
Abstract..................................................................................................................................................................3
Acknowledgements...............................................................................................................................................5
Introduction ..........................................................................................................................................................6
Materials and Methods........................................................................................................................................9
Results ..................................................................................................................................................................16
Discussion ............................................................................................................................................................20
References............................................................................................................................................................24
Literature Review...............................................................................................................................................28
The effects of road construction and habitat clearing on arboreal marsupials in Western Australia .28
Abstract................................................................................................................................................................29
Introduction ........................................................................................................................................................29
The Bunbury Outer Ring Road Project .........................................................................................................30
Study Species.......................................................................................................................................................31
Road construction – effects on the environment ...........................................................................................33
Translocations.....................................................................................................................................................40
Conclusions..........................................................................................................................................................42
References............................................................................................................................................................43
Appendix 1: Project Proposal...........................................................................................................................47
2 Abstract
The construction of roads and the associated vegetation clearing that comes with it can
cause negative environmental effects on the local wildlife. To mitigate these effects the
translocation of wildlife has often been adopted. This consists of removing the animals
and placing them within continuous undisturbed vegetation unaffected by the negative
impacts of the road. The outcomes of these translocations, however, have not been
studied extensively and little is known about their conservation value, especially for
threatened fauna.
The construction of the Bunbury Outer Ring Road in Bunbury provided the opportunity
to study the effects of this practice on translocated western ringtail possums
(Pseudocheirus occidentalis). Five western ringtail possums (two males and three
females) were removed from the road alignment immediately prior to the clearing or
during the clearing and relocated in vegetation close to the alignment. They were fitted
with radio transmitter collars that had a mortality switch, so they changed the transmitting
frequency if the animal did not move for 3-4 hours, allowing us to detect and collect dead
individuals and investigate the cause of mortality.
Three male western ringtail possums were found near the road alignment and were radio
collared but instead of being relocated they were released at the point of capture. Six
additional western ringtail possums, three males and three females, were captured and
radio collared at a reference site, which was approximately 500m, away from the road
construction site and therefore was not disturbed by the road construction operations.
Within the group of five animals that were relocated three western ringtail possums died,
two from possible fox predation as established by DNA swabs of the retrieved collars.
The other one possibly died because the collar was entangled in a tree branch. No other
western ringtail possums died during the study within the other two locations.
Two female and five male common brushtail possums (Trichosurus vulpecula
hypoleucus) were captured and radio collared at the reference site, but none could be
captured on or near the road alignment, so these possums were still radio tracked
regularly but because they all lived in the undisturbed site they could not be compared to
3 a similar group of animals living near the disturbance caused by the road construction.
One of them, a male, died from presumed fox predation while all the other animals
survived until the end of the study.
Similarly we were able to capture and radio collar four female and one male phascogale
(Phascogale tapoatafa) at the reference site, and only one near the road. However, this
animal could not be studied because its collar was found on the ground the day after its
release. The phascogales were radio tracked at the reference site but due to size
limitations the collars did not have a mortality switch, so often it was unclear if animals
were dead up in a tree of if they had managed to remove their collars, which we observed
they were capable of doing throughout the study. Only two of the phascogales were
confirmed dead, one from fox predation and the other due to the male die-off, a
phenomenon exhibited in this species where all males die after the mating season.
The results of this study support the notion that survivorship of relocated western ringtail
possums is low, especially in the presence of introduced predators and road constructions
activities. Significantly non-relocated possums living next to the road alignment were
still alive at the end of the study despite the disturbance going on around them.
Further studies during road construction events and with greater number of animals
should be undertaken to clarify if the survivorship of relocated ringtail possums will
improve if fox control operations are in place.
4 Acknowledgements
First and foremost I would like to expressly thank my supervisors Dr Roberta Bencini
and Dr Brian Chambers for all their help during this year both with the setting up of the
experiment and the numerous revisions of the final article.
To Kaori Yokochi for her help with the statistical analyses, for allowing me to look at her
own project to get better ideas on how to conduct my own and for her amazing darting
skills, that allowed me to obtain some of the study animals. To Steve Correia and
Roberta Bencini for catching many of the study animals while I was away on a study
camp. To Greg Harewood for help during trapping and capture of western ringtail
possums.
I would also like to thank Main Roads Western Australia for the funding to conduct this
work, for their commitment to the environment and particularly Alan Grist for his
practical help throughout the project. To Fulton Hogan’s Environmental and Safety
Officer, Jacob Cumberworth for his tireless commitment to endless radio-tracking nights,
and help with the trapping.
Finally to my family and friends, to all those people who came and helped me radio-track
the study animals: I hope you had an enjoyable time, as I enjoyed having you there.
Special thanks to my family for putting up with my ranting and raving about my project
for all this year as well.
5 Introduction
Road infrastructure is an important part of the progression and development of human
society, because roads allow the transportation of goods and people in a quick and easy
fashion over the country. However, the development of roads can have a negative impact
on the environment, and is thought to be a contributing factor to species decline
(Burbidge & McKenzie, 1989; Bennett 1990; Keller et al. 2004; Fahrig & Rytwinsk
2009; Soanes & van der Ree 2009). Mortality due to habitat clearing, habitat loss and
fragmentation, edge effects, barriers to movement, invasion of undesirable flora, fauna
and disease, vehicle collisions, and environmental pollution are all factors that can affect
the native wildlife found in the vicinity of any road, or road construction site (Goosem
2007). For fauna in particular, all of these factors contribute to make a very challenging
environment for any animals remaining in the area, with possible changes in their
survival ability and abundance (Fahrig & Rytwinski 2009).
These effects, in particular barrier effects and road mortality, can be devastating for
arboreal mammals. As these animals depend on trees for their survival, the effects of
roads on the nearby vegetation can be of increased significance to the ability of these
animals to survive in the wild (Goosem et al. 2005).
Among the arboreal mammals living in the southwest of Western Australia, the western
ringtail possum, Pseudocheirus occidentalis, is especially susceptible to the clearing
necessary to build roads and other infrastructure because they are strictly arboreal, they
have small home ranges and they are threatened (de Tores et al. 2008; Clarke, 2011). To
manage them during developments western ringtail possums are generally translocated,
often to inappropriate areas, without lack of adequate research to determine whether this
is the best course of action (A. Wayne pers. comm.). For example, at the Leschenault
Peninsula many translocations have been attempted but after initial successes, these
populations crashed due to unknown causes (McCutcheon et al. 2007).
6 While generally used for the conservation of threatened or endangered species,
translocations are used increasingly to resolve human-animal conflicts such as that
generated by the development of roads (Lindenmayer & Burgman 2005). These
translocations, while they have the best interests of the animal at heart, may not prove to
be the best option for their continued survival.
Studies by Griffith et al. (1989) showed that the translocation of threatened species had a
success rate of only 44%, which may actually be lower depending on the species. While
this can be attributed in part to lack of criteria for success or failure of the translocations,
in most cases it was caused by lack of proper research into whether this practice would
actually benefit the animals being translocated in the first place (Griffith et al. 1989;
Fischer & Lindenmayer 2000; Sheean et al. 2012).
In general, when translocations are being attempted, initial studies of any possible
suitable area should be undertaken to determine their suitability for the introduction of
further animals. One of the conditions of undertaking a translocation within government
guidelines is for any threatening processes of the translocated species to be eliminated
from the translocation area (Shea 1995). Detailed research needs to be undertaken before
any translocation attempt in order to determine whether the site is possible for a
successful translocation attempt, but with the development of roads where the
progression of the project is on a very tight schedule, this research is not always possible.
While many studies have looked at better ways to translocate or relocate animals, very
few have looked at the actual benefits of moving these animals away compared with
keeping them in their original areas of occupation (Griffiths et al. 1989; Cowan 2001;
Eymann et al. 2006; Roselan 2009; Clarke 2011). These studies showed that habitat
quality, and the reduction and/or control of the threatening process is of utmost
importance in attempting a successful translocation (Cowan 2001; Eymann et al. 2006).
