Table Of ContentAustral Ecology (2004) 29, 585–599
Survival and recruitment of seedlings and suckers of trees
and shrubs of the Australian arid zone following habitat
management and the outbreak of Rabbit Calicivirus
Disease (RCD)
ANDREW J. DENHAM* AND TONY D. AULD
New South Wales National Parks and Wildlife Service, PO Box 1967, Hurstville NSW 2220, Australia
(Email: [email protected])
Abstract Recruitment in many shrub and tree species in arid Australia is limited by domestic and feral animal
grazing. The outbreak of Rabbit Calicivirus Disease (RCD or RHD) in late 1995 at Kinchega National Park (NP)
in arid western NSW, Australia, coincided with intensified warren ripping and other rabbit (Oryctolagus cuniculus)
control efforts. We monitored changes in rabbit numbers in response to these factors using spotlight counts and
dung-pellet counts. Rabbit populations declined initially, and were low for the remainder of the study. To assess the
impact of rabbit control on vegetation, we examined whether recruitment of seedlings or vegetative suckers was
occurring at Kinchega NP, following the arrival of RCD; how long such recruits were likely to survive under current
grazing pressures; and whether the magnitude of current seedling and vegetative sucker recruitment and survival is
greater than available data prior to RCD and extensive warren control. We followed the survival of individual
seedlings and newly emerged suckers of six shrub or tree species, most of which are considered to be at risk of local
decline. Three levels of caging were used to exclude all mammalian grazers; all mammalian grazers except rabbits;
and no mammalian grazers. Pooled cohorts of seedlings and suckers were analysed using failure-time analyses.
During the course of the study, suckering was observed in four species (Acacia carneorum, Alectryon oleifolius,
Casuarina pauper and Santalum acuminatum), with seedlings observed in the other two species (Dodonaea viscosa
and Acacia ligulata). Survival of suckers of some species was slightly greater than pre-RCD estimates, but in general,
a pattern of little or nil recruitment is likely in suckers exposed to rabbits or to all grazing mammals. Seedling survival
was particularly low regardless of the level of exclusion, largely due to desiccation. Reduction of grazing impacts
may only allow recruitment into populations of species reliant on seedlings under more favourable climatic
circumstances than experienced in this study, while the probability of successful recruitment into populations of
suckering species in western NSW continues to be low even at very low rabbit densities.
Key words: demography, grazing, rangeland, RCD, recruitment, survival analysis.
INTRODUCTION that recruitment events may occur continuously at low
levels or episodically over time scales that are difficult
Tall shrubs and trees are an important component of to evaluate (Watson et al. 1997a,b). Thus many reports
vegetation in arid and semiarid Australia, providing on the population viability of such species rely on
habitat and environmental heterogeneity that allows the anecdotal evidence (Griffin & Friedel 1985), chance
persistence of associated floral and faunal biodiversity observational data (Read 1995), or size frequency
(Aguiar & Sala 1999 and references therein; Facelli & distributions (Crisp & Lange 1976; Crisp 1978; Friedel
Brock 2000; Pugnaire & Lázaro 2000). There are 1985; Woodell 1990). Attempts to overcome these
concerns, however, that recruitment in these species inadequacies include combining size frequency
has been limited or nonexistent for many decades, distributions with observations on seedling or sucker
resulting in long-term declines and local extinctions as survival over the short term (Auld 1995a; Watson et al.
the existing plants senesce and die (Hall et al. 1964; see 1997a; Yates & Broadhurst 2002), increasing the
below). Uncertainty in the degree of recruitment size or the duration of the study, increasing the
needed to maintain populations lies in the long-lived frequency of observations or modelling populations
nature of many of these species, and the observation using data obtained over short time scales (Wiegand
et al. 2000).
*Corresponding author. Grazing by rabbits (Oryctolagus cuniculus (L.)) has
Accepted for publication January 2004. long been considered a primary cause of limited shrub
586 A. J. DENHAM AND T. D. AULD
and tree recruitment in semiarid and arid Australia 2. Is recruitment of seedlings or vegetative suckers
(Johnston 1969; Lange & Graham 1983; Silander occurring at Kinchega NP, following the arrival of
1983; Friedel 1985; Auld 1990, 1993, 1995a,b; Cohn RCD?
& Bradstock 2000). More recently, Eldridge and Simp- 3. How long are recruits likely to survive under
son (2002) found that rabbit warrens resulted in current grazing pressures?
changes to soil structure and plant and cryptogram 4. How does the current level of seedling and vege-
cover, favouring introduced weedy species over native tative sucker recruitment and survival compare
perennial species. Other feral grazers, domestic stock with available data on survival at Kinchega NP
and kangaroos are also reported to have had an impact prior to both extensive warren control and the
on shrub and tree recruitment (Lange & Purdie 1976; outbreak of RCD?
