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RESEARCH ARTICLE
Current and predicted future distributions of wallabies in
mainland New Zealand
A. David M. Latham, M. Cecilia Latham and Bruce Warburton
Wildlife Ecology & Management, Manaaki Whenua Landcare Research, Lincoln, New Zealand
ABSTRACT
Bennett’s(Notamacropus rufogriseus) and dama (N. eugenii) wallabies
have been increasing in numbers and distribution in mainland New
Zealand. Here, we update their current distributions; estimate
current rates of spread to predict their future distributions; and
describe the extent of suitable habitat for each species. Current
distributions based on breeding populations and probable
distributions based on outlier confirmed sightings resulting from
natural dispersal and illegal liberations suggest that Bennett’s and
dama wallabies currently occupy between 5322–15,229 km
2
(532,200–1,522,900 ha) and 1865–4126 km
2
(186,500–412,600 ha),
respectively. In 50 years, best- and worst-case estimates predict
that they could occupy as much as 5883–44,226 km
2
(588,300–
4,422,600 ha) and 1912–40,579 km
2
(191,200–4,057,900 ha),
respectively. Habitat suitability was predicted to be high in the
North and South islands, except for areas of high elevation,
intensive agriculture with limited woody vegetative cover and
large urban centres. In the absence of widespread intensive
control, the ranges currently occupied by Bennett’s and dama
wallabies are predicted to increase by up to 7- and 20-fold
(respectively) in 50 years. As their distributions continue to expand,
they will become more difficult to control and their impacts more
widespread. We suggest that progressively containing wallabies
within increasingly smaller distributions and reducing their
numbers to minimise damage within those distributions should be
considered the top management priorities. If progressive
containment is feasible it could lead to eventual eradication.
ARTICLE HISTORY
Received 9 January 2018
Accepted 25 April 2018
ASSOCIATE EDITOR
Mandy Tocher
KEYWORDS
Bennett’s wallaby; dama
wallaby; habitat suitability;
invasive species;
Notamacropus eugenii;
Notamacropus rufogriseus;
pest control; range
expansion; unwanted
impacts
Introduction
Introduced wild mammalian herbivores pose a significant threat to native ecosystems and
production landscapes in many invaded areas (e.g. Pimental et al. 2000; Vázquez 2002; van
Wieren & Bakker 2008; Latham et al. 2017). Their feeding and activity habits impact native
vegetation, plantation forests, and agricultural and horticultural crops (Leader-Williams
1988; Williams et al. 1995; Vázquez 2002), as well as intensify soil erosion (Bayne et al.
2004; Gangoso et al. 2006). Their impacts are often most severe on island ecosystems
(Courchamp et al. 2003) and this is evident in New Zealand where the flora evolved in
the absence of mammalian herbivory (Greenwood & Atkinson 1977).
© 2018 The Royal Society of New Zealand
CONTACT A. David M. Latham lathamd@landcareresearch.co.nz
This article has been republished with minor changes. These changes do not impact the academic content of the article.
NEW ZEALAND JOURNAL OF ZOOLOGY
https://doi.org/10.1080/03014223.2018.1470540
Excluding moose (Alces alces), 22 mammalian herbivores have extant wild populations
in New Zealand following introductions that occurred mostly during the mid to late 1800s
and early 1900s (King 2005). Many of these species have been shown to cause significant
damage to agriculture and the environment (see Cowan 2005; Forsyth & Tustin 2005;
Norbury & Reddiex 2005; Nugent & Fraser 2005; Parkes 2005; and references therein).
Among these are five species of wallabies that have extant wild populations in New
Zealand following liberations that occurred in the late 1800s for private collections and
sport hunting (King 2005). Two species, Bennett’s wallaby (Notamacropus rufogriseus)
and dama (or tammar as it is commonly called in Australia) wallaby (N. eugenii), have
well-established populations in mainland New Zealand (Warburton 2005a,2005b) and
a further three species occur only on Kawau Island in the Hauraki Gulf, North Island
(Warburton 1986). One of the species now confined to Kawau Island was eradicated
from two islands (Rangitoto and Motutapu) in the Hauraki Gulf in the 1990s
(Mowbray 2002).
Bennett’s and dama wallabies have increased in numbers and distribution since their
initial releases, and like some species of macropods (kangaroos and wallabies) in their
native Australia (Caughley et al. 1987; Di Stefano 2004), they have had significant
unwanted impacts in some invaded areas. The primary impacts that Bennett’s and dama
wallabies have in New Zealand are damage to agriculture, seedlings within exotic planta-
tion forests (mostly Pinus radiata) and native vegetation. Bennett’s wallaby has been recog-
nised as a pasture pest since the 1940s (Warburton 2005b). The plant species that form the
bulk of their diet are the same as those grazed by sheep (McLeod 1986). Accordingly, if
wallabies are allowed to reach moderate-to-high densities (estimated at c. ≥2 per ha, or
level 3 on the Guilford Scale; an index based on wallaby faecal pellet density and sign
and sightings), they may reduce stocking rates of livestock (Warburton 2005b; NPCA
2015); although this relationship needs to be quantified. Dama wallabies are also primarily
grazers, with up to c. 70% of their diet comprising pasture species (Williamson 1986).