Due to the very low success rates of translocations we would expect that animals that are
kept within part of their original home range would have a better chance of surviving than
those that have been moved (McCutcheon et al. 2007; Roselan 2009).
7 Roselan (2009) conducted the only study so far that has examined the survival rates of
south-western common brushtail possums (Trichosurus vulpecula hypoleucus) that were
not moved during a road clearing. She found that all brushtail possums were still alive at
the end of the five month study in contrast with Pietsch (1994) who found that only two
out of ten translocated common brushtail possums (Trichosurus vulpecula) survived for
more than ten weeks. However, in Roselan’s (2009) study none of the animals were
actually moved away from the area for comparison with those managed in situ.
Additionally no similar study has been conducted on the western ringtail possum.
In Bunbury, a developing city in the southwest of Western Australia, the demand for the
construction of roads has increased as more people have moved into the area. Small town
roads, once used only by residents are more commonly now being utilised by large goods
trucks. In an aim to divert these trucks directly to the harbour a series of roads were
commissioned for construction including the Bunbury Outer Ring Road Stage 1, which
entailed clearing 5.5ha of western ringtail possum habitat and a further 14.3ha of
potential western ringtail possum habitat (McCarthy 2011).
One of the conditions for the approval for the construction of the Bunbury Outer Ring
Road Stage 1 was the production and implementation of a management plan for the
western ringtail possums living in this habitat. Main Roads WA designed the plan in
close collaboration with researchers from The University of Western Australia, who
undertook to conduct research to study the effects of the road construction on the arboreal
mammals found in the area.
Extensive searches and trapping revealed that three arboreal mammal species were
present in the area: the western ringtail possum, the common brushtail possum and the
brush-tailed phascogale (Phascogale tapoatafa), which are all within the Critical Weight
Range mammals (Burbidge & McKenzie 1989). As these are arboreal species, spending
most of their lives in trees, the loss of these trees or the continuity of the canopy can have
negative effects on any local populations (Laurance 1996). The management plan
included the removal and relocation of animals found directly on the road alignment to
undisturbed vegetation within the study area. At the same time an offset bushland
purchase by Main Roads WA half km north of the road alignment provided an excellent
8 opportunity to study the same species at a reference site away from the disturbance of the
road construction.
We hypothesised that animals that had to be moved away from the clearing site would
have a lower survival rate than those that were kept within their home ranges, both within
the road construction area and at the reference site where animals were left completely
undisturbed.
Materials and Methods
Study species
The three study species are arboreal and spend most of their lives in trees. Like most
other Australian mammals, they are nocturnal, foraging in the canopy at night and resting
during the day.
The western ringtail possum is considered ‘Vulnerable’ under the Environment
Protection and Biodiversity Act 1999, and ‘Rare or likely to become extinct’ under the
Wildlife Conservation Act 1950, despite its common appearances in southwest Western
Australia. Averaging between 820 and 1020g for both sexes, the western ringtail possum
is a folivore that is able to construct its own nest, called a ‘drey’, composed of twigs and
leaves from surrounding vegetation, so is not reliant solely on tree hollows for nesting
locations (Thomson & Owen 1964). They very rarely descend from the trees where they
live to travel along the ground, which increases their susceptibility to negative effects due
to habitat clearing and tree loss (Clarke 2011). The western ringtail possum occurs in
patchy distributions south from the Collie River, east to Two Peoples Bay in Western
Australia, in areas dominated by peppermint trees. This patchy distribution and specific
habitat means that all populations of western ringtail possums are essential for the
continual survival of this species (Clarke 2011).
In Southwest Western Australia, the subspecies of the common brushtail possum T. V.
hypoleucus is found, averaging 1616g for males, and 1470g for females (Wayne et al.
2005). Although the brushtail possum is common, it has been in decline since European
settlement. Once widespread throughout the whole of Western Australia, it is now
9 restricted to the southwest of Western Australia (Bennett et al. 1991). Omnivorous in
diet, eating the occasional insect, or scavenging for meat when available, this possum is
more willing to travel along the ground to move from location to location, unlike the
western ringtail possum. This species nests in tree hollows, or rabbit burrows and
underground networks when no other nesting sites are available (Wayne et al. 2005).
The brush-tailed phascogale is thought to be a separate subspecies of the eastern variety,
although not yet classified as such, and is thought to be under the same ecological stress
as the western ringtail possum and the brushtail possum (Rhind & Bradley 2002). A
small (110g and 150g for females and males respectively), carnivorous dasyurid, the
brush-tailed phascogale is highly threatened in many areas of the south-west and little
researched, so its conservation is important (Soderquist 1995; Rhind & Bradley 2002).
Nesting exclusively in tree hollows, and feeding primarily on invertebrates found
underneath the bark of trees, the brush-tailed phascogale is highly vulnerable to habitat
loss and fragmentation, particularly of those trees bearing tree hollows (Rhind 1996;
Scarff et al. 1998; van der Ree et al. 2001; van der Ree et al. 2006).
Study Site
This study was conducted between February 2012 and September 2012 within the
Bunbury regional area, where the construction of the Bunbury Outer Ring Road, Stage 1
was occurring. This area was situated in eucalypt woodland with groves of sheoak
(Allocausarina fraseriana), paperbark (Melaleuca rhaphiophylla), banksia (Banksia spp.)
and peppermint (Agonis flexuosa). The area also comprised previously grazed farmland,
which created breaks in the native vegetation. An existing gravel road, powerlines and
their maintenance tracks also dissected parts of this eucalypt woodland.
Three different sites were chosen: a release site, a treatment site and a representative
reference site, as pre-construction monitoring of the arboreal animals could not occur
(Figure 1).
Animals that were found directly in the area to be cleared for the road were released in
the release site, a patch of uncleared vegetation adjacent to the treatment area. These
10 animals were in immediate danger of being harmed during the clearing process if not
removed, and thus became the relocated group of animals.
The treatment site was the area indirectly impacted by the road construction because it
was immediately adjacent to the road alignment. Any animals that were found on the
treatment area but outside of the clearing area were returned to their capture locations, as
they were not in immediate danger from the clearing process and became the treatment
group of animals.
The reference site was situated in a 100 ha bushland purchased by Main Roads WA as an
offset, approximately half a kilometre away in undisturbed eucalypt woodland where any
animals captured would not be immediately impacted by the construction of the road.
This area was skirted on two edges by power-line maintenance tracks providing easy
access to this undisturbed area.
Capture of animals
Trapping was conducted for a total of 744 trap nights in the treatment site, and 315 traps
nights in the reference site to capture any brushtail possums and brush-tailed phascogales
using Sheffield live-capture wire cage traps (60 cm x 20cm x 20cm; Sheffield Wire
Products, Welshpool WA) placed in a grid with 50 m spacing between traps (Figure 1).
The greater number of trap nights conducted at the treatment site was due to low trapping
success in this area. The traps were baited with pieces of apple and universal bait
consisting of peanut butter, rolled oats and sardines, and were covered with hessian sacks
to provide shelter and some form of protection for the animals caught. The traps were
checked early every morning, and any new arboreal animals caught were placed in dark
cotton bags.
Western ringtail possums were searched extensively at night through spotlighting and
captured using a pole syringe or a dart gun to inject the intramuscular anaesthetic Zoletil
at a nominal dose of 12 mg/kg.
11 Figure 1: Aerial photograph of the study site. The blue dots represent the trap sites. Animals captured on
the road alignment, marked in yellow, were relocated to the release site. The reference site was
chosen as it is continuous vegetation that was unaffected by the road construction activities, while
the release site was chosen as this was the nearest continuous vegetation that was as close as
possible to the road alignment where the animals were originally captured.
12 All captured animals were placed in bags and transported to the DEC’s Lechanault field
station in Australind where they were anaesthetised using isoflurane gas anaesthesia. The
animals were first induced with 5% isoflurane supplied with 2 litres of oxygen per
minute. When the animals were fully unconscious, the isoflurane was reduced to 1.5%
when anaesthetising brushtail possums, and 2% when anaesthetising western ringtail
possums or brush-tailed phascogales. The oxygen was kept constant at 2 litres of oxygen
per minute. Breathing and heart rate were regularly monitored throughout the
anaesthesia.