Silander 1983; Brown 1985; Wisniewski & Parsons
1986; Pickard 1993; Tiver & Andrew 1997; Cohn &
Bradstock 2000; Letnic 2000; Hunt 2001).
METHODS
The escape of the Rabbit Calicivirus Disease (RCD,
otherwise known as Rabbit Haemorrhagic Disease,
RHD) onto mainland Australia from an offshore island Study Area
in October 1995 (Pech & Hood 1998) and its rapid
spread to arid and semiarid areas, including Kinchega Kinchega National Park (32(cid:2)28(cid:3)S 142(cid:2)20(cid:3)E) lies
National Park (NP), has created renewed interest in adjacent to the Darling River in western NSW,
rabbit control and shrub regeneration. At Kinchega NP, Australia, 113 km south-east of Broken Hill. The park,
a rabbit control program including warren ripping covering 44 182 ha, was gazetted in 1967.
(using a deep plough to destroy burrows) commenced
in 1995, and continues whenever resources are
Climate
available. To date, approximately 25 000 warrens have
been ripped in an effort to reduce suitable habitat for The climate at Kinchega NP is one of low, erratic
rabbits and thus reduce recolonization rates (Foran rainfall with no predictable seasonality, combined with
et al. 1985; Mutze 1991) between outbreaks of RCD. high summer temperatures often in excess of 40(cid:2)C
To monitor changes in vegetation and recruitment of (Wellard 1987). Rainfall at nearby Menindee (average
long-lived shrubs as a result of reduced rabbit numbers, 244 mm per annum, Commonwealth Bureau of
a series of monitoring plots including exclosures was Meteorology, Melbourne, 2002) over the study period
set up at Kinchega NP in December 1996. In this was typically variable, with large falls in the summers
paper, we report the results of studies of the survival of of 1997–1998 and 1999–2000, and a number of above
vegetative suckers and seedlings in six long-lived average monthly records (Fig. 1). Of the 7 years pre-
species since 1996. We ask: sented in Figure 1, two had well above average rainfall
1. Have rabbit densities declined following the (1996 and 2000), 1997 and 1998 were close to average,
outbreak of RCD and extensive warren control 1999 was below average, while 2001 and 2002 were
measures? relatively dry with only 152 and 107 mm, respectively.
Fig. 1. Rainfall data for Menindee (source, Commonwealth Bureau of Meteorology, Melbourne). Bars, actual rainfall; solid
line, mean monthly rainfall; dotted line, median monthly rainfall.
REGENERATION OF ARID TREES AND SHRUBS AFTER RCD 587
Vegetation in an area where the target species dominated the
overstorey and included some suitable habitat into
The vegetation of Kinchega NP includes a number of
which the target species could spread.
broad communities. The most widespread of these are
Eucalyptus largiflorens open woodland on floodplains of
the Darling River and Tandou Creek; low open shrub-
Exclusion treatments
lands of Maireana pyramidata on ancient weathered
dunes bordering the floodplain; open shrublands of
An exclusion experiment was established to determine
Acacia spp., Eremophila sturtii, Dodonaea viscosa ssp.
whether survival of seedlings or vegetative suckers was
angustissima and Senna artemisioides; and open wood-
affected by mammalian (goat, kangaroo or rabbit)
lands of Casuarina pauper and Alectryon oleifolius ssp.
grazing. At each site, two or three 25 m (cid:4) 25 m plots
canescens on the dune system in the south-west of the
were established for each fencing treatment.
park (Westbrooke et al. 2001). These open woodlands
Treatments were exclusion of all mammalian grazers
require specific management action to address the lack
(No Access); exclusion of all mammalian grazers
of recruitment in the overstorey species (Auld &
except rabbits (Rabbit Access); and no exclusion (All
Denham 2001). Kinchega NP also supports a high
Access). No Access and All Access plots were estab-
diversity of ephemeral plant species (annuals and
lished in December 1996 for A. oleifolius, C. pauper,
short-lived perennials) (Robertson 1987, 1988;
A. carneorum, A. ligulata and D. viscosa ssp. angustis-
Westbrooke et al. 2001). The distribution, abundance
sima, while for S. acuminatum, one No Access treat-
and composition of this component of the flora vary
ment was established in January 1995, and the other in
greatly in response to seasonal rainfall and flooding
March 1998. Allocation of plots to No Access or All
(Robertson 1987, 1988).