Although they are better adapted to living in forest interior than Bennett’s wallabies,
forest-dwelling dama wallabies tend to be smaller and have lower kidney fat reserves
than individuals with ready access to grassed clearings or pasture (Williamson 1986).
Bennett’s wallabies browse a number of native plant species (McLeod 1986), and can
contribute significantly to the depletion of forest understorey and prevent regeneration
of palatable species (Warburton 2005b). Although dama wallabies prefer edge habitats
they are able to live solely within native podocarp/mixed hardwood forests (Williamson
1986). In forested habitats with few grassy clearings, the diet of dama wallabies comprises
a high percentage of native plants. For example, Williamson (1986) reported that only 3%
of the diet of forest interior dama wallabies was composed of grasses, whereas native
kāmahi (Weinmannia racemosa) and māhoe (Melicytus ramiflorus) together made up
c. 50% of their diet. Knowlton & Panapa (1982) erected three permanent 20 m × 20 m
plots in the Lake Okataina Scenic Reserve, North Island. One plot excluded only dama
wallaby, one plot excluded red deer (Cervus elaphus) and dama wallaby, and one plot
was left open to both mammalian herbivores as a control. Results from these plots
showed that browsing of seedlings by dama wallabies prevented the regeneration of the
most palatable species and that plant species diversity was 57% higher where dama walla-
bies were excluded (Knowlton & Panapa 1982; Wallace & Wallace 1995; Wright 2017).
Benes (2001) and Wright (2017) concluded that the combined effects of dama wallabies
2A. D. M. LATHAM ET AL.
and red deer noticeably altered forest understorey composition and structure, and that the
regeneration of the most palatable plant species will require the removal or reduction to
low densities of both taxa. At high densities, wallabies can also impact soils and accelerate
erosion rates (Warburton et al. 1995).
The management of wallabies in New Zealand is the responsibility of affected regional
councils (Warburton 2005a,2005b) and all affected regions include wallabies in their
regional pest management plans (RPMP). These plans have objectives to keep wallabies
at low abundances, prevent spread outside of containment areas delineated by councils
and eliminate isolated populations outside of containment areas (Figure 1). Importantly,
the responsibility for wallaby control on private land lies with the landowner and regional
councils can issue ‘notices of compliance’to landowners when wallaby numbers on their
properties are assessed and found to be too high (e.g. if Bennett’s wallaby density is ≥level
4 on the Guilford Scale; NPCA 2015; G. Sullivan, Environment Canterbury, pers. comm.,
15 January 2016). The most common methods currently used for reducing wallaby
numbers include shooting (primarily from the ground) and poisoning using sodium fluor-
oacetate (compound 1080) or potassium cyanide in Feratox pellets deployed in bait
Figure 1. Historical and current distribution and confirmed sightings and kills (since 2000) of Bennett’s
wallabies in South Island and dama wallabies in North Island. The black dots represent the sighting and
kill locations used to estimate probable distributions using kernel density estimators (see Methods for
details); the white dots (Bennett’s wallaby only) represent outlying observations suspected to be illeg-
ally-liberated individuals. Containment areas were delineated by regional councils. They were designed
to contain all known breeding populations of wallabies and have geographic boundaries, such as large
rivers, urban areas and mountains that provide defensible boundaries against range expansion by wal-
labies. The details associated with confirmed wallaby sightings and kills were provided by regional
councils and the New Zealand Department of Conservation.
NEW ZEALAND JOURNAL OF ZOOLOGY 3
stations (Choquenot & Warburton 2006; Shapiro et al. 2011; NPCA 2015). If landowners
fail to manage wallabies in accordance with RPMP rules they can be subject to legal action
under the Biosecurity Act 1993.
As of 2016, Environment Canterbury’s annual budget for Bennett’s wallaby manage-
ment included c. $66,000 for compliance inspections, c. $16,000 for faecal pellet transect
monitoring and c. $20,000 for incursion response (G. Sullivan, Environment Canterbury,
pers. comm., 15 January 2016). Similarly, Bay of Plenty and Waikato Regional Councils
had a joint dama wallaby management programme with a combined annual budget of
c. $210,000, plus an additional contribution of c. $10,000 from the New Zealand Depart-
ment of Conservation (DOC) (G. Corbett, Bay of Plenty Regional Council, pers. comm.,
15 January 2016). This expenditure did not include control work paid for by landowners
affected by wallabies, for which we do not have estimates. The combined council and
agency budget of c. $320,000 per year for control and surveillance of wallabies in mainland
New Zealand, and additional landowner expenditure for control, have probably been effec-
tive at slowing their rate of spread, but it has failed to prevent them spreading via natural
dispersal from their designated containment boundaries (Figure 1). For example, Bennett’s
wallabies have used bridges and dams to cross the Waitaki River at the southern edge of
their containment area and established a breeding population that straddles southern Can-
terbury and northern Otago (Warburton et al. 2014). Moreover, the number of enforcement
notices issued to properties with higher than permissible numbers of Bennett’s wallabies
has on average increased since 2004 (none prior to 2004; 5.2 enforcement notices from
2004–2017; max. = 16 in 2015), despite comparable effort being put into compliance
inspections during that period (B. Glentworth, Environment Canterbury, unpubl. data).