We weighed the animals and measured their body length, tail length, head length and
right pes length. Females were also checked for the presence of pouch young while for
males measurements were taken of the length and width of the right testis, and the width
of both testes.
Animals were micro-chipped with Passive Integrated Transponders (PITs, Trovan,
Central Animal Records, Keysborough, Victoria) and the possums also received a
reflective ear tag to distinguish between those already caught, and whether they were
male (tag in the left ear) or female (tag in the right ear). The animals were then fitted
with radio-transmitter collars so they could be tracked throughout the clearing and the
road construction. The possum species were fitted with movement sensitive mortality
collars (Biotrack Ltd, Dorset, UK; or AVM Instrument Company, Ltd, Colfax, California,
USA), which changed the frequency emitted when the animals had remained still for
more than three to four hours, while the brush-tailed phascogale collars (Biotrack Ag357)
were too small to be fitted with this function. All collars weighed less than 3% of the
total body weight of the animals.
The release of the animals occurred at night, under red light so night vision was not
affected at least 8 hours after recovery from the anaesthesia.
Clearing for the construction of the road commenced on the 19th of March, 2012, with a
fauna relocator on site charged with spotting and relocating any animals still left in the
vegetation. Any trees that were within the clearing alignment were pushed over in the
hope any animals would be able to escape or to be caught by the fauna relocator to be
13 collared and relocated. Indeed two of the relocated animals were found on vegetation
that had been pushed over and captured by UWA researchers or the fauna relocator the
night after the vegetation was cleared.
Radio-tracking
Each animal was radio-tracked once a week, finding both the day/nesting location and
their night/active location. A handheld Sirtrack 3 element Yagi antenna and battery
operated ‘Biotel’ RX3 radio receiver (Bio Telemetry Australia, St Agnes, SA) were used
to determine these locations. Differential GPS locations of the trees on which were found
were recorded using a Thales Mobile Mapper Pro GPS and post processed using the
MobileMapper Office software (Thales Australia, NSW). Each tree that was also marked
using flagging tape.
While day tracking usually involved triangulating animals to a specific tree where a
possible nesting site would be, night tracking involved visually spotting the animals to
confirm that they were alive. Any collars of possums that had not moved for several
hours would emit a mortality signal, and would be tracked until they were spotted to be
alive, or the collar was recovered. When a mortality event occurred, the radio collar and
any remains of the animal were collected. Radio collars were swabbed for saliva and
these samples were analysed at the Helix DNA laboratory (Helix Molecular Solutions Pty
Ltd, Leederville WA) to determine the predator species involved (fox, cat, dog or
chuditch) through melt curve analysis (Berry and Sarre 2007).
Treatment of animals
Before and during the clearing operations we captured a total of five western ringtail
possums that were found directly on the alignment and relocated them. Three were
captured near the alignment and released at their point of capture and another six were
captured at the reference site, one in March and the other five in July. Brushtail possums
and brush-tailed phascogales were captured at the reference site, but the only phascogale
that was caught near the road alignment managed to remove its radio collar within a day
of release and was therefore excluded from the study (Table 1). Due to this unbalance in
14 numbers it was possible to an extent to compare the data that were collected on western
ringtail possums but not for the brushtail possums and phascogales.
Table 1. Arboreal mammals captured and radio collared at the reference, treatment and relocations sites
within the study area.
Site Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Reference
Relocation
Relocation
Relocation
Relocation
Relocation
Treatment
Treatment
Treatment
Treatment
Species Brushtail possum
Brushtail possum
Brushtail possum
Brushtail possum
Brushtail possum
Brushtail possum
Brushtail possum
Phascogale
Phascogale
Phascogale
Phascogale
Phascogale
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Ringtail possum
Phascogale
Ringtail possum
Ringtail possum
Ringtail possum
Animal ID BCF006
BCF012
BCM009
BCM013
BCM017
BCM018
BCM019
PCF008
PCF010
PCF011
PCF015
PCM014
RCF026
RCF027
RCF028
RCM021
RCM029
RCM030
RTF005
RTF022
RTM001
RTM002
RTM024
PTM003
RTM004
RTM020
RTM025
Name Desiree
Anna
Jack Harkness
The Doctor
Rory
Ricky
Micky
Donna Noble
Martha Jones
Rose Tyler
Sarah Jane Smith
Iantho
Houdinette
Big Mumma
Mary
Stretch
Bruce
Alan
Amelia Pond
Lucky
Michael Jackson
Enrico
Felicity
Conrad
Romeo
Othello
Primo
Sex Female
Female
Male
Male
Male
Male
Male
Female
Female
Female
Female
Male
Female
Female
Female
Male
Male
Male
Female
Female
Male
Male
Female
Male
Male
Male
Male
Data analysis
Survivorship of the animals at the three different sites was determined using the known
fate model in MARK (Cooch & White 2010). Additional factors were included to
determine whether anything else other than the animals’ location influenced survival in
any of the locations. These additional factors included sex, weight of the animal, headbody length, average maximum and minimum temperatures between radio-tracking
periods and rainfall. Not all of these factors were found to be important in the models, so
15 only those with a delta AICc value below 2 were included in the final analysis. The AICc
value is the corrected or adjusted Akaike’s Information Criterion and is derived using the
equation:
AICc = -2 log [L(Ô)] + 2K [2K(K+1)/n-K-1]
Where [L(Ô)] is the value of the likelihood function, K is the number of parameters in the
model and n is the sample size. The AICc is the value used to determine the best model, as
the lower the AICc value, the better the fit of the model. No significance is formulated with
these models as the program MARK uses a theoretical model based approach in order to
achieve the best survivorship outcome, which is different from the null hypothesis testing,
which includes significance testing. Instead, a null model is hypothesised, and if any other
models are able to explain any variation in the data better than the null model there is found
to be a difference in the survivorship.
The home ranges of the animals were determined using the 95% kernel contours found in
Ranges 8 (Anatrack Ltd, Dorset, United Kingdom), as this has been determined to be the
most accurate type of home range analysis when appropriate sample sizes and smoothing
parameters are used (Comport et al. 1996). The home ranges of the individuals were then
grouped into their respective species and areas, and ANOVA performed to test for
differences in home ranges between species and within species.
Other variables of the area where the arboreal animals were found were also considered, such
as the animals’ preferred day/rest location, the species of tree on which the animals were
found most often in, how high up the animals were in the tree and general characteristics of
the animals that were collared. These variables can help determine the requirements to
maintain an arboreal population in any area and the differences found between this
population of arboreal animals and populations in other locations.
Results
Survival of western ringtail possums in the different sites
Three of the five relocated western ringtail possums died during the course of the study,
two due to presumed predation by foxes and one because its collar became stuck on a
branch. Survivorship was therefore markedly different within all models for the relocated
16 animals, with probabilities of surviving the whole of the period on average below 25%
(Table 2). This was the only group of western ringtail possums that experienced any
deaths within the period of the study, so all other groups had 100% probability of
surviving the extent of the study.
Table 2. Survivorship models for western ringtail possums between sites in Bunbury below delta AICc
values of 2.0. Relocated animals had much lower survival rates than the other two groups.
Factors included in model AICc Treatment Relocation Reference Site + Mean min temp + rainfall 35.960 1.000 0.198 1.000 Site + rainfall 37.337 1.000 0.208 1.000 Site 37.559 1.000 0.201 1.000 Site + Mean max temp + rainfall 37.932 1.000 0.213 1.000 Individual variables, such as the sex, weight and length of the animals had no impact on
the survivorship between these groups.
Survival of the study species in the reference area
None of the western ringtail possums living at the reference site died during the study.
However, one male brushtail possum died during the study. This was probably due
predation given the nature of the remains and bite marks found on the collar, but DNA
analysis was inconclusive. Of the five brush-tailed phascogales found at the reference
site, two were confirmed dead, and another two lost their signal at some point towards the
end of the study. The only other brush-tailed phascogale collar that was found to still
have a signal at the end of the study was only ever found in the same location, both day
and night for four months, suggesting the brush-tailed phascogale had either been able to
take off the collar, or had died in the tree where the signal came from.
Radio tracking and home ranges
The western ringtail possums radio tracked during the study were found at three distinct
locations corresponding to the treatment, relocation and reference sites (Figure 2). This
shows that the animals did not move between these sites throughout the study and that
17 relocated animals moved large distances of up to 1.2km from the original release site.
Figure 2. The locations of individual western ringtail possums radio tracked throughout the study were
clustered in three separate areas corresponding to the treatment (red), relocation (orange) and
reference sites (blue). Animals moved up to 1.2km away from the relocation site.
The western ringtail possums moved to the relocation sites had the largest home range, of
37.3+29.50 ha compared to 8.8±5.40 ha at the treatment site and 2.6±2.00 ha at the reference
site. However, these differences were not significant (p=0.371; Table 3).
Table 3. Home range estimates (ha) and standard errors of western ringtail possums living at the three
different sites.
Site Mean (ha) Standard error Minimum Maximum Treatment 8.8 5.40 0.7 19.2 Relocation 37.3 29.50 0.8 155.0 Reference 2.6 2.00 0.2 12.7 18 Incremental area plots demonstrated that new locations for night tracking were very
frequent by the end of the study, showing that we had not yet reached the extent of their
levels of movement, or the size of their home ranges.
Of the species living at the reference site the brush-tailed phascogales had the largest
home ranges, approximately three times the size of the home ranges of the brushtail
possums and over 15 times the size of the home ranges of western ringtail possums
(Table 4).
Table 4. Home ranges (ha) and standard errors of the study species living at the reference site.
Species Mean (ha) Standard error Minimum Maximum Western ringtail possum 2.7 2.00 0.2 12.7 Brushtail possum 11.2 2.60 4.1 24.3 Brush-tailed phascogale 35.0 22.60 5.4 124.8 Habitat utilisation
While the western ringtail possums sometimes used dreys as their day nesting location,
brushtail possums and brush-tailed phascogales exclusively used tree hollows as their
nesting locations. These tree hollows were usually found in dead trees, but occasionally a
live eucalyptus tree provided an adequate tree hollow space for occupation. Tree sharing
was often observed within these arboreal species, not only within species, but between
species, with a male western ringtail possum and a male brushtail possum found
numerous times to be nesting in the same tree. The most common trees animals were
found in were the peppermint, banksia, eucalyptus and sheoak.
Other species captured
Other than the target species, no other native mammal species were caught during
trapping within any of the sites. Lizards, such as the bobtail lizard (Tiliqua rugosa) and
the king skink (Egernia kingii) were frequently caught in the traps during the summer
months, but were inactive when the winter round of trapping was commenced. Magpies
19 (Gymnorhina tibicen) and crows (Corvus coronoides) were also found occasionally in the
traps, both through the winter and summer rounds of trapping. Non-native mammals
included the mouse (Mus musculus), the black rat (Rattus rattus), and one cat (Felis
catus).
Discussion
The results of this study suggest that relocating western ringtail possums to an area
unaffected by road development might not be beneficial this species, especially in the
presence of introduced predators. Two western ringtail possums were killed by foxes
soon after they were relocated within the release site. The other western ringtail possum
that died during the study probably died from strangulation or starvation after its collar had
been caught in a branch. This western ringtail possum had moved away from the release site,
suggesting that relocated western ringtail possums tend to move around perhaps in search of
a new home range or perhaps in an attempt to return to their original home range. Indeed all
the relocated possums moved rapidly away from the relocation site and it is possible that
during these movements they had to come to the ground where they were easy prey to foxes.
Foxes were regularly sighted in the area, and half way through the study period, a fox baiting
and shooting program was introduced in an attempt to reduce the impact of foxes on native
animals within the area. Local farmers also reported that foxes were prevalent in the area,
and implemented their own fox control strategies, but maintained that the fox problem had
not been eliminated, and that foxes were still seen moving through their lands.
Foxes have been known to prey on western ringtail possums despite the western ringtail
possum’s arboreal lifestyle (Bennet 1990; May & Norton 1996). Fox predation has been
the cause of many translocation failures in Australia, and it has been shown that with
proper exotic predator control most conservation efforts in Australia would be successful
(Cowan 2001; Sheean et al. 2012).
The construction of roads can increase the predation risk because predators, in particular
foxes, have been known to utilise clearing tracks and roads to move around (Ramp et al.
2006; Goosem 2007). The facilitation of foxes into the area may explain the lack of
ground-dwelling native animals when the trapping was conducted. Clearing for access
20 ways to the site had been conducted before trapping had started, and could have allowed
foxes and other predators easy access into the area. The presence of introduced predators
at the treatment site may have also caused brushtail possums that are not known to be
trap-shy to become so. If foxes were attracted the to the trap sites, they might have
discouraged the brushtail possums from entering the traps. The powerlines maintenance
tracks at the reference site may have also allowed the expansion of the introduced
predators into this area, where one brushtail possum succumbed to fox predation during
the study.
No deaths of radio collared western ringtail possums occurred in either the reference or
the treatment sites, even though the treatment site was directly affected by the road
construction operations. So it appears that the vicinity of heavy machinery, people and
noise during the day when the road construction took place did not affect the short term
survival of these possums.
Unfortunately there was no replication for this study and sample sizes were extremely
small to draw firm conclusions. The duration of this study was also insufficient to
include all of the seasons, as this study was mostly conducted in winter. For this reason,
estimates of the survivorship could not be properly examined based on environmental
variables. Had the study been continued, we might have observed more mortality events,
thus achieving a better understanding of the factors that influence any species ability to
survive both during a road clearing process, and in an undisturbed environment.
Initial models that included time as a variable also were not better at predicting the
survivorship of the different groups of western ringtail possums, supporting the concept
that the relocation of animals was the main reason for the differences in survivorship.
A longer study would have been required to improve the estimates for the home ranges of
the study species because incremental area plots showed that asymptotes in the home
ranges had not been reached with the number of locations collected with this study.
Additionally because three of the western ringtail possums died within a couple of weeks
of being relocated their home ranges could not be determined.
21 Many animals have been known to wander great distances for some time after they have
been translocated to find suitable home ranges (Cowan 2001; Massei et al. 2010). This
movement is similar to animals undertaking dispersal from their natal home ranges, but is
more risky for translocated animals because they cannot go back to their original home
range if a direction does not lead to suitable habitat. In this study the relocated animals
moved to the very edge of the vegetation, or crossed wide stretches of open ground to
reach a different patch of vegetation, which would have increased the risk of
encountering predators.
The home range of an animal should provide all the different things that the animal may
need in its lifetime (Worton 1989). While western ringtail possums are able to make their
own nesting sites in the form of dreys (Thomson & Owen 1964), the western ringtail
possums in this study preferred to sleep in tree hollows where they were available. For
the release animals that were placed in a suitable tree hollows can be very difficult to find
(van der Ree et al. 2006).
There have been very few studies of the difference in survivorship for animals that were
relocated to another area compared with animals that were left within at least part of their
original home range. Roselan (2009) reported advantages of leaving animals as close to
their original home range as possible in the only study that attempted to manage brushtail
possums on site during the construction of the Mandurah Entrance Road. Cowan (2001)
showed that brushtail possums moved extensively, and sometimes travelled up to 7km to
return to their original home range.
While the western ringtail possums in this study did not appear to exhibit homing
behaviour, they moved away from the release site. By contrast, possums that were
released in their capture locations remained in the same area throughout the study.
There have been no other studies of western ringtail possums managed on site during
clearing of their habitat for the construction of a road and further studies with greater
numbers of animals might clarify if this is a viable management strategy.
22 Differences between species at the reference site
The brush-tailed phascogales moved great distances throughout the study as shown by
their large home ranges. The only male brush-tailed phascogale radio tracked in this
study was also found in one of the treatment traps, over one and a half kilometres away,
during the second round of trapping. Its death in July was an expected event, as brushtailed phascogales exhibit male die-off after the mating season (Cuttle 1982; Rhind
2002).
The female phascogales also travelled great distances, with one individual even leaving
the reference site to move into a small group of eucalypts 300 metres away. The brushtailed phascogales proved exceptionally hard to radio track as their collars were smaller
and so their signal strength was weaker. This reduced signal strength, combined with
their wide ranging movements meant they were frequently out of range.
Compared to the western ringtail possum, brushtail possums are a more mobile species,
and spend time on the ground moving from tree to tree (Wayne et al. 2005). This
movement along the ground may have been the cause of the only brushtail possums death
during the study. The western ringtail possums in comparison spend no time on the
ground, which could explain the lack of deaths in the reference site for this species.
Unfortunately, no brushtail possums or were caught near the road alignment and the only
brush-tailed phascogales captured there removed its collar within a day, so we were
unable to study how the road construction affects these species. Therefore the original
hypothesis about the effects of translocation on these arboreal animals could not be
tested.
Conclusions
Translocation as a management strategy needs to be assessed on an individual species
basis as different species might respond to different stressors and disturbances in different
ways. For instance, omnivores have been shown to have a greater ability to survive
translocations than herbivores and carnivores, and generalists also fare better than
specialists (Massei et al. 2010).
23 Translocations are no longer considered adequate management strategies for the
conservation of arboreal species. Translocations not only have deleterious effects on the
individuals, but up to 70% of common brushtail possums were reported to die within the
first week (Eymann et al. 2006; Pietsch 1994). Translocations therefore, should only
ever be used when there is no other alternative, when certain death would result from the
animal staying in that area, and after all in situ methods of management have been
exhausted (Burbidge & de Tores 1998).
The management of western ringtail possums in situ during road construction remains an
option that still needs to be investigated in further studies with more animals and better
replication.
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27 Faculty of Natural and Agricultural Sciences Research Project
The University of Western Australia
Literature Review
The effects of road construction and habitat clearing on arboreal
marsupials in Western Australia
Jessica Hunter
Supervisors:
Associate Prof. Roberta Bencini
Dr. Brian Chambers
28 Abstract
Road construction is a vital part of economic development but the development of roads
can have very detrimental effects on the surrounding environment. These include habitat
loss, fragmentation, and barriers to movement, creation of isolated populations, edge
effects, increased road mortality and increased predation to name a few. These effects
have been extensively studied in other countries, but they are still relatively unknown on
the Western Australian wildlife. Of particular concern are the arboreal species found in
the south west of Western Australia, which are endemic to the area and threatened by
habitat destruction.
The translocation of animals out of areas earmarked for the construction of roads can
prove to lower the rate of survival of many arboreal species, but these effects are still
largely unknown. This study aims to reduce the effects of road construction on arboreal
mammals by moving them as close to their original locations as possible.
The construction of the Bunbury Outer Ring Road provided the opportunity to conduct a
comparative survivorship study of three arboreal marsupials found in the area: the
western ringtail possum (Pseudocheirus occidentalis), the brushtail possum (Trichosurus
vulpecula) and the brush-tailed phascogale (Phascogale tapoatafa).
Introduction
As the human population increases, further development is needed to accommodate for
the additional needs (Roedenbeck et al. 2007). Roads and other transport provide new
developments with future income through the transport of goods and safer ways to travel
between locations. These developments cause additional costs to the environment
however, which impacts on both plant and animals species within the area. Many small
mammals within the Critical Weight Range, between 50-5500g in weight, have been
found to be at extreme risk of extinction, with the main cause being environmental
change (Burbridge & McKenzie 1989). Environmental change due to road construction
includes the clearing of habitat to make way for the road leading to loss of resources and
29 homes for many animals, fragmentation, isolation and barriers to existing population of
animals, and increased risk of vehicle collisions and predation due to the establishment of
a road (Goosem 2007). The clearing due to development is generally unavoidable, as the
prime locations for many new roads and housing developments is generally within
existing pristine vegetation supporting many different species of wildlife. To be able to
establish conservation practices that will actually help the survival of these different
animal species, we need to better understand the underlying processes that are causing the
changes in the species populations. This literature review will discuss these associated
environmental impacts of road construction, and some potential ways to mitigate these
effects to improve the overall survivorship of any species within the area.
The Bunbury Outer Ring Road Project
The Bunbury Outer Ring Road will provide an alternative route for truck drivers
travelling to and from Bunbury Port. Its construction was subject to conditions to
minimise some of the environmental impacts on the western ringtail possum
(Pseudocheirus occidentalis).
The area to be cleared for its construction includes the clearing of 5.5 ha of western
ringtail possum habitat. Additionally the brushtail possum (Trichosurus vulpecula) and
the brush-tailed phascogale (Phascogale tapoatafa) are also found in the area. These
species are at particular risk from this road construction, due to the loss of continuity
within their home ranges as a road gets built in the middle of it and the destruction of
habitat necessary to build the road. Currently, no research on the impacts of road
construction and the removal of animals from these areas has been conducted on the
brush-tailed phascogale, and little research has been conducted on the two possum
species. Thus the Bunbury Outer Ring Road project provides an excellent opportunity to
study these effects on the nocturnal arboreal marsupial populations found in the clearing
area.
30 Study Species
The effect of clearing on arboreal animals is well documented, but this information
mostly revolves around the clearings conducted in plantations in the eastern states of
Australia (Laurance & Laurance 1996; Forman & Alexander 1998; Lindenmayer et al.
1999; Goosem 2007; Lada et al. 2008; Soanes & van der Ree 2009). These areas are
generally then left to re-establish as vegetation before the clearing occurs again. With
road construction, however, there will be no regeneration of vegetation after clearing, and
there may even be suppression techniques employed to keep vegetation near the road
edge to a minimum (Goosem 2007). Information about how arboreal species respond to
permanent clearing procedures is not as well documented. As they are nocturnal animals,
these species are at rest while the road construction operations occur during the day. This
makes them susceptible to accidental killings when the clearing occurs.
In this study, three nocturnal arboreal species will be studied to determine their response
to the road construction. These three species are the western ringtail possum, the
brushtail possum and the brush-tailed phascogale.
The western ringtail possum
Listed as Vulnerable under the Environmental Protection and Biodiversity Act 1999, and
rare or likely to become extinct under the Wildlife Conservation Act 1950, this species is
a medium sized folivorous marsupial, weighing up to 1.3kg and approximately 40cm in
body length. It has dark brown fur on top with cream coloured fur on its belly. This
species generally constructs nest-like structures in the tree canopy using sticks and leaves,
called a ‘drey’, which allows it to establish home ranges out of areas limited in potential
tree hollow nesting locations. Breeding occurs during March to April only once a year,
producing generally one but up to three pouch young. The western ringtail possum’s
distribution occurs from the Collie River, down to Two Peoples Bay in Western
Australia, with a very patchy distribution most commonly occurring in areas dominated
by peppermint trees (Agonis flexuosa). This has resulted in a decline in their abundance
in recent decades due to deforestation and clearing for development purposes, which can
limit their dispersive abilities due to their avoidance of the ground. All known
populations of the western ringtail possum are considered essential for the conservation
31 of this species due to their low breeding capacity, short lifespan and susceptibility to
predation, which means that the population existing at the Bunbury Outer Ring Road
needs to be preserved as much as possible (de Tores et al. 2008; Clarke 2011).
The brushtail possum
The south-western subspecies of the common brushtail possum (T. vulpecula hypoleucus)
is grey in colour on their backs with cream coloured fur underneath and weighs up to
1.5kg. This subspecies was once widespread over much of the West Australian
landscape, but is now restricted to the drier forests and woodlands of the south-west
(Bennet et al. 1991). Not strictly folivorous in diet, eating the occasional insect, or
scavenging for meat, this species is able to travel along the ground more easily and safely
than the western ringtail possum, which has an increased risk of predation when on the
ground (Clarke 2011). The brushtail possum does not construct dreys, preferring hollows
as diurnal sites but when hollows are unavailable, can use rabbit burrows or underground
networks for their nesting sites (Clarke 2011). Breeding occurs generally only once a
year, producing mostly one offspring at a time, but has been known to produce twins.
This species, like the western ringtail possum, is very susceptible to habitat loss and
fragmentation due to clearing procedures, but little is known of their actual survival rates
during the clearing procedure. Roselan (2009) studied the survival rates of brushtail
possums through the construction of the Mandurah Entrance Road in Western Australia,
and found that they had high survival rates when not removed from their original home
ranges. No similar studies have been conducted on brushtail possums or any of the other
arboreal marsupial species found in the Bunbury Outer Ring Road area, so research into
these species will provide valuable insight into how these other species are affected by
road construction.
The brush-tailed phascogale
This small carnivorous marsupial species has suffered recent declines due to clearing for
development, much like the western ringtail possum. In the eastern states the brush-tailed
phascogale weighs approximately 200g and eats mostly invertebrates; they have grey fur
above, and cream coloured fur on their bellies, naked ears and a black ‘bottle-brush’ tail
32 (van der Ree et al. 2001). Currently split into two subspecies, the north and south
subspecies, there has been calls for a reclassification into three subspecies due to
morphological and genetic differences between the south-western and south-eastern
populations of phascogales (Soderquist 1995).
Little is known about the south-western population of phascogales, as most research was
conducted on the Victorian populations. Thought to exist within only fifty per cent of its
former range, the brush-tailed phascogale has only recently been rediscovered within
Perth, and has now also been found as far down south as Albany (Soderquist 1995).
Breeding generally occurs between mid-May to early July, and males have larger home
ranges during this time. Females can have up to eight pouch young, and the males die-off
after mating (Soderquist 1995). This species is vulnerable to habitat fragmentation and
degradation, in particular the removal of tree hollow den sites, and predation by feral
animals (Rhind 1996; Scarff et al. 1998; van der Ree et al. 2001; van der Ree et al.
2006). Both of these threats are expected to increase with the construction of the
Bunbury Outer Ring Road, so monitoring of this population needs to be conducted to
ensure that this population survives well into the future.
Road construction – effects on the environment
The persistence of a population depends on the populations’ ability to survive, and move
freely through a landscape (Soanes & Van der Ree 2009). When road construction is
started in an area, this can limit these abilities of a population in many different ways.
Habitat is lost, reducing the amount of resources and shelter available, fragmentation of
landscapes occur, where the animal is no longer able to move through continuous
vegetation, creating isolated populations, and the road itself represents a barrier that
prevents or limits the ability of animals to move to the other side, and predators increase
in abundance due to easy access. These effects will be described within this section.
These effects need to be taken into consideration when roads are constructed. Some
mitigation techniques, outlined in Goosem (2007), can be used to try to alleviate these
effects, but little research has been conducted to see how beneficial these techniques are
to the wildlife populations found in the area.
33 Most of the information available on habitat loss, fragmentation and other effects
associated with clearing processes comes from studies done on arboreal marsupials found
in eastern Australia (e.g. Andrews 1990; Bennett 1990; Laurance & Laurance 1996;
Lindenmayer et al. 1999; Goosem 2007; Lada et al. 2008). While some wisdom can be
drawn from these studies, there are substantial differences in the species found in the
south west of Western Australia that make separate research necessary. The south west
of Western Australia is Australia’s only mainland biodiversity hotspot (Myers et al.
2000), and the risk of extinction for many small mammals in this region is high
(Burbridge & McKenzie, 1989). For these reasons, research needs to be conducted into
the effects of road construction on the species and vegetation found in Western Australia
and on arboreal mammals in particular.
Habitat loss
One direct result of road construction is the clearing of vegetation, resulting in habitat
loss and is one of the causes of decline for some species (Keller et al. 2004; Goosem
2007; Lada et al. 2008). Habitat loss can lead to the loss of food resources and of
sheltering sites for the animals living in the clearing, with the level of impact increasing
with animal body size (Bennett 1990; Bennett et al. 1991; Lindenmayer et al. 1999). If
they can, the species affected will generally move to another location, and this will cause
a change in the distribution, abundance and persistence of species within the area.
Folivores, such as the western ringtail possum, and the brushtail possum, are particularly
sensitive to clearing as it results in a direct reduction in the food resources available to
them. As the clearing actively removes trees from the area, energy expenditure increases
within these species because the animals have to travel further to find adequate food
supplies (Laurance & Laurance 1996). The same can be said for carnivorous species, as
the impact of tree loss with produce a direct effect on the prey of the carnivores (van der
Ree et al. 2001).
Direct mortality during clearing operations can cause significant reductions in the
abundance of species within a road construction area (Goosem 2007). Arboreal animals
may be severely affected due to their nocturnal nature. These species are at rest during
the day when the clearing operations usually occur, so they may not be fully aware of
34 what is happening around their rest sites during the day. Although it could be assumed
that animals are killed as the tree falls during the clearing procedure, some may actually
survive, and may find themselves trapped in their resting sites with no way to get out and
later get killed in the mulching machines used in the clearing procedures. In support of
this we have personally observed western ringtail possums emerging at night from
cleared vegetation that had been left on the ground.
For brush-tailed phascogales habitat loss can have a very large effect on the existing
population (Scarff et al. 1998; van der Ree et al. 2001; van der Ree et al. 2006). This
species relies heavily on dead trees for tree hollows for nest sites and to forage for food.
If the area is totally cleared the phascogale will not be able to survive in the area. Van
der Ree et al. (2001) found that the phascogale was able to persist in small areas of
uncleared vegetation, but only if the large hollow trees were not cleared. These larger
trees were found to have a greater diversity of microhabitats containing many
invertebrate food source species than smaller trees, as well as providing adequate nesting
sites. This means that if the remaining vegetation contains large trees with tree hollows,
the phascogale population may persist after the construction of the Bunbury Outer Ring
Road.
With habitat loss come many other environmental impacts, such as edge effects and
disturbance effects. When roads are developed, they not only affect the immediate
vegetation structure, but can cause changes in the hydrology of the area, and increase the
effects of erosion on the sides of the road (Andrews 1990; Goosem 2007). These effects
are worse when there is no tree canopy cover to give some form of protection to the
ground underneath. With this loss of the tree canopy also come increases in light
intensity at the ground level, which also causes an increase in the temperature and
moisture stress at road edges. This increase in light, temperature and moisture stress
generally leads to floristic composition changes near the road edge, as these conditions
favour weeds and invasive species. In turn, these changes in the floristic composition on
the road edge can eventually lead to changes in the whole floristic composition within the
region, ultimately changing the distribution and abundance of animals found in these
areas (Goosem 2007).
35 Fragmentation
Fragmentation is defined as ‘the loss of continuity’ between once connected habitats
(May & Norton 1996), and it is considered one of the major threats to wildlife
populations (Lindenmayer et al. 1999). When an area is cleared for development,
populations of animals and habitat both become fragmented. How a landscape is
spatially structured has a major influence on the distribution, abundance and persistence
of many species within the wild, so this clearing will have either direct or indirect
implications for wildlife (Bennett 1990; Lindenmayer et al. 1999).
The effects of fragmentation and habitat loss can be ameliorated through the construction
of vegetation corridors through cleared areas (Andrews 1990; Goosem et al. 2005; Lada
et al. 2008). In the case of roads, this could mean suitable vegetation in the median strip
of the road so the animals have a ‘safe’ haven to rest in before continuing along their
journey to the other side. This allows dispersal and gene flow to occur between
populations but it could also result in increased road kills (Bennett 1990). The positive
effect of corridors depends on the width, the connectivity and usage intensity of the
vegetation found in these areas (Andrews 1990; Bennett 1990; Forman & Alexander
1998). While new vegetation corridors can be established after the road construction, this
vegetation is generally kept at an earlier successional stage, where mature, large trees are
not able to grow. Therefore it is better to include these vegetation corridors into the
original plans for the road, allowing mature vegetation to stay (Andrews 1990).
If not managed correctly however, these wildlife corridors can be sources of invasive
vegetation and predation, which can greatly affect the surrounding populations (Andrews
1990; Cork & Catling 1996). There is also some risk of the animals staying in these
habitat corridors, creating pockets of high density populations in unsuitable habitats, and
road mortality, but this can also be reduced by the implementation of fauna crossing
structures and reduced speed limits through high animal traffic areas.
Barriers to movement
There are both functional and behavioural barriers to movement, which affect any
population living on or near a road. The effect of the barrier is determined by the width
36 of the road, the volume of traffic, how likely mortality is when crossing the road, and
how much the species knows about avoidance of these roads (Soanes & van der Ree
2009). Most species tend to avoid the edge of the roads, even if the width of the road is
relatively small (Andrews 1990; Bennett 1990). This can result in isolated populations
where little to no gene flow occurs, as has been shown by Keller et al. (2004) on a study
of the flightless ground beetle Abax parallelepipedus. This species, once separated by
habitat fragmentation showed low levels of genetic differentiation, with the potential for
speciation to occur if this species is unable to disperse across the road. For arboreal
animals, the barrier effect is even greater than for strictly terrestrial animals, as any large
gap in the canopy constitutes a barrier to movement. These arboreal species do not like
coming down to the ground for prolonged periods of time with no tree coverage, and so
are less likely to cross roads than other species of terrestrial mammals (Soanes & van der
Ree 2009).
Barriers to movement can include physical barriers, such as fences and ridgelines built
purposely to prevent animals from crossing the road. While this would decrease the
amount of road mortalities found in the area, unless other structures are put in place to
allow animal movements, isolated populations are created where no gene flow can occur.
If there are no fauna passes put in when a fence or ridgeline is constructed, major losses
of animals can result when a major disturbance, such as fire, comes through the area.
Fences and other such barriers not only cause the loss of movement of animals but can
also change water supplies to an area, create overpopulation due to lack of dispersive
abilities, increased human access through maintenance roads, and disruption of seasonal
movements (Andrews 1990).
Predation
The development of roads also causes an increased risk of predation for the species living
in vegetation left undisturbed by the clearing and construction process. May & Norton
(1996) reported that in fragmented landscapes possums became the main food sources of
the dingo, the fox and the cat, when rabbits were not in abundance in any particular area.
Animals in the Critical Weight Range are considered at increased risk from predation by
feral animals, and include both species of possum and the brush-tailed phascogale found
37 in the study area (Kinnear et al. 2002; May & Norton 1996). Foxes and cats use new
clearings and paths as easy access movement routes to move between areas, especially
when these paths lead to areas that the predators may not have been able to get to before
(Laurance & Laurance 1996; May & Norton 1996; Goosem 2007). Roads not only
provide easy travelling conditions for predators, but also a potential food source in the
form of road kills (May & Norton 1996). How these predators use the surrounding
vegetation compared to the roads is not known, however, with little research conducted as
a comparison between roads and other habitat lacking roads (May & Norton 1996).
A survivorship study, while not examining directly the impact of predators, may also
shed light on this matter, as the cause of death of the animals might be determined. This
will, in turn, determine whether there is a more likely risk of predation in areas that have
been recently cleared compared with areas that have been left undisturbed.
Fauna crossing structures, such as underpasses and overpasses and fences to stop the
animals from crossing the road, are a frequent inclusion into the current construction of
roads. These fauna crossing structures are, however, being increasingly recognised as
high predator interaction sites, known as the prey-trap hypothesis proposed by Little et al.
(2002). This hypothesis predicts that predators will use fences and crossing structures to
capture prey moving through or along these boundaries causing a problem for the
conservation of the animals using these facilities. Mitigation techniques designed to
alleviate the effects of road construction may instead enhance the negative effects such as
predation of threatened fauna. Evidence for the prey-trap hypothesis unfortunately is
sorely lacking, with most information being anecdotal and opportunistic, but still needs to
be considered when constructing amelioration structures (Little et al. 2002; Harris et al.
2010). Goosem et al. (2005), however, found that the use of logs and rocks to provide
cover for animals passing through the fauna crossing structure reduced the impacts of
predation. Careful consideration of the effects of predators, and possible ways to reduce
these effects need to be considered before any mitigation technique is used.
It may be thought that arboreal animals will not be affected by an increase in predators
due to their arboreal nature, but unfortunately this is not the case. When road clearings
occur, this can destroy the whole or part of the animals’ home range, which includes their
38 food sites, sheltering sites, and potential mates (Richardson et al. 1997). These animals
would then have to find a new area, which generally means crossing cleared vegetation
and puts them at risk of predation (Clarke, 2011). Foxes and cats are opportunistic
feeders that are able to swap to the more abundant food source found in any area. This
means that these populations of predators will not decrease with a decrease in native
animal abundance but will simply switch prey (Bennett 1990).
Road mortality
One of the more obvious direct impacts of road construction is vehicle collisions with the
local wildlife, and is now considered the leading human direct cause of vertebrate
mortality on land (Forman & Alexander 1998; Rowden et al. 2008). With upwards of
1000 deaths per km annually (Goosem 2007), vehicle collisions can be a major threat
affecting many animal populations. The degree of mortality on roads is dependent on the
width of the road, the type of adjacent vegetation, and the landscape structure found
within these areas. For wider roads there is a less likely chance of movement across the
road, as most animal species tend to avoid large stretches of open space (Goosem 2007).
If the vegetation is pristine, previously undisturbed vegetation that now has a road
through the middle, there will be a greater abundance of animals found in the vegetation
with the ability to cross the road, and therefore a more likely chance of a road collision.
Unless some mitigation techniques are put in place, such as diversion runways, bridges,
fauna crossing structures and reduced speed limits, the level of road mortality can be very
high (Goosem 2007). Arboreal species tend to avoid crossing roads, due to their
avoidance of the ground. Vehicle collisions do occur, however, particularly in areas
where arboreal mammals are abundant (Trimming 2010).
Other effects
The construction of roads causes many other ecological impacts that have not been listed
above. The impacts that have been mentioned above are those that have been deemed the
most destructive to wildlife populations, but other effects are still harmful to the
environment. Greenhouse gases and other air pollutants increase with the construction of
roads as heavy machinery is needed to construct the road, and then vehicles will drive
39 along the road when finished (Forman & Alexander 1998). Chemical pollutants have the
potential to increase harmful effects on native vegetation and wildlife, as oil, metals and
other chemicals from vehicles spill onto the road and any roadside vegetation (Forman &
Alexander 1998). Noise and light effects also increase on and around roads as car
headlights and streetlights appear on roads and the sounds of the cars coming past
disrupts the animals found in the area (Goosem 2007). Nocturnal species, like most of
those found in Australia, are particularly sensitive to light emissions, as any bright lights
sweeping the area can hinder their night vision, leaving them at risk of predation, or
unable to forage for a period of time (Goosem 2007).
Road impacts not only occur directly around the road, but the effects can sometimes be
felt in the landscape hundreds of metres into the surrounding vegetation. For roads the
greater the speed allowed, and the greater the traffic on the road, the larger and longer the
effects last into the surrounding vegetation. Forman & Alexander (1998) found that for
woodlands, at a speed of 120km/h with high vehicle traffic, the effects of the road went
as far as 810m into the surrounding vegetation. This means that for any road built, the
effects of this road will not only last as far as the immediate edge of the vegetation, but
will continue on into the vegetation for some distance before the effect is no longer felt.
For any wildlife living in the area, these effects can be destructive in terms of population
health, density, and composition.
