Rangel. J 13(2) 1991,96-106
LONG-TERM EFFECTS OF WARREN RIPPING FOR RABBIT CONTROL
IN SEMI-ARID SOUTH AUSTRALIA
G.J. Mutze
Animal and Plant Control Commission, GPO Box 1671,Adelaide SA. 5001
Abstract
Warren ripping and poisoning were used to control rabbits on the flood-out plain of a major
creek system on Manunda Station, a sheep-grazing property near Yunta in semi-arid South
Australia. Rabbit numbers were initially reduced by >99 per cent, as indicated by the number of
active entrances remaining in rabbit warrens. After nearly 10 years without follow-up control
work, ripped warrens had only two per cent of the pre-control number of active entrances.
Poisoning effectively reduced rabbit numbers in the short-term, but had no long-term effect on
the number of active entrances, either in ripped or unripped warrens. Perennial shrubs regenerated
on and around ripped warrens. Warren ripping on this part of Manunda is a cost-effective
management option.
Introduction
Rabbits (Oryctolagus cuniculus) cause severe economic losses in the semi-arid pastoral zone of
southern Australia by competing with sheep for pasture (Cooke and Hunt 1987). Rabbit
grazing also prevents regeneration of palatable shrubs and trees, such as Acacia aneura (Wood
1984) and Acacia papyrocarpa (Lange and Graham 1983). Extreme economic and environmental
consequences could result if palatable, perennial plants are eventually lost from rabbit-infested
areas in the pastoral zone.
Rabbit numbers can be controlled in these areas by warren ripping (Parker et al. 1976, Martin
and Eveleigh 1979, Wood 1985) and poisoning (Cooke and Hunt 1987), but rabbit control is
rarely incorporated in standard station management because of doubt about its economic benefits
(Parer and Parker 1986). Ripping is cost-effective in some semi-arid areas where land tenure
allows increased stocking-ratesto take advantage of increased pasture availability (Wood 1985,
Cooke and Hunt 1987). However, the benefits accruing from control work also depend on the
period for which population levels remain low, before further expenditure is necessary to again
reduce rabbit numbers to acceptably low levels.
Even without rabbit control, arid zone rabbit populations undergo major fluctuations in
response to seasonal conditions. During severe droughts, as few as three per cent of warrens
may remain active in sandy country, inactive warrens being rapidly covered by drifting sand
(Myers and Parker 197517). Rabbits survive best during drought in large, deep refuge warrens
located where drainage from surrounding areas provides some plant growth in response to minor
rainfall events (Myers and Parker 1975a, 1975b). It is important to destroy such refuge warrens
during control work in arid areas, and indeed they were the main target in previous experiments
(Parker et al. 1976, Martin and Eveleigh 1979, Wood 1985). This paper reports on rabbit
coptrol conducted in an area of prime refuge warrens bordering an extensive flood-out plain, to
determine whether a single treatment can provide effective long-term rabbit control. Only Parer
and Parker (1986) have previously reported the long-term effect of rabbit control in arid areas.
Rabbit control in semi-arid South Australia is most effectiveat the end of summer, because dry
soil allows ripping to break up the warren structure very thoroughly, and starvation and
myxomatosis have usually reduced rabbit numbers to low levels (Cooke 1981, Cooke and Hunt
1987). At the start of this study the soil was very dry, but there was some green vegetation
available and rabbit numbers were still extremely high. Therefore, it was considered that
poisoning might be a necessary adjunct to warren ripping in order to prevent immediate
reinvasion of the warrens by rabbits living on the surface nearby.
Long-term effects of warren ripping for rabbit control
Materials and methods
Study site
Manunda Station is a sheep grazing property of 640 km2 near Yunta, South Australia. Under
leasehold agreements the Station is permitted to carry 10,000 sheep, a maximum average
stocking rate of 15.6 sheep/km2, but more normally it carries 7,000 - 8,000 sheep. The
majority of the property is undulating chenopod shrubland, dominated by bluebush, Maireana
sedifolia, black bluebush, Maireana pyramidata and saltbush, Atriplex spp. A major creek
system, Manunda Creek, with a catchment area of approximately 4,000 km2 in the hills
surrounding Yunta, runs through the centre of the property. Water flows in the creek only
briefly following heavy rain. When the creek does flow, water often spreads out over extensive
creek flats which are in places >1 krn wide. Such floods produce tremendous growth of weeds
and pasture so that the creek flats, although a small proportion of total station area, provide a
relatively large proportion of the potential carrying capacity.