Despite recent range expansions, no primary research has been done on either species
of wallaby on mainland New Zealand since the 1980s, and no reviews have been under-
taken since 2005 (Warburton 2005a,2005b). The objectives of this work were: 1. to
update the current distributions of Bennett’s wallaby and dama wallaby in mainland
New Zealand; 2. estimate current natural rates of spread and use these to predict future
distributions over the next 50 years; and 3. describe the extent of suitable habitat for
each species in mainland New Zealand.
Methods
Wallaby distributions and rates of spread
Historical and current distributions for Bennett’s and dama wallabies have been delineated
by affected regional councils (Environment Canterbury, Otago Regional Council, Bay of
Plenty Regional Council and Waikato Regional Council) based on qualitative and quan-
titative surveys conducted by them and DOC. We define these as ‘current distributions’of
known breeding populations of wallabies. However, since the early 2000s regional councils
and DOC have collated numerous confirmed sightings and kills of wallabies outside of the
current distributions and these records are likely the result of illegal liberations and/or
natural dispersal (Figure 1). As Lindström et al. (2013) point out, including outlying obser-
vations of individuals may be particularly important for estimating population rates of
spread and predicting future distributions, because these estimates will include individuals
dispersing at the invasion front, as opposed to residents located proximally to the front. In
4A. D. M. LATHAM ET AL.
this sense, omitting dispersing individuals may underestimate actual rates of spread at the
leading edge of the invasion front. Accordingly, we used 265 and 82 outlying sighting/kill
locations (all of which were assumed to have originated from natural dispersal) for Ben-
nett’s wallaby and dama wallaby, respectively (see Figure 1,‘Animals shot or sighted’), to
estimate the current ‘probable distribution’. These boundaries were estimated as the 95%
isopleth of kernel density estimators (KDE) around the outlying observations plus 200
random points drawn within the current distribution. The 200 points were used to
depict the area identified as currently occupied by wallabies and were randomly drawn
to avoid biasing the KDE towards specific areas within this distribution. The 2015 bound-
aries of the current and probable distributions represent best- and worst-case management
scenarios, respectively, from which to predict future distributions of wallabies.
We calculated natural rates of spread (m yr
−1
) for Bennett’s wallaby for each of the
three time periods for which distributions had been delineated: 1916‒1956, 1956‒1975
and 1975‒2015. Similarly, we used four time periods for which distributions had been deli-
neated for dama wallaby: 1947‒1954, 1954‒1979, 1979‒2000 and 2000‒2007. In addition,
we also calculated rates of spread between the latest (pre-2015) council delineated distri-
butions (1975 for Bennett’s and 2007 for dama) and the probable distributions constructed
using the additional confirmed outlying sightings and kills. Rates of spread for each period
were estimated by calculating the straight-line distance between polygon boundaries
depicting two consecutive time periods (e.g. 1916‒1956). Because range expansion does
not occur at the same rate around the boundary, we sampled the distance between the
two polygons every 100 m and then constructed a distribution of annual rates of spread
for each period. For this analysis we excluded all established populations that have resulted
from known illegal liberations (e.g. Bennett’s wallaby population at the northern end of
Lake Pukaki; Warburton et al. 2014).
We predicted wallaby range expansion under four natural spread scenarios: 1. low rate
of spread (1st quartile annual rate of spread) from the current distribution; 2. high rate of
spread (3rd quartile) from the current distribution; 3. low rate of spread (1st quartile) from
the probable distribution; and 4. high rate of spread (3rd quartile) from the probable dis-
tribution (see values highlighted bold in Table 1). We used 1st and 3rd quartiles instead of
the mean or median of the distribution of rates of spread because the former was inap-
propriate given that the data were right-skewed and the latter did not depict the large
rates of spread reported from some parts of the invasion front of both species. We used
the 2015 polygon boundaries (current or probable) as the starting distribution from
which to incrementally model annual spread. To include biological realism to the model-
ling process, we constructed layers that biased rates of spread in ArcGIS 10.2 (ESRI 2015).