Access treatments was made at random after marking
out both plots except for one of the two S. acuminatum
Grazing history
sites (Site 8), where the No Access fence was
constructed prior to the experiment. Rabbit Access
Grazing by domestic stock occurred from the 1860s
plots were constructed in March 1999 at sites of
until the gazettal of Kinchega National Park in 1967.
The major native mammalian grazers on the park are A. oleifolius, C. pauper, A. carneorum and A. ligulata.
red, Macropus rufus, and western grey, Macropus In locating all plots within a site, we attempted to
minimize landscape variations (slope, aspect, and
fuliginosus, kangaroos, while euros, Macropus robustus,
overstorey species density) at a local scale.
and eastern grey kangaroos, Macropus giganteus, are
less common. Rabbits first reached the area in about
1881 (Caughley 1987) and are widespread and
abundant, while feral goats are uncommon, but Vegetation sampling
variable in abundance (Auld & Denham 2001).
Rabbits and goats have been linked to seedling/sucker After the plots were established, all individuals of the
mortality and lack of recruitment in many long-lived target species were tagged and their height and number
shrubs at Kinchega NP (Auld 1993, 1995a,b; Auld & of stems emerging from the soil were recorded. The
Denham 2001). primary purpose of these tags was to enable us to
distinguish these individuals from new recruits
observed in subsequent censuses. We monitored the
Study sites and species growth and survival of these existing individuals, but
the results are not presented here.
Two sites were located for each of the target overstorey On the first and subsequent censuses (approximately
shrub/tree species. Study species were Acacia carneo- every three months from March 1997 until November
rum Maiden (Fabaceae: purple-wood wattle, formerly 2002), the plots were searched for new recruits
A. carnei, Sites 1 and 2) a shrub to 4 m; Casuarina (seedlings or suckers that had emerged since the
pauper F. Muell. ex L. Johnson (Casuarinaceae: belah, previous census). With the exception of A. ligulata,
Sites 3 and 4) a tree to 15 m; Alectryon oleifolius (Desf.) where germination was sporadic and new recruits were
S. T. Reynolds ssp. canescens S. T. Reynolds (Sapin- observed on only a few occasions, some new recruits
daceae: rosewood, Sites 5 and 6) a tree to 7 m; were observed on most censuses. For each of these
Santalum acuminatum (R. Br.) DC. (Santalaceae: cohorts, all or a sample of the new recruits were tagged.
quandong, Sites 7 and 8) a shrub or small tree to 6 m; Where time constraints limited the number that could
Acacia ligulata A. Cunn. ex Benth. (Fabaceae: sandhill be tagged, new recruits were chosen to result in an even
wattle, Sites 9 and 10) a shrub to 4 m. A single site was distribution across the plot and in relation to the adult
also located for Dodonaea viscosa Jacq. ssp. angustissima plants. Sample sizes for each species/site/treatment
(DC.) J.G. West (Sapindaceae: hopbush, Site 11) an group for each cohort ranged from 1 to 66. Growth
erect shrub to 4 m. Nomenclature follows Harden and survival of tagged suckers and seedlings were
(1990, 1992, 1993, 2002). Replicate sites were chosen monitored in subsequent censuses.
588 A. J. DENHAM AND T. D. AULD
When tagged plants had died we attempted to compared with Weibull models using log-ratio com-
determine the likely cause of death, by inference from parisons (G-tests) (Fox 2001). To facilitate comparison
what remained of the individual. If there was no sign of of survival across recruit type, treatment and sites,
the plant it was scored as ‘gone’. Plants in this category graphs of the non-parametric Kaplan-Meier estimates
may have been eaten or died by other means and the are presented.
remnants may have broken off and blown away. Plants
that were clearly dead, but otherwise undamaged were
scored as ‘withered’. Plants that were dead, but had Rabbit abundance sampling
evidence of grazing damage, or were noted as having
grazing damage in the previous census, were scored as Two indices of rabbit abundance were obtained in
‘eaten’. Where tags were not located, or were found out conjunction with the exclusion experiment. Firstly, an
of the ground, the plants were scored as ‘missing’. index based on spotlight counts was obtained from data
These plants contributed to survival analyses up to the collected by NSW National Parks and Wildlife Area
last census where they were located. From then on the staff. These data were gathered in a three monthly
individual was censored from the data set. This spotlight count along a 5-km transect on three con-
classification of plants allowed some interpretation of secutive nights at three locations within the park, one of
whether or not the loss was likely to be independent which was within 3 km of our study sites. Counts were
of the fencing treatment (e.g. if desiccation was the started in September 1996, 3 months prior to the start
cause). of this research, and then repeated approximately every
Combining cohorts within species, site and treat- 3 months, until October 2002. Counts of rabbits per
ment groups generally provided sufficient numