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Habitat preference of the Australian water rat (Hydromys
chrysogaster) in a coastal wetland and stream, Two Peoples Bay,
south-western Australia
Peter C. Speldewinde
A,D
, Paul Close
A
, Melissa Weybury
B
and Sarah Comer
C
A
Centre of Excellence in Natural Resource Management, The University of Western Australia,
Albany, WA 6330, Australia.
B
The University of Western Australia, Perth, WA 6009, Australia.
C
Department of Environment and Conservation, Albany, WA 633, Australia.
D
Corresponding author. Email: peter.speldewinde@uwa.edu.au
Abstract. This study provides a preliminary investigation of the home range and habitat selection of the Australian water
rat (Hydromys chrysogaster) in Two Peoples Bay Nature Reserve near Albany, Western Australia. Six individuals were
captured (trap success 1.9%) from 810 trap-nights. This low number suggests that the water rat population in Two Peoples
Bay Nature Reserve is much smaller than anecdotal evidence would suggest. Home-range size (neighbour-linkage method)
averaged 18.9 ha (11.6). Individuals preferentially utilised wetland habitats characterised by dense, low-lying vegetation
(0–30 cm from ground), low-density canopy cover and shallow, narrow water bodies.
Additional keywords: water rat, radio tracking, habitat use, home range.
Received 6 January 2012, accepted 30 April 2013, published online 14 June 2013
Introduction
An estimated 27 small native mammals have become extinct
in Australia since European settlement, 15 of which had
Western Australian populations (DEWHA 2009). Increasing
rates of extinction require conservation strategies to protect
the remaining species and their habitats. Options for conservation
strategies such as habitat protection (Margules and Pressey
2000), rehabilitation of degraded habitats (Lindenmayer et al.
2002) or translocation of extant populations (Dodd and Seigel
1991) will benefit from knowledge of an animal’s home range and
habitat preferences (Margules and Pressey 2000).
First described in 1804, the Australian water rat (Hydromys
chrysogaster) (or ‘rakali’by its Aboriginal name) (Atkinson et al.
2008) inhabits waterways throughout Australia (Vernes 1998)
(including Tasmania) and some coastal islands (Scott and Grant
1997). Although, nation-wide, the H. chrysogaster population
is considered stable (although most studies have been carried
out in eastern Australia), individual populations face significant
threats. Populations in Western Australia, particularly in the
wheatbelt region, have undergone significant declines (Atkinson
et al. 2008) that have been attributed to salinisation and an
associated decline in preferred prey species (Scott and Grant
1997), waterway pollution and degradation (Lee 1995), and
reclamation of wetlands for urban development (Lee et al.
2006).
Despite its widespread distribution and local abundance
much of the available knowledge relates to populations from
south-eastern Australia, with knowledge of populations in
Western Australia contrastingly limited (Atkinson et al. 2008;
Chapman and Chuwen 2010; Smart et al. 2011). Limited
information on the species’home range and habitat requirements
suggests that only localised movements, concentrated around
nesting sites and foraging areas, are undertaken (Harris 1978).
Preferred habitats tend to be complex (Harris 1978), with a
diversity of structural habitats, including sunken logs, roots
and vegetation (Harris 1978; Smart et al. 2011). Stable stream
banks with dense riparian vegetation (Smart et al. 2011) provide
protection from introduced predators such as foxes and cats,
as well as native raptors (Scott and Grant 1997).
Relationships between structural habitat and home-range
size, however, remain unclear and constrain the development
of management strategies that aim to protect suitable habitats
within appropriately sized reserves. In south-western Australia,
basic information on habitat use and home-range size is also
currently lacking and, given the isolation of populations within
south-western Australia and the unique coastal habitats in which
they occur, this knowledge is critical to their management.
To address these knowledge gaps, this study investigated
the home range and habitat requirements of H. chrysogaster
within a small coastal reserve on the south coast of Western
Journal compilation Australian Mammal Society 2013 www.publish.csiro.au/journals/am
CSIRO PUBLISHING
Australian Mammalogy, 2013, 35, 188–194
http://dx.doi.org/10.1071/AM12001
Australia. We hypothesised that: (1) individual home ranges
will be centred on particular aquatic habitats (river, lakes
and wetlands) and associated riparian vegetation, and (2)
individuals will preferentially use habitats with high vegetation
density close to the ground that provides effective cover from
predators. The implications of our results for the development
and implementation of appropriate management strategies for
Western Australian populations are briefly discussed.
Methods
Study site
The study was conducted along the Gardner Creek and Gardner
Lake, located in the Two Peoples Bay Nature Reserve (TPBNR),
~40 km east of Albany, Western Australia (Fig. 1). The Gardner
Creek complex drains a small coastal catchment comprising a
relatively extensive water body (Gardner Lake) that drains into a
LAKE
GARDNER
GARDNER RIVER
TWO PEOPLES BAY
MT GARDNER
SOUTHERN OCEAN
WESTERN
AUSTRALIA
TWO PEOPLES BAY
0 0.5 1 2 3 4
N
Kilometers
Fig. 1. Map of Two Peoples Bay Nature Reserve, Western Australia, showing approximate location of trap
lines (indicated by asterisks). Shaded area indicates Gardner Lake, whilst the crosshatch areas indicate the two
swamplands associated with the study area.
Water rats in south-western Australia Australian Mammalogy 189
short (2 km) and narrow (~10 m) creek that occasionally flows
out to the sea. The catchment contains numerous small
perennial and ephemeral wetland areas adjacent to both the creek
and lake. TPBNR supports intact and relatively undisturbed
natural habitats that provide regionally important refuge for
the critically endangered Gilbert’s potoroo (Potorous gilbertii)
and the endangered noisy scrub bird (Atrichornis clamosus).
The principal vegetation types found in riparian areas of the
reserve include Taxandria juniperina Low Forest and Thicket,
Melaleuca Low Forest and Banksia littoralis Low Woodland
in the interdunal swales. Taxandria juniperina communities are
characterised by a variable mid-stratum of species including
Callistachys lanceolata, and all have lower strata of sedges
and shrubs such as Lepidosperma gladiatum and Myoporum
caprarioides. The riparian Melaleuca communities are
dominated by M. rhaphiophylla,M. preissiana with Agonis
flexuosa and M. cuticularis closer to the margins of Gardner
Creek, and have a dense understorey of sedges.
Trapping
Sheffield traps (300 300 700 mm with treadle), baited with
pilchards, were set within the riparian vegetation of Gardner
Creek, Gardner Lake and associated wetlands (Fig. 1). Traps were
placed within 2 m of the waters’edge, facing the water and
covered on one end with a hessian bag. Traps were checked daily,
within 2 h of dawn, for the presence of animals. Trapping was
conducted between April and October for a total of 810 trap-
nights. All captured animals were weighed using a spring balance,
sexed and measured for long pes (mm) and head length (mm).
Animals were ear-tagged for identification purposes and released
at the point of capture.
Radio-collaring
Only individual H. chrysogaster large enough to carry collars
were retained for the tracking study. For all collared animals, the
weight of the collar was less than 5% of the total body weight (i.e.
animal weight >240 g) (Neubaum et al. 2005). Each animal was
sedated using Isofluorane (Lichtenberger and Ko 2007) and fitted
with a two-stage transmitter radio-collar (Sirtrack ).
Anaesthetised animals were allowed to recover before being
released at the point of capture at dusk.
Radio-tracking
Radio-collared individuals were given several days to adjust to
the presence of the collar before commencement of the tracking
study. Tracking commenced in early May and continued until
mid-July 2010. Animals were located hourly over a period of
6–8 h, approximately three times per week. To ensure that a full
24-h period of activity was observed, each tracking session
occurred over different times of the day and night. The location of
each animal was estimated from three separate compass bearings
to allow for triangulation. On several occasions the individual was
tracked to its burrow.
Habitat data
Habitat data were collected at 1-m intervals along 10-m transects
(n= 114). Each transect was spaced at ~50 m intervals and
positioned perpendicular to the waters’edge. From the mouth of
Gardner Creek to Gardner Lake, transects extended 10 m on both
sides of the creek. Along the lake, transects included only the
south-eastern shore (Fig. 1). Transects were also positioned
within the two wetland areas, at ~50-m intervals following a
grid pattern that allowed for comparable quantification of
structural habitat with the remaining river and lake habitats. At
each 1-m offset, vegetation density (number of vegetation
touches) was recorded at 30-cm intervals along a vertical 2-m
staff. In addition, dominant plant species were recorded
between each 1-m offset. Canopy cover (%) was estimated
using a gridded cylinder at 0, 5 and 10 m along each transect.
Width and depth of the main source of standing water was
noted. In wetland areas, the depth of standing water at each
transect offset was also recorded. Major sediment type was
recorded using a modified Wentworth scale (sand: 0.5–1 mm;
gravel: 2–8 mm; and mud: 3.9–62.5 mm in diameter) (Wentworth
1922).
Home-range estimation
Each animal location was calculated by plotting triangulated
bearings and using the Maximum Likelihood Estimator function
of LocateIII(Pacer Computing). The home range of each
animal was calculated using both the minimum convex polygon
(MCP) and the Kernel Density (fixed) method using Ranges VI
(Kenward and Hodder 1996). For comparison, estimates were
made using the neighbour-linkage analysis of RANGES
IV. Instead of calculating ellipses and contours based on densities,
this analysis aimed to minimise the summed distances between
points. The neighbour-linkage analysis removes the large ‘buffer
zones’around animal location points (Kenward et al. 2001).
Home ranges were also manually estimated on the basis of
observations that movements of the individuals were largely
restricted to the waterways and associated riparian vegetation.
The calculation assumed that the animals located along Gardner
Creek and Gardner Lake would be confined to the riparian
vegetation (which generally extends ~10 m from the waters’
edge) and that the entire area of each wetland in which animals
were located provided suitable habitat. The manual estimation of
home range therefore consisted of the length of river used
multiplied by 20 m plus the length of the lake shore multiplied by
10 m plus the area of any small wetland used.
Habitat preference
Four main habitat types were identified based on observations
made during trapping sessions: Riverine (R), Lacustrine (L) (i.e.
lake), Palustrine (P) (i.e. swamp) and Coastal Palustrine (CP).
Principal Component Analysis (PCA) and subsequent Analysis
of Similarity (ANOSIM) were conducted using PRIMER (ver.
5.0) (Clarke and Warwick 1994) to ensure that sufficient
differences existed between these sites and to confirm their a
priori classification. The R-statistic was used to indicate
significant differences between habitats. All data were log(x+1)
transformed and normalised before analysis. The PCA was also
used to determine which habitat variables differentiated the
habitat types.
The habitat used by each water rat was determined by
summing the number of location fixes within each habitat type.
The availability of each habitat type was determined through the
190 Australian Mammalogy P. C. Speldewinde et al.
transect data where each transect was classified as a particular
habitat type (total n= 114). A Chi-square analysis was conducted
using the frequency data for habitat used versus available for each
animal.
Results
Six individuals were captured (trap success of 1.9%), including
two females (mean weight = 317.5 42.5 (s.e.) g) and four males
(mean weight = 663.7 66.0 g). An insufficient number of fixes
were available for analysis for the individual identified as Male 1
and the two females. The combined trapping and radio-tracking
data from the three remaining males yielded 135 locations.
Male 2 and Male 3 were both recollared during the course of the
study due to faulty and dropped collars.
Average home-range size varied among estimation methods
(Table 1). The MCP and 50% Kernel methods estimated the
largest home ranges (45.6 12.5 ha and 42.8 13.7 ha,
respectively). The 95% and 80% kernel estimations were
32.0 19.7 ha and 27.3 8.6 ha, respectively. The neighbour-
linkage analysis and manual calculation method provided much
smaller estimates of home range (18.9 11.6 ha and 7.4 1.4 ha,
respectively) (Table 1; Fig. 2).
Habitat selection
ANOSIM indicated that all the a priori habitat types (Inland
Swamp, Coastal Swamp, River, Lake) were significantly
different (P0.03) from one another on the basis of structural
habitat attributes (Fig. 3; Table 2). The first two principal
components together explained 59.1% of the variation in the
original data matrix. The two palustrine habitats were
characterised by low-lying dense vegetation, sparse canopy cover
and narrow, shallow bodies of standing water. The riverine and
lacustrine habitats were characterised by higher levels of canopy
cover, higher average vegetation density above 30 cm and deeper,
wider bodies of standing water (Fig. 3; Table 3).
Ground cover in preferred habitats was dominated by mainly
sedge and grass species such as Lepidosperma gladiatum,
Meeboldinea scariosa,Juncus kraussii and Gahnia aristata.
Additionally, some taller species such as Taxandria juniperina
and juvenile paperbarks (Melaleuca ericifolia) were also
common.
The frequency of habitats used by two water rats differed
from that expected on the basis of availability (Male 3: c
2
= 35.8,
d.f. = 3, P<0.01; Male 4: c
2
= 689.0, d.f. = 3, P<0.01)whilst one
did not differ (Male 2: c
2
= 6.1, d.f. = 3, P= 0.11), possibly due to
low sample size. Males 2 and 3 used the coastal palustrine
habitat more frequently than expected on the basis of its
availability (Fig. 4), and neither used the lake or inland swamp
habitat. Male 4 preferentially used river habitat and was not
recorded in the coastal palustrine habitat (Fig. 4).
Discussion
This study represents one of only a few on the home range and
habitat use by H. chrysogaster in Western Australia. While a
limited number of studies have been conducted on eastern
Australian populations (Atkinson et al. 2008), differences in
environmental conditions, including habitat, limit the extent
that these findings can be generalised to Western Australian
Table 1. Home-range estimates (ha) for male H. chrysogaster at Two
Peoples Bay Nature Reserve, Western Australia
MCP95 represents the minimum convex polygon estimate at the 95% isopleth.
K95 represents the kernel density estimate at the 95% isopleth
Animal No. of
locations
Neighbour-
linkage (ha)
K95 (ha) MCP95 (ha)
Male 2 12 6.2 71.0 25.5
Male 3 48 8.4 17.4 42.8
Male 4 75 42.1 7.5 68.5
Mean ± s.e. 18.9 ± 11.6 32.0 ± 19.5 45.6 ± 12.5
LAKE
GARDNER
TWO PEOPLES BAY
GARDNER RIVER
N
0 250 500 1000 1500 2000 Meters
Fig. 2. Map of Gardner River (Two Peoples Bay Nature Reserve) showing approximate
area used by Male 3 (stipes) and Males 2 and 4 (crosshatch) (note use by Males 2 and
4 similar).
Water rats in south-western Australia Australian Mammalogy 191
populations. This has implications for management and
conservation of the species in south-western Australia, where
there has been substantial reduction in suitable habitat as a result
of both urban expansion and clearing of land for agriculture.
Knowledge of both habitat preferences and home range
represent key knowledge gaps for the implementation of effective
management and conservations efforts.
In riverine habitats, H. chrysogaster remains close to the water
and within the riparian vegetation (Fig. 2) (see also Harris 1978;
Serena and Gardner 1995). In the case of Gardner Creek, the
riparian zone extended ~10 m to either side of the creek. Estimates
of home-range size, based on neighbour-linkage analysis
(18.9 11.6 ha), are relatively large compared with previous
studies that describe home ranges of 1.2–11.5 ha (Harris 1978;
Serena and Gardner 1995). The manual calculation provided an
estimate much closer to those described in previous studies, as the
area calculated included only known suitable habitat. Most
studies have avoided these complications by reporting the lengths
of river, or river area (e.g. ha) utilised by individuals (Serena and
Gardner 1995). However, these calculations, like those of the
MCP method, can be strongly influenced by outlying fixes. MCP
and Kernel estimates of home-range size assume that the area
between individual locations is utilised by the animal. These
methods can overestimate home-range size when bends in the
river place estimates of home range boundaries well beyond the
terrestrial extent of the riparian vegetation. Our study included
only three individuals tracked over ~10 weeks so further studies
are required to provide a better understanding of home-range size
and use.
Our finding that H. chrysogaster prefers low-lying, dense
vegetation with little canopy cover supports previous
observations that the species is found only in wetlands with high
vegetation density (>50%) (Smart 2009). Similar observations
have been made for aquatic marsupials, such as the platypus
(Scott and Grant 1997). Selection of, and/or preference for, dense
vegetation close to the ground appears to be widespread among
small mammal populations (Stewart 1979; Lunney and Ashbey
1987; Ford et al. 2003), including H. chrysogaster (Scott and
Grant 1997), as these habitats provide shade, shelter, protection
and resources for prey species such as insects and frogs. Whilst the
water rat is mostly documented sheltering in burrows dug into
river banks (Serena and Gardner 1995), it is also known to use
reeds as nests, highlighting the potential importance of ground
5
0
–5
–5 0 5 10
PC1
PC2
River
Lacustrine
Palustrine
Coastal
Palustrine
Fig. 3. PCA ordination of Principal Components 1 and 2 on habitat data
collected at Two Peoples Bay Nature Reserve, Western Australia.