These included a hard ‘barrier to movement’layer that completely prevented the extension
of wallaby range and included the following features: the South Island main divide (Ben-
nett’s wallaby only); major rivers; man-made canals in the Canterbury High Country
(Bennett’s wallaby only); large lakes; and large towns. However, we allowed spread to
occur through this barrier at bridges and dams, i.e. wallabies could extend their range
beyond barriers but this was biased towards and slowed by crossing features. In addition
to hard barriers, we also included a semi-permeable barrier for mountain ranges (exclud-
ing the main divide) ≥1500 m above sea level (a.s.l.). This layer allowed wallaby range
expansion to occur across these high elevations, but the rate of spread was set at half
that of elevations < 1500 m a.s.l. Using this modelling approach, we ran each of the
NEW ZEALAND JOURNAL OF ZOOLOGY 5
four scenarios previously described for 50 years to estimate Bennett’s wallaby and dama
wallaby distribution at 5, 10, 20 and 50 years into the future. Range expansion for each
wallaby species was modelled separately. Finally, we ran a fifth scenario to estimate the
impact of human-assisted spread on the future distribution of Bennett’s wallaby. In this
case, we included recent confirmed sightings and kills that occurred distally to the area
included in the probable distribution assuming they represented illegally-liberated estab-
lished populations (based on information from regional councils and DOC), and allowed
these to expand following the rules described above and using a high rate of spread. We
did not conduct a similar analysis for dama wallaby because the worst-case scenario
(expansion from the probable distribution at a high rate of spread) resulted in a distri-
bution that already included all extreme outlying observations.
Wallaby habitat suitability
The range expansion models described above depict future distributions at a given point in
time representing first-order selection (sensu Johnson 1980) by wallabies. These models
do not describe habitat use within predicted future distributions. To identify suitable
habitat for Bennett’s and dama wallabies and predict potential distribution in New
Zealand, we used Maxent 3.3.3 (Phillips et al. 2006). Maxent is a machine learning
approach for modelling species distributions from presence-only records. Maxent has
been shown to perform as well, or better than, other methods for modelling these kind
of data (Elith et al. 2006). In addition to presence data, Maxent uses a number of randomly
selected points (called ‘pseudo-absences’) that are then combined with biophysical covari-
ates to construct an index of habitat suitability for each cell ranging from 0 (least suitable
habitat) to 1 (most suitable habitat). Presence data for Bennett’s and dama wallabies were
obtained from three sources. First, locations of incidental observations or where animals
had been killed outside known wallaby distributions were obtained from regional councils
and DOC (Figure 1). Second, we randomly drew 300 and 267 locations for Bennett’s
wallaby and dama wallaby, respectively, from within their current distributions to
Table 1. Bennett’s wallaby and dama wallaby natural rates of spread (m yr
−1
) in mainland New Zealand
for three and four different time periods, respectively. Estimates were derived for the known historical
distributions as well as the ‘current’and ‘probable’(defined as the 95% kernel density estimator around
sightings and kill locations) distributions. Rates of spread for a given period were calculated as the
distance between the two distributions, measured every 100 m along the distributional boundary.
We estimate best-case (current distribution) and worst-case (probable distribution) rates of spread
(see descriptions in Methods). Values used to model wallaby range expansion under four different
scenarios are shown in bold: we refer to the 1st quartile as a low rate of spread, whereas the 3rd
quartile as a high rate of spread.
Wallaby species Distribution Time period No. of years Min. 1st quartile Median 3rd quartile Max.
Bennett’s Current 1916–56 40 0.3 318.8 697.3 1067.0 1703.5
1956–75 19 0.2 27.2 244.9 620.5 1555.3
1975–2015 40 0.0 16.5 61.3 181.7 647.5
Probable 1975–2015 40 178.0 353.5 515.0 827.8 1232.5
Dama Current 1947–54 7 246.7 521.7 657.9 1057.6 1857.1
1954–79 25 0.5 84.1 192.0 273.4 523.6
1979–2000 21 0.8 160.8 300.2 450.9 773.3
2000–07 7 0.0 3.1 3.5 92.9 976.9
Probable 2007–15 8 22.3 592.9 1106.6 1981.3 3883.8
6A. D. M. LATHAM ET AL.
represent occurrence within these areas. Finally, for Bennett’s wallaby we also included the
locations of 27 pellet transects surveyed since 1993 and a further 11 transects surveyed
since 2008, all of which recorded wallaby presence (G. Sullivan and B. Glentworth,
Environment Canterbury, unpubl. data). In total, we used 631 and 349 presence-only
records for Bennett’s wallaby and dama wallaby, respectively. Each wallaby species was
modelled separately.
Based on wallaby biology in New Zealand (Warburton 2005a,2005b), we identified 14
biophysical variables as potentially important predictors of habitat suitability (Table 2).
These variables were generated from three different datasets: LENZ (Land Environments
of New Zealand, Leathwick et al. 2003); LCDB (Land Cover Data Base v4.1, www.lcdb.
scinfo.org.nz); and a digital elevation model (DEM) of New Zealand (www.lris.scinfo.
org.nz). All these datasets were in raster format except for LCDB, which was converted
to a 25 m resolution raster prior to analyses. Biophysical variables were collected within
1km
2
(100 ha) cells; this cell size was chosen as a compromise between estimates of
home range size for wallabies in New Zealand (only available for dama wallaby:
c. 0.4 km
2
[40 ha]; Williamson 1986) and a practical unit size for conducting field
surveys (Karanth et al. 2009).
We assessed the performance of the Maxent models by determining how well the model
discriminated between suitable and unsuitable habitat over a range of thresholds. To do
this, we estimated the area under the receiver operating characteristic curve (ROC–
AUC), which represents the probability that a randomly chosen presence site will be
ranked as more suitable than a randomly chosen pseudo-absence site (Fielding & Bell
1997). A model with AUC = 0.5 performs no better than random, a model with an
AUC value between 0.7 and 0.8 has acceptable discrimination, and a model with an
AUC value > 0.8 has excellent discrimination and indicates that the model has high pre-
dictive power (Fielding & Bell 1997). An additional measure of performance is the
Table 2. Biophysical variables used to model habitat suitability for Bennett’s wallaby and dama wallaby
in mainland New Zealand using incidental observations in Maxent. Variables were collected from the
source layer within 1 km
2
(100 ha) cells. The last two columns show the relative contributions of each
biophysical variable included in each model. Models were fitted separately for Bennett’s wallaby and
dama wallaby. Variables with a high relative contribution to model fit are shown in bold.
Variable Source Pixel size (m)
Percent contribution
Bennett’s wallaby Dama wallaby
Average annual temperature (°C) LENZ 100 3.2 12.6
Average elevation (m) DEM 25 0.9 5.0
Average slope (°) LENZ 100 16.9 21.0
Distance to cover (m) LCDB 25 3.2 0
Distance to river (m) LCDB 25 0.2 0.8
Min. temperature of coldest month (°C) LENZ 100 48.9 33.0
% agriculture in flat areas
a
LCDB 25 0.1 0.1
% cover (forest, shrubs, tall tussock) LCDB 25 5.7 0.3
% forest LCDB 25 1.6 18.4
% pasture/grasses LCDB 25 0.4 4.3
% pasture/grasses in hill and high country
a
LCDB 25 18.0 2.7
% shrubs LCDB 25 0.5 0.2
% urban LCDB 25 0.2 0.3
% water LCDB 25 0.2 1.3
a
Flat areas are defined as those between 0 and 100 m a.s.l., hill and high country areas are defined as those above 100 m
a.s.l.
NEW ZEALAND JOURNAL OF ZOOLOGY 7
regularised training gain, which describes how much better the Maxent distribution fits
the presence data compared to a uniform distribution, where all cells have the same prob-
ability and a training gain of 0. The exponential of the training gain is a measure of how
many times higher the sample likelihood is compared to a random cell (Yost et al. 2008).
For example, if the gain is 2, it means that the average sample likelihood is exp(2) ≈7.4
times higher than that of a random background pixel.
Results
Wallaby distributions and rates of spread
Chronological distributions for Bennett’s wallaby and dama wallaby show significant geo-
graphical range expansion since early 1900s, and recent (post-2000) confirmed sightings
and kills outside of current containment areas indicate continuing expansion (Figure 1).
The predicted future distributions of Bennett’s and dama wallabies are shown in
Figures 2–3and Table 3. Currently, Bennett’s and dama wallabies occupy c. 5322 km
2
(532,200 ha) and c. 1865 km
2
(186,500 ha), respectively. Assuming low natural rates of
spread (Table 1) from the current distributions, future distributions for Bennett’s and
Figure 2. Predicted distributions of Bennett’s wallaby at five time periods using four different estimates
of rate of spread and two distributional polygons. A–B, Expand Bennett’s wallaby 2015 ‘current distri-
bution’using either a low (16.5 m yr
−1
) or a high annual rate of spread (181.7 m yr
−1
), respectively; C–
D, expand Bennett’s wallaby 2015 ‘probable distribution’using either a low (353.5 m yr
−1
) or a high
annual rate of spread (827.8 m yr
−1
), respectively; E, predicted distribution of Bennett’s wallaby includ-
ing six illegally-liberated outlying populations. The 2015 ‘probable distribution’was allowed to expand
using a high annual rate of spread (827.8 m yr
−1
).
8A. D. M. LATHAM ET AL.
Figure 3. Predicted distributions of dama wallaby at five time periods using four different estimates of
rate of spread and two distributional polygons. A–B, Expand dama wallaby 2015 ‘current distribution’
using either a low (3.1 m yr
−1
) or a high annual rate of spread (92.9 m yr
−1
), respectively; C–D, expand
dama wallaby 2015 ‘probable distribution’using either a low (592.9 m yr
−1
) or a high annual rate of
spread (1981.3 m yr
−1
), respectively.
NEW ZEALAND JOURNAL OF ZOOLOGY 9
dama wallabies are predicted to increase by c. 550 km
2
(55,000 ha) and c. 50 km
2
(5000 ha), respectively, over 50 years (Figures 2A,3A;Table 3). This represents an increase
of 10% for Bennett’s wallaby, but only a trivial increase (2.5%) for dama wallaby. Assum-
ing high rates of spread (Table 1) from the current distributions, distributions are pre-
dicted to increase to c. 9621 km
2
(962,100 ha) for Bennett’s wallaby and c. 3265 km
2
(326,500 ha) for dama wallaby (Figures 2B,3B;Table 3).
The probable distribution of Bennett’s wallaby is 14,135 km
2
(1,413,500 ha), which is
almost three times larger than the current distribution (Figure 2;Table 3). Assuming a
low or a high rate of spread from the probable distribution, Bennett’s wallaby range is pre-
dicted to increase in 50 years to 20,631 km
2
(2,063,100 ha; Figure 2C) or 28,447 km
2
(2,844,700 ha; Figure 2D), respectively. The probable distribution of dama wallaby is
4126 km
2
(412,600 ha), which is more than double the size of the current distribution
(Figure 2;Table 3). Assuming a low or a high rate of spread from the probable distribution,
dama wallaby range is predicted to increase in 50 years to 11,070 km
2
(1,107,000 ha;
Figure 3C) or 40,579 km
2
(4,057,900 ha; Figure 3D), respectively.
Including illegally-liberated established populations, the probable distribution of Ben-
nett’s wallaby is 15,229 km
2
(1,522,900 ha; Figure 2E). Predicting range expansion using
these populations as the starting range and a high rate of spread resulted in large range
expansions into northern Otago and mid-Canterbury (Figure 2E; Table 3). The distri-
bution in 50 years under this scenario is predicted to be 44,226 km
2
(4,422,600 ha) or
55% larger than under the maximum size predicted under a solely natural spread scenario
(28,447 km
2
[2,844,700 ha]).
Wallaby habitat suitability
The Maxent model for Bennett’s wallaby predicted abundant suitable habitat within most of
the eastern parts of the South Island and some suitable habitat in West Coast and Southland
regions (Figure 4A). The primary lower elevation areas from which Bennett’s wallabies are
predicted to be absent are associated with high production exotic grassland (e.g. dairy
farms) and larger urban centres. Similarly, the model predicted low habitat suitability at
Table 3. Size of predicted distributions (km
2
) for Bennett’s wallaby and dama wallaby in mainland New
Zealand. Distributions are predicted at five time periods using four different estimates of rate of spread
for each species and two different distributions, ‘current’and ‘probable’.
Wallaby species Type of distribution Rate of spread (m yr
−1
)
Year
2015 2020 2025 2035 2065
Bennett’s Current 16.5
b
5322 5395 5443 5553 5883
181.7
c
5322 5947 6477 7434 9621
Probable 353.5
b
14 135 14 925 15 703 17 018 20 631
827.8
c
14 135 15 949 17 444 20 257 28 447
Probable, IL
a
827.8
c
15 229 18 328 21 529 28 048 44 226
Dama Current 3.1
b
1865 1873 1881 1887 1912
92.9
c
2051 2165 2283 2508 3265
Probable 592.9
b
4126 4789 5489 6780 11 070
1981.3
c
4126 6335 8649 13 988 40 579
a
Probable, IL is predicted Bennett’s wallaby range expansion from the 2015 probable distribution using a high rate of
spread (827.8 m yr
−1
) and assuming spread by illegally-liberated established populations in addition to spread by the
main population within the current distribution.
b
Low rate of spread, corresponding to the 1st quartile of the distribution of rates of spread.
c
High rate of spread, corresponding to the 3rd quartile of the distribution of rates of spread.
10 A. D. M. LATHAM ET AL.
high elevations in the South Island’s mountains, with the caveat that most valleys provide at
least moderately suitable habitat for Bennett’s wallaby. Three variables (minimum tempera-
ture of coldest month, percent pasture in hill and high country, and average slope) had a
combined relative contribution of 84% (Table 2). The AUC (0.92) and Gain (1.31) values
indicated that the Maxent model for Bennett’s wallaby performed well.
The Maxent model for dama wallaby predicted abundant suitable habitat within most
central parts of the North Island (Bay of Plenty, Waikato and Manawatu–Wanganui),
much of Hawke’s Bay and Gisborne regions, and smaller, but not insignificant parts of
Northland, Auckland, Taranaki and Wellington regions (Figure 4B). High production
exotic grassland (e.g. dairy farms) was the primary variable responsible for dama
wallaby absence at lower elevations. The model also predicted low habitat suitability at
high elevations and around major urban centres. Four variables (minimum temperature
of coldest month, average slope, percent forest and average annual temperature) had a
combined relative contribution of 85% (Table 2). The AUC (0.96) and Gain (1.72)
values indicated that the Maxent model for dama wallaby performed well.