Rabbits are present throughout the property but they are most common along Manunda Creek.
Huge warrens are dotted all along the low rises on the edge of the flood plain. There the rabbits
are able to take advantage not only of major floods, but also of minor flushes in vegetation
when creeks from the hills on either side of the flood plain irrigate small run-on areas on the
edge of the flood plain.
The study area stretched 2 km north-south along Manunda Creek and extended from the creek,
across the river flats, to the watershed on a small range of hills approximately 3 km to the east.
The area was divided according to the main topographic and vegetation characters into four main
habitats: river flats, lower slopes, upper slopes and stony hills (Fig. 1). The river flats support
an abundant and diverse array of exotic weeds, but within the study area the vegetation was
Fig. 1. Manunda study site showing
(a) habitat types: F, creek flats; LS,
lower slopes; US, upper slopes; H,
hills; and (b) treatment blocks A, B,
and C, within which warrens were
ripped, with poisoned areas stippled.
G.J. Mutze
predominantly Lincoln weed, Diplotaxis tenuifolia, dense patches of African boxthorn, Lycium
ferocissimum, native Sclerolaena spp. and lush medic pastures after floods. The lower slopes
are open Myoporum platycarpurn woodland with Sclerolaena spp., grasses and sparse chenopod
shrubs in the understorey; the upper slopes are dense chenopod shrubland with thickets of black
oak, Casuarina cristata, particularly towards the base of the hills; and the boundary between
upper and lower slopes was defined as the boundary of dense shrubland. On the hills, grassland
and sparse shrubs are interspersed with bare rock outcrops and black oak in the creeklines.
Botanical nomenclature used in this paper follows that of Jessop and Toelken (1986).
Assessment of rabbit numbers
Two methods were used to obtain indices of rabbit numbers.
(i) The number of rabbit warrens and burrow entrances.
Rabbit warrens were counted and mapped by walking transects across the whole study area,
except for a small area of impenetrable boxthorns on the river flats. Within each warren,
all open burrow entrances were counted and classified as: active, if they were free of wind
blown debris and cobwebs and/or had evidence of use such as tracks or fresh rabbit dung;
or inactive. Warren and burrow densities were obtained by dividing counts within each
habitat by the area of the habitat, as determined from aerial photographs.
(ii) Spotlight counts of rabbit numbers.
Set 400 m transects were marked out and rabbits were counted at night by spotlight from a
slow moving vehicle (approximately 8 km/h). All rabbits seen within approximately
50 m of the vehicle were included, but the field of view was restricted to c50 m in the
boxthorns on the flats. Visibility of rabbits also was lower in 1989 than 1980 because of
the increased vegetative cover. Warrens in an area immediately to the north of the study
area, left as an untreated experimental conuol, were ripped a year later by the station
manager. Therefore, in 1989, for comparison with the treated area, transect counts of
rabbits were made in a similar unripped area on the flats and lower slopes to the south of
the study area. Furthermore, the original transects within the ripped area could not be
relocated in 1989, so the 1989 counts only approximately match those of 1980 for all
sites.
Rabbit control
Rabbit control was conducted on the heavily infested river flats and lower slopes, and also on
the small portion of the upper slopes that extended into the creek paddock. The main objective
of the control work was destruction of the huge warrens on the lower slopes. Poisoning was
used in some treatments to test the importance of low rabbit numbers for preventing warren
reopening. The experimental area was divided into three blocks and treatments were applied as
in Fig. 1; A, poisoned and ripped, and also poisoned on the adjacent flats; B, ripped only, but
poisoned on the adjacent flats; and C, ripped only. Within the treatment blocks, 18 warrens
were left unripped, including seven accidentally missed or inaccessible small warrens and 11
relatively large warrens on the boundary of the treatment blocks and the flats, which were
omitted by the ripper operator. These 18 warrens were used as experimental controls to assess
the effect of ripping.