River Lacustrine Palustrine Coastal
Palustrine
Habitat type
90
80
70
60
50
40
30
20
10
0
Percentage available/used
% available % used male 2
% used male 3 % used male 4
Fig. 4. Habitat use of Males 2, 3 and 4 at Two Peoples Bay Nature Reserve,
Western Australia, showing the percentage of habitat available to male
H. chrysogaster and the percentage of radio-telemetry location fixes within
each habitat type.
Table 2. ANOSIM test statistics and significance levels demonstrating
differences between each of the habitat types used by H. chrysogaster at
Two Peoples Bay Nature Reserve, Western Australia
Groups RP
Global 0.389 0.01
River, Lake 0.343 0.01
River, Swamp 0.361 0.03
River, Coastal 0.363 0.01
Lake, Swamp 0.739 0.01
Lake, Coastal 0.876 0.01
Swamp, Coastal 0.386 0.01
Table 3. Eigenvectors displaying the main influences in each principal
component
Coefficients shown in bold contribute most to determining the difference
between habitat types present at Two Peoples Bay Nature Reserve, Western
Australia
Variable PC1 (44.7%) PC2 (14.4%)
Height at maximum density –0.357 –0.009
Mean density at 180 cm –0.353 –0.063
Mean density at 150 cm –0.396 –0.157
Mean density at 120 cm –0.423 –0.137
Mean density at 90 cm –0.418 –0.186
Mean density at 60 cm –0.365 –0.124
Mean density at 30 cm –0.206 0.255
No. of plant species –0.1 0.166
Mean canopy cover 0.083 –0.324
Width of standing water 0.117 –0.621
Depth of standing water 0.176 –0.565
192 Australian Mammalogy P. C. Speldewinde et al.
cover as alternative shelters for the species (B. Johnson, pers.
comm.). The selection of habitats with dense vegetation close
to the ground would provide protection against a variety of
predators, including foxes, cats and also raptorial birds (McNally
1960).
Permanent water is a critical component of water rat habitat
(Scott and Grant 1997; Valentine et al. 2009). A distinguishing
feature of the preferred habitats identified in this study, e.g. coastal
and inland swamps, was the presence of small (shallow and
narrow) bodies of standing water. This preference for palustrine
habitats has also been noted by Woollard et al. (1978).
Reclamation of swamp and wetland areas for development (Lee
et al. 2006), and increased groundwater extraction for urban
areas and agriculture are expected to represent significant
threats to remaining populations due to reduction of permanent
shallow water bodies. The loss of riparian vegetation (through
clearing and salinisation) occurring throughout Western
Australia (Hancock et al. 1996) is also a significant threat to
populations of H. chrysogaster. Habitat disturbance has been
implicated in the decline of a variety of other aquatic mammals
such as the otter (Barbosa et al. 2001) and the platypus (Grant and
Temple-Smith 2003). Considering the wide range of habitat
types in which H. chrysogaster has been observed (Atkinson
et al. 2008), an overall ‘species preference’would be difficult to
derive. Rather, when selecting vegetation types to support
H. chrysogaster, conservation priorities should focus less on
particular species and perhaps focus on the particular ‘structural’
characteristics that certain species exhibit (e.g. dense vegetation
along shallow water bodies). Although the results of this study
are preliminary they go some way to identifying critical habitats
for this species.
Acknowledgements
Thanks to Keith Morris and Brent Johnson from the Western Australian
Department of Environment and Conservation (Woodvale) for provision of
radio-tracking equipment and advice on project design. We are grateful to
several volunteer radio-trackers. This study was undertaken as an honours
project by MW under the supervision of the coauthors. Financial support for
MW was provided by the University of Western Australia Honours Research
program and we are grateful for the administrative support of the Centre of
Excellence in Natural Resource Management. All fieldwork was conducted
under approval from the UWA Animal Ethics Office RA/3/100/888 and DEC
permits SF007023 and CE002513. We would like to thank the anonymous
reviewers who made comments on previous versions of this manuscript who
made suggestions to improve the paper.
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