Discussion
Empirical estimates of rate of spread associated with the current distributions were highly
variable across time periods for both species. Additionally, historical rates of spread were
Figure 4. Predicted habitat suitability derived using Maxent, estimated using incidental observations of
wallabies (i.e. presence-only data). A, Bennett’s wallaby in South Island; B, dama wallaby in North
Island. Bright red and orange colours indicate areas predicted to be excellent to good wallaby
habitat (HSI = 1), pale blue indicates moderately suitable habitat and dark blue represents poor
habitat (HSI = 0).
NEW ZEALAND JOURNAL OF ZOOLOGY 11
usually far greater than more recent estimates. This could arise from three non-mutually
exclusive factors. First, rates of spread could have slowed recently as the wallaby invasion
front met unsuitable habitat. Second, control of wallabies, particularly at the invasion
front, may have been adequate to halt or minimise dispersal and thus population
spread. This seems unlikely (at least) for Bennett’s wallaby, because their distribution
and numbers have increased since the disbandment of the wallaby control unit in
South Canterbury in 1992 (Choquenot & Warburton 2006). Third, estimates of rate of
spread associated with the current distributions may be representative of movement by
individuals that reside on the proximal edge of the core breeding population, rather
than of movement by dispersers (that generally occur at low densities) at the invasion
front (e.g. Lindström et al. 2013). Interestingly, we found that recent rates of spread esti-
mated using the probable distributions were comparable to the earliest historical estimates,
suggesting that estimates using the current distributions do not accurately depict dispersal
behaviour/movement at the invasion front. Greater rates of spread than those used here
have been estimated for brushtail possums (Trichosurus vulpecula) in some areas
(e.g. c. 8400 m yr
−1
in Northland [Te Kao to Te Paki], 1983–late 1986; Clout & Ericksen
2000). Similarly, the estimated rates of spread for several wild ungulate species introduced
into New Zealand are either much larger (e.g. 8690 m yr
−1
, chamois Rupicapra rupicapra)
or comparable (e.g. 1770 m yr
−1
, red deer; 805 m yr
−1
, fallow deer) (Caughley 1963) to the
spread values we used. As with possums in New Zealand (Cowan 2005), dispersal of wal-
labies in Australia is strongly biased towards young males attempting to avoid male–male
competition (Eldridge & Coulson 2015), although the biological drivers of wallaby disper-
sal in New Zealand remains a knowledge gap.
High variation in estimated rates of spread created difficulty in determining appropri-
ate values with which to predict future distributions. We consider that projections using
low rates of spread (1st quartile) from the current distributions are unrealistic, as they do
not account for the large number of recent confirmed sightings and kills outside of the
current distributions. In the absence of widespread future control they are, at best, opti-
mistically low. It is also unclear whether it is more appropriate to model future wallaby
distributions using the current breeding distributions delineated by councils or (what
we term) the probable distributions (i.e. including the large number of confirmed sightings
and kills that occur outside council delineations). If these outlying locations represent
breeding populations, even at low densities, then the use of the higher estimated rates
of spread to model future distributions (Figures 2C or 2D,3C or 3D) may provide
more realistic projections. New surveillance methodology may help confirm the presence
of low density breeding populations outside of containment areas, and thus refine esti-
mates of rates of spread. In the absence of these data, we speculate that future wallaby dis-
tributions are likely best represented by Figures 2B or 2C,or3B or 3C. Although not
worst-case scenarios, these represent significant range increases for both species. For
example, in 50 years the distribution of Bennett’s wallaby is predicted to increase in
area by 81%–288% with respect to the currently delineated range. For dama wallaby,
the range increase would be between 75%–494%. If existing illegal liberations (assuming
they are established breeding populations) are included into the estimate for Bennett’s
wallaby, then the distribution at 50 years is predicted to increase in area by c. 730%.
Further evidence for potential large increases (under current control levels, which
appear insufficient to contain populations) in the distributions of both wallaby species
12 A. D. M. LATHAM ET AL.
comes from our habitat suitability modelling. Bennett’s wallaby has been confirmed south
of the Waitaki River (Warburton et al. 2014), and the availability of suitable habitat for
wallabies invading southwards suggests that their range will increase substantially in the
absence of effective control. There is also ample suitable habitat north of the current dis-
tribution of Bennett’s wallaby, so we would also predict continuing expansion in that
direction. The availability of suitable habitat for Bennett’s wallabies means that they
could occupy most of the South Island if allowed to expand. Similarly, the predicted con-
tinued expansion of dama wallabies into Mamaku and Horohoro, and invasion of places
like Oropi and Te Urewera, seems reasonable given that forest types around these
locations are similar to those already occupied by dama wallabies (Williamson 1986).