Translocations
When a road is being constructed any animals found on the proposed alignment are
generally relocated to a different area where it is thought they would have a better chance
of survival because it is thought that they could get killed in the clearing process if left
alone (Goosem 2007; Roselan 2010). This type of relocation is not the same as
conservation translocation, which is defined as the movement of species from one
irreversibly declining population to another location where the threatening processes are
controlled or eliminated to conserve and extend the distribution of the species (Clarke
2011). In order for a successful translocation to occur, many different variables need to
be examined first to make sure that the location, and density of species being released is
40 enough for a sustainable population with greatest survivorship (Clarke 2011; Sheean et
al. 2012). The size of the area where the animals are to be translocated to needs to be
examined in order to make sure that it is adequate enough to support a sustainable
population. The average density of this species within the area needs to be considered, as
well as the home range, and whether there are enough resources available within the area
to support the population. This area also has to have adequate nearby vegetation for
population expansion if the translocation proves to be successful, and for the general
dispersive nature of the animals being translocated. The number of animals to be
translocated is generally the minimum number of breeding individuals able to start and
sustain a growing population without causing any decline in the population the animals
are being removed from. All, or the greatest majority of the threats to these species need
to be eliminated from the area so there is the greatest chance of survival for the species in
their new area (Clarke 2011). This all needs to be taken into account, along with how
receptive the species are to translocations, before a translocation proposal can be
approved by the authorities (Department of Conservation and Land Management 1995).
Sheean et al. (2012) reported that only 46% of conservation translocations in Australia
were actually successful, with the lack of adequate predator control recognised as a major
factor resulting in the failure of many of these plans. Many of these programs also had
different criteria for whether the relocation went ahead or not, showing very large
dissimilarities within the guidelines for relocation attempts.
During road construction the stringent criteria for translocation proposals are not applied
because the imperative is to remove fauna that would otherwise be killed or injured
during the clearing operations. Therefore animals are removed from their existing
habitat, and placed in habitat thought to be suitable, often with no long term monitoring
programs, or criteria to determine whether the relocation was successful or not.
If translocations are not conducted properly there is increased risk of overcrowding due to
new animals being introduced into an area that is already populated with animals. Any
new animal that is translocated to a different animals home range could potentially end
up fighting, with potentially harmful effects for one or both of the animals (Cowan 2001;
Eymann et al. 2006). Therefore, when translocating animals into new areas, the density
41 of the new area needs to be examined to make sure overcrowding will not occur. These
new animals need to then establish their own home ranges, so there is increased risk of
mortality as they move around more than usual to establish their new home ranges
(Massei et al. 2010). This type of movement is different from dispersal as not only the
juvenile animals move but the adults also try to find their new home ranges. Increased
movement causes increases in the risk of predation, as safe locations, and escape routes
have not been established (Cowan 2001). If predators were in low abundance in their
previous location, these animals may not exhibit predator avoidance behaviours. If the
animal is then able to survive all this, it may still suffer from malnutrition, dehydration,
and decreased immunocomptence, suggesting that translocations may not be the best
option for many species(Massei et al. 2010).
Barker & Gemmell (1999) found that when a translocation occurs, brushtail possums
become stressed, potentially losing body weight or their young, with the possibility of
mortality for many of these animals. If the animal was relocated to an area that was close
to their previous home ranges, however, this stress was reduced, and the probability of
survival was higher. Because of this, and the low survival rate of possums that have been
translocated, translocation is no longer a conservation option for this species (Eymann et
al. 2006; Massei et al. 2010). Another study by Cowan (2001) in New Zealand, showed
that brushtail possums also showed homing behaviours after being relocated, some
moving up to 12.5km to get close to their original home ranges. This means that even if
we relocated some of the animals out of the clearing area for the road construction, they
just might end up trying to move back to their original locations.
Conclusions
The construction of Bunbury Outer Ring Road will include mitigation techniques to
minimise the negative effect of the road construction on the animal populations and
research into their outcomes. The animals found in this area therefore will be monitored
to study their survival during and after the clearing and determine whether these
mitigation strategies will have any positive effect, and possibly develop further strategies
in light of the results of this study. This project is only one stage of a many stage
42 development of what will eventually be a major road, and any information gathered here
can help to reduce the impacts on the animals in the following stages of the road
construction.
It is hoped that through this study further information will be gathered on the effects of
road establishment on arboreal mammals. These effects, such as habitat loss,
fragmentation, reduction in resource availability, barrier effects, predation, and road
mortality, have caused very devastating impacts on many animal populations. Our
knowledge of the effects of roads is still increasing, and with this increase in knowledge
will come a better understanding of the best ways to mitigate and reduce these effects.
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46 Appendix 1: Project Proposal
Student: Jessica Hunter
Clearing procedures such as for road construction or housing developments are
sometimes an unavoidable process of development. This can have potentially devastating
effects on any native animal populations within the area leaving many animals, especially
arboreal marsupials like the western ringtail possum (Pseudocheirus occidentalis),
brushtail possum (Trichosurus vulpecula), and the brush-tailed phascogale (Phascogale
tapoatafa) without a home or killing them during the process.
In order to try to mitigate the effects of the clearing procedure, research needs to be
conducted to understand the best method of management of animals during the process.
The aim of this project is to examine the effects of the construction of the Bunbury Outer
Ring Road on arboreal marsupials by comparing their populations in the road
construction area with those found in a reference site situated in nearby unaffected
vegetation purchased as an offset by main Roads WA. Animals affected by the clearing
procedure will be placed in uncleared vegetation adjacent to the clearing alignment, so
they stay as close as possible to their original home ranges. This will help us to
understand whether the local populations are better able to adapt to the changes in the
landscape if they are kept within the same relative area of the clearing event.
Many impacts develop from clearing procedures, including fragmentation, reduced
resource availability, increased risk of predation and habitat loss, which can increase the
risk of mortality in any animal population (Soanes & Van Der Ree 2009; Lindenmayer et
al. 1999). Road construction in particular results in the creation of isolated populations,
and barriers to movement as there can be adequate vegetation for sustainable populations
on either side of the road, but no safe way to cross the road. This can result in an increase
in mortality due to road kills if movement between populations occur if no other forms of
management like rope bridges or faunal underpasses are included within the road
construction (Goosem 2007).
47 For arboreal animals clearing can be very devastating to the population as the clearing
also involves the potential loss of nesting areas, home ranges, and means of movement
from area to area. Methods to best alleviate the effects of the clearing and road
construction will minimise the losses to any animal populations that were found in the
clearing area.
During most clearing events any animals that are displaced from the clearing area are
relocated to a completely different location to try to reduce the risk of the animals being
killed during the development process. It has been shown, however, that translocating
some animal species may reduce the survivorship of the population due to the increased
risk of predation brought about by the animals having to re-establish suitable nesting,
breeding and feeding sites, and the possibility of these animals trying to move back to the
areas they were removed from (Cowan 2001).
Only one study to date examined the survivorship of an arboreal marsupial when
individuals were kept within their original areas of occupation during the construction of
a road (Roselan 2010). In this study common brushtail possums were still alive at the end
of the study in contrast to translocated possums that were reported to succumb in large
numbers within the first week (Eymann et al. 2006; Pietsch 1994). This research aims to
determine whether this is a better way to manage populations of arboreal mammals that
have been affected by the clearing procedure. Unfortunately, we are not able to keep all
of the animals within their original home ranges, but it is thought that if these animals are
kept as close as possible to their original home ranges, this may increase the survivorship
of the displaced population.
Survivorship of the arboreal marsupials will be established by tagging and radio collaring
arboreal marsupials within the clearing alignment. These animals will then be radiotracked during and after the clearing event to determine their survivorship. This will then
be compared with a cohort of arboreal marsupials obtained from a population nearby to
the clearing site to determine whether the relocations of these animals have been
successful. The program MARK will be used for analysis of the survivorship of the
arboreal marsupials and minimum convex polygon (MCP) range estimators will also be
used to determine the home ranges of these populations.
48 Project timeline
Task
Mar
Apr
Trapping
X
X
Radio-tracking
X
X
Research
Proposal/Seminars
X
X
Jul
Aug
Sep
X
X
X
X
Data Analysis
X
X
X
X
Report Writing
X
X
X
X
Literature Review
Seminar
X
May
Jun
Nov
X
X
X
X
49