The rabbits were poisoned on 20 March 1980, using oats treated with 1080 (monosodium
fluoroacetate) at 0.04 per cent w:w, laid at 3 kg/km from a vehicle mounted baitlayer, in a
furrow, in areas where rabbits were active. Three pre-feeds of unpoisoned oats were laid at
approximately three day intervals before poisoning. Ripping was conducted between 5 March
and 3 April, poisoned areas being ripped only after poisoning. Warrens were cross-ripped at 1 m
intervals with a single-tine ripper mounted behind a 36 kW, two-wheel drive tractor used for
grading roads on the property.
Long-term effects of warren ripping for rabbit control
The long-term effect of treatment on the final number of active burrow entrances remaining in
each warren was tested by a generalized linear model using the initial number of active entrances
as a covariate. An initial model was fitted with poisoning as a treatment factor of three levels
(corresponding to the treatment blocks A, B and C), with ripping as treatment factor of two
levels (ripped or not ripped), and including all possible interaction terms. Non-significant terms
were then sequentially deleted from the model, beginning with highest order interaction terms
and those with low estimate : s.e. ratios. The test for significance when considering each
deletion used the error mean square and error degrees of freedom from the model remaining after
the previous deletion. Data were log-transformed before analysis because of the exponential
relationship between active burrow entrances and rabbit numbers (Parer 1982), and because the
multiplicative effect of poisoning and ripping made this the most appropriate model (see Cooke
and Hunt 1987).
If, following recolonization of ripped warrens over a number of years, rabbit numbers are
temporarily low because of adverse seasonal conditions, the total number of open burrow
entrances (active plus inactive) may provide a better estimate of long-term recolonization rates
than active entrances. Therefore, the above analyses were repeated using total open entrances in
place of active entrances.
Photographs of general views across ripped warrens were taken to monitor warren
recolonization and vegetation changes after rabbit control. Photopoints were marked with
hardwood pegs for ease of location.
Seasonal conditions
Rainfall at Manun& homestead in 1979, the year before control work began, was slightly
above the 200 mm average (Table 1). Heavy rain at the beginning of that year produced good
pasture growth, but only 64 mm of rain fell in the six months immediately preceding control,
by which time the soil was very dry and much of the pasture had dried off. Annual rainfall
recordings exceeded the long-term average in seven of ten years following the control work
(Table 1). During that period the creek flowed on 18 occasions and probably inundated the
flood-plain most of those times (G. Shephard, personal communication). Sheep were removed
from the study area for ten years following rabbit control, but some grazing was continued by
red kangaroos (Macropus rufus), grey kangaroos (M. fuliginosus), euros (M. robustus) and feral
goats (Capra hircus).
Table 1. Rainfall at Manunda Station from 1979 to 1989, and long-term mean rainfall.
Year
mean 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989
Rainfall (mm) 200 231 221 221 78 228 221 136 181 256 235 395
Results
At the beginning of the study the lower slopes of the study site adjacent to the flood-out flats of
Manun& Creek supported a large number of enormous rabbit warrens (Fig. 2). The average
density was almost 40 open burrow entrances per ha. On the creek flats there was a greater
number of warrens, but of smaller size, and the average density was 10 open burrow entrances
per ha. Warrens on the upper slopes and hills were much sparser and moderately small (Table
2).
Before treatment, rabbit numbers on the flats and lower slopes were very high. On the flats
many rabbits were living on the surface, taking shelter under boxthorn bushes. This is
G.J. Mutze
Table 2. Pre-control density of rabbits in four habitats on Manunda Station
Area
Flats
Lower slopes
Upper slopes
Hills
(ha)
Warrens
total
Warrens
per ha
97
86
269
118
182
100
63
33
1.9
1.2
0.2
0.3
Entrances/warren
total active
Entrancesfia
total active
5.6
33.0
10.9
9.4
10.5
38.4
2.6
2.6
4.1
23.5
7.0
6.2
7.7
27.3
1.6
1.7
Entrances
(% active)
74
71
65
66
Fig. 2. Distribution of rabbit warrens on the Manunda study site before treatment. The
shaded area contained impenetrable boxthorn thickets and could not be surveyed. Solid lines
represent creekbeds unless otherwise indicated.
emphasised by the high number of rabbits seen on the flats (Table 3) in relation to the number
of active burrow entrances (Table 2).
Effects due to poisoning
Transect counts indicate that poisoning provided effective short-term reduction of rabbit
numbers on the flats (Table 3). In the long-term, however, for the 87 ripped and 18 unripped
warrens between the flood out flats and the fenceline of the creek paddock (areas A, B and C,
Fig I), poisoning had no effect on the number of active entrances in ripped or unripped warrens
(poisoning F2,99= 0.7, NS; poisoning * initial active entrances F2,97
= 0.9, NS; poisoning *
ripping F2,95= 0.5, NS; poisoning * ripping * initial active entrances F2,93= 2.49, NS).
Long-term effects of warren ripping for rabbit control
Table 3. Spotlight transect counts of rabbits at Manunda. Data are shown as mean rabbits
sighted per km travelled (#ban),with bracketed numbers indicating the number of different
nights on which the4nx 400m transects in each habitat were counted to obtain the mean value
shown.
Area
Pre-treatment
1980
Mar
(#/km)
(#/w
2
2
419 (2)
12 (3)
26 (2)
13 (1)
8 (1)
5
2
4
81 (1)
8.5 (1)
0 (3)
103 (3)
2 (2)
81 (2)
7 (1)
Transects
(n)
Post-treatment
1980
1980
May
Aug
1989
Sep
(#/km) (#/km)
Flats
Poisoned
Outside of study area
Lower slopes
Treated
Untreated
Outside of study area
59 (1)
The eflect of ripping warrens
Ripping of warrens, either with or without poisoning, was extremely effective for reducing
rabbit numbers. Eight weeks after control work was completed only 1512882 burrow entrances
in ripped warrens had reopened, 211257 in block A, 101662 in block B and 31963 in block C,
and no rabbits were sighted in spotlight transects in the ripped area (Table 3).
Ten years after treatment, the mean number of active entrances in the ripped warrens was still
only two per cent of pre-treatment levels (Table 4). Only 7 of 87 ripped warrens were active,
while a further five warrens had either one or two open burrow entrances but were inactive. By
contrast, unripped warrens within the treatment area had only slightly fewer open entrances than
before treatment (Table 4). Of these, most of the 11 warrens on the edge of the flats appeared to
have been covered by fast flowing water during a major flood six months before, and were
probably still being re-established at the time of the final survey.
For individual unripped warrens, the final number of active entrances was closely related to the
initial number of active entrances. The number of active entrances in reopened ripped warrens
also was positively correlated with the initial size of the warren, but ripping had greatly reduced
the number of active entrances present (regression slopes from generalized linear model:
unripped = 1.04 k 0.13 (s.e.), ripped = 0.10 0.05; difference between slopes term, ripping *
initial active entrances Fl,lol = 49.5, Pc0.001; R~ = 0.75). When the analysis was repeated on
changes in the total number of open entrances, rather than active entrances, the results were
substantially unchanged. All poisoning terms were non-significant, but ripping * initial burrow
entrances was highly significant (Fl,lol= 30.8, Pc0.001).
+
A large proportion of the burrow entrances re-opened in the ripped warrens were beyond the end
of the rip marks on the original warren. One warren of more than 100 entrances originally
straddled the fenceline, and it was possible to rip only the half of the warren on the western side
G.J. Mutze
of the fence. Although the eastern portion of the warren remained active, only two active
entrances were re-established in the ripped portion of the warren.
Changes in rabbit numbers outside of the ripped area
At the time of the final survey in October 1989, warrens on the flats were extremely difficult to
locate because of the dense annual vegetation, but there appeared to be fewer warrens than in
1980. A few small warrens were located and 27 per cent of the burrow entrances in these
warrens were active. Only 50 per cent of the warrens in the upper slopes and hills area were
resurveyed in 1989 (Tables 2,4), but this was sufficient to show that there had been little
change in rabbit numbers during the ten year period. In the resurveyed warrens of the upper
slopes there had been an eight per cent decrease in total number of burrow entrances but a small
increase in the proportion of these which were active, so that rabbit activity remained more or
less the same. In the hills, the total number of burrow entrances decreased by ten per cent and
number of active entrances decreased by 12 per cent, indicating that a small decline had occurred
in rabbit activity.
Table 4. Size of rabbit warrens and activity level of burrow entrances at Manunda, in warrens
which were surveyed before and 10 years after control work.
Habitat
Ripping
treatment
Warrens
Before control
After control
(entrancesharren) (entrancesharren)
Total
Active Total
Active
Change
(W
Total
Active
Treatment area (mainly Lower slopes)
block A
Ripped
Not ripped
block B
Ripped
Not ripped
block C
Ripped
Not ripped
A+B+C
Ripped
Not ripped
Upper slopes
Hills
Vegetation changes
Following removal of the rabbits considerable regeneration of palatable shrubs occurred on and
adjacent to the ripped warrens. Regenerating species included Maireana pyramidata, Maireana
brevifolia, Myoporum platycarpurn and Atriplex spp. Regenerating Maireana was up to 1 m
tall, and sufficiently dense to almost obscure the warrens in some cases. From photopoints it
102
Long-term effects of warren ripping for rabbit control
Fig. 3. Photopoint number 17 (a) in May 1980, showing two large, ripped warrens in the
foreground, and (b) in October 1989, showing the extensive shrub regeneration following
removal of sheep and rabbits. The standing tree in (a) has fallen to the left, and the hills on the
left horizon in (b) are faintly visible on the original slide of (a).
103
G.J. Mutze
was also evident that, near the southern end of the study area, the boundary of the dense
chenopod shrubs (i.e. that originally used as the boundary between the upper slopes and the
lower slopes) had shifted more than 100 m towards the creek (Fig.3).
Discussion
Warren closure and subsequent reopening
The rate of reopening of ripped warrens recorded in this study is extraordinarily low, compared
to that recorded in previous studies. In sandy soils in western New South Wales, 62 per cent
(Wood 1985) and 30 per cent (Martin and Eveleigh 1979) of warrens were reopened within six
months of ripping. Comparatively fewer, 15 per cent, were reopened in steep areas of the
southern Flinders Ranges in South Australia (Cooke and Hunt 1987). On Byrnedale Station,
near Broken Hill, rabbits were virtually eliminated by a ten year programme of warren
destruction (Parker et al. 1976); however, failure to conduct follow-up work allowed rapid
reinvasion from surrounding properties, and the number of open burrow entrances reached 50
per cent of pre-control levels in the space of only three good seasons (Parer and Parker 1986).
Nevertheless, the current owner of Byrnedale, Mr. Colin Caskey, believes that the published
records indicate a more rapid rate of reinvasion by rabbits than is normally the case on his
property. He estimates that in an average five year period only ten per cent of the original (precontrol) number of burrow entrances are re-established, and he uses a periodic ripping
programme to keep rabbit numbers low (C. Caskey, personal communication). This level of
reinvasion is still substantially higher than at Manunda.
The reasons for the low rate of warren reopening on Manunda are not clear, but in the short
term, may have been related to very efficient warren destruction (see Cooke and Hunt 1987).
The failure to reopen burrow entrances in the ripped portion of warrens, even where only half of
a large warren was destroyed, indicates that the soil texture probably made burrow establishment
difficult. Conditions were very dry during ripping and the soil was churned into a fine powder,
into which the tractor sank to its axles and often bogged. As a consequence, the ripping tine
penetrated to a considerably greater depth than the nominal 45 cm.
At the time that warrens were ripped, European rabbit fleas, Spilopsyllus cuniculi, were
released near the study area. Following the introduction of rabbit fleas at Belton and Witchitie
Station, 100 km north-west of Manunda, the effectiveness of myxomatosis was greatly
enhanced and rabbit numbers remained low for ten years (Cooke 1983). Rabbit fleas are now
well established at Manunda, and may have indirectly limited rabbit numbers sufficiently to
restrict the long-term re-establishment of ripped warrens.
Economics of control
The data show that long-term rabbit control by warren ripping is potentially a cost-effective
management tool on Manunda. Assuming one rabbit for every two active burrow entrances
(Parer 1982) and 12 rabbits as one sheep equivalent (B.D.Cooke, unpublished data), grazing
pressure from rabbits in the ripped warrens was equal to about 1.0 sheeplwarren or 1.1
sheepha, constituting seven times the average stocking rate for the property. Each warren took
approximately 30 minutes to rip with the old tractor used, and the work was laborious because
of frequent bogging. At current rates, contract ripping with a crawler tractor would have cost
about $10 per warren, as the warrens were very large but close together (see Cooke and Hunt
1987). If the rabbit infestation at the time of ripping was indicative of the average rabbit
grazing pressure, an extra 1.0 sheep per ripped-warren could have been grazed for the 10 years
following control, without further expenditure. Using an annual gross margin of $12 per sheep
(M. Michelmore, personal communication), costs would have been recovered in the first year.
Further benefits would have accrued beyond the ten years because rabbit numbers were still low.
Long-term effects of warren ripping for rabbit control
In addition, the large refuge warrens on the lower slopes were probably the breeding source from
which most of the rabbits on the flats originated. During the initial survey most of the small
warrens on the flat appeared to be shallow, and it is probable that many were destroyed by
floods during the study period and temporarily re-established during periods of high rabbit
numbers. If most of the surface-living rabbits on the river flats dispersed to there from the large
warrens on the adjacent lower slopes, the above estimate of the competitive grazing pressure
exerted by rabbits from the ripped warrens is conservative.
Substantial increases in stocking rates following rabbit control have been recorded on other
stations in South Australia. Most recently, the control of rabbits on Arkaba Station, initiated
by Cooke and Hunt (1987) and continued by the landholder, allowed sheep numbers to be
increased by 3,000 for a cost of $150,000 (L. Hunt, personal communication). Again,
assuming a gross margin of $12 per sheep, the work yielded a return of about 24 per cent on
investment. Manunda Station is usually lightly stocked relative to the maximum stocking rate
allowed under leasehold conditions. This would have allowed an increase in stock carried to take
advantage of the extra pasture available after rabbit control. However, the new manager chose
instead to destock the area and allowed the vegetation to regenerate.
Regeneration of shrubs
The regeneration of perennial shrubs on and adjacent to the ripped warrens has been substantial.
This cannot be unambiguously attributed to rabbit control because warren ripping coincided
with destocking of the area as well. It is not known what impact continued grazing by
kangaroos and feral goats had, or whether regeneration would have occurred after rabbit control
if the sheep stocking rates had been maintained at previous levels, or if they had been increased
to take advantage of extra pasture availability. However, rabbit control on Arkaba allowed,
simultaneously, both rapid cost recovery and improvement in range condition (L. Hunt,
personal communication). Because of their selective grazing of seedling shrubs, even low
density rabbit populations are able to completely prevent regeneration of palatable shrubs
(Lange and Graham 1983). Therefore, it is reasonable to assume that shrub regeneration was not
possible with the high rabbit numbers present before control. In reviewing range regeneration
programmes in Western Australia, Hacker (1989) concluded that the worst degradation of
rangelands in the arid winter rainfall zone occurred along the floodplains of major rivers that
were formerly occupied by chenopod shrublands, and that regeneration in response to exclusion
of grazing would generally be unacceptably slow. This study clearly shows that rapid chenopod
shrub regeneration can sometimes occur in such areas when severe grazing pressure is removed.
Regeneration of palatable shrubs is vital for the conservation of semi-arid shrublands, to
maintain their productivity during drought and to prevent soil erosion. Grazing competition
from the few rabbits surviving in refuge warrens during drought also may be critical to droughtproneness of the pastoral enterprise, because refuge warrens occur in the drainage run-on areas
which provide the best growth of annual plant species during periods of low rainfall.
Previous authors have argued that warren ripping is economically worthwhile in heavily rabbitinfested, semi-arid, sheep-grazing areas with sandy soils (Wood 1985) and steep, stony hills
(Cooke and Hunt 1987). The results from Manunda provide further evidence that, in some semiarid areas, it can be highly cost-effectivein intermediate soil types.
Acknowledgments
I thank the owners, the Duncan family, and managers, Garnharn Skipper and Garry Shephard, of
Manunda Station for their willing cooperation and for the use of station property and facilities
for this work. Tony Adams and Ros Watkins were co-workers in the initial survey and control
work, Murray Whitehead assisted with the final survey and David Chinner laid the poison trails.
I also thank Brian Cooke for advice throughout the study, and Brian Coman, Bill Low and an
anonymous reviewer for helpful comments on previous drafts of this manuscript.
G.J. Mutze
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Manuscript received 26 April 1991, accepted 11 September 1991.