Although the beech and podocarp dominated forests on the upper slopes of Te
Urewera may be less suitable, lower altitude forests dominated by kāmahi (Weinmannia
racemosa) and māhoe (Melicytus ramiflorus) are likely to be suitable habitat for dama wal-
labies (Williamson 1986). The habitat suitability model for dama wallaby supports these
predictions, with this species likely to occupy most of the North Island (except for areas
heavily influenced by intensive agriculture) if not contained.
Our rate of spread models predict future distributions of Bennett’s and dama wallabies
at five different time periods. This is important for management agencies because it incre-
mentally illustrates the extent of each region likely to be impacted by wallabies if they are
not sufficiently controlled. However, these models were not designed to assess habitat pre-
ferences, occupancy or densities within future predicted distributions. We acknowledge
that the future predicted wallaby distributions shown here will probably overestimate
the actual area occupied by wallabies. For example, dama wallabies are believed to
avoid areas of intensive dairying with inadequate scrub/forest cover and thus those
should be subtracted from their predicted range, that is 15% (5983 km
2
[598,300 ha] of
40,579 km
2
[4,057,900 ha]) of their predicted range in 50 years. Even accounting for
this, dama wallaby are still predicted to increase by c. 1750% under a worst-case scenario.
It would have been useful to constrain predicted future distributions by estimated habitat
suitability to estimate actual area impacted by wallabies at various points in the future.
However, we did not do this because of the subjectivity in choosing an appropriate cut-
off value to translate habitat suitability into occurrence (Liu et al. 2005), and future
changes in land use that could alter the availability and extent of suitable wallaby habitat.
We used environmental data surrounding confirmed wallaby locations to estimate suit-
able habitat for Bennett’s and dama wallabies, and extrapolated this information to get an
indication of areas that may be affected by wallabies in the future. Because these analyses
used environmental data from wallaby presences (i.e. within the areas they currently
occupy), they are probably biased towards predicting similar habitats elsewhere in each
of the islands as being the most suitable. For areas that have not yet been invaded by wal-
labies it is more difficult to predict their suitability because of a lack of data. Thus, habitat
suitability models should be considered to represent the minimum area likely to be occu-
pied by wallabies in the absence of effective control. Moreover, because wallabies have not
reached their equilibrium distribution in New Zealand, predictions from the Maxent
model should be taken cautiously (Elith et al. 2010). One way of addressing these limit-
ations would have been to fit the suitability models with wallaby presence data from
their native Australia and use these models to extrapolate suitability in New Zealand.
However, many uninvaded habitat types in New Zealand differ substantially from those
NEW ZEALAND JOURNAL OF ZOOLOGY 13
in their native ranges, suggesting that such an approach may not be very informative;
although some environmental variables, such as variation in rainfall in Tasmania, could
be. Nevertheless, both models performed well (based on AUC), and thus we can be con-
fident that areas where wallabies are likely to occur have been highlighted. Wallabies are
likely to occur throughout the New Zealand mainland, with the exception of high
elevations, urban areas and high production exotic grassland (e.g. dairy farms). It is poss-
ible that suitable habitat might have been underestimated in high production exotic grass-
land because the lack of fine-scale vegetation data meant that refugia (patches of scrub,
hedgerows and riparian corridors) within these areas could not be easily identified.
However, the spatial and temporal resolution of our analyses should be sufficient to
guide wallaby management in both islands.
Populations of Bennett’s and dama wallabies currently persist outside of containment
areas and these, as well as confirmed new out-of-containment-area sightings and kills,
appear to be increasing. Based on moderate empirical estimates of rates of spread, and in
the absence of widespread sustained control or containment, the distributions of both
species are predicted to increase substantially over the next 50 years, resulting in further
impacts on agriculture, native vegetation and, potentially, contributing to soil erosion
within newly invaded areas. If allowed to increase in distribution and numbers, they may
have additional unwanted impacts through, for example, increased rates of collisions with
motor vehicles as occurs in their native Australia (e.g. Bond & Jones 2014). The current
levels of surveillance and control done by agencies and landowners is likely insufficient to
prevent further geographical spread of wallabies. Significantly more funding and a well-coor-
dinated strategy will be needed to achieve this. Effective management of these species will also
require surveillance, detection and control methods that are fit for purpose, and these areseen
as the highest priorities for the progressive containment of wallabies in New Zealand.
Acknowledgements
We thank personnel from Environment Canterbury, Bay of Plenty, Otago and Waikato regional
councils, and the New Zealand Department of Conservation for providing data and valuable
discussion. P. Cowan (Manaaki Whenua Landcare Research), E. van Eyndhoven (Ministry for
Primary Industries) and two anonymous reviewers provided invaluable comments on earlier
versions of the manuscript.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This study was supported by funding provided by the Ministry for Primary Industries, Manaaki
Whenua Landcare Research and Strategic Science Investment Funding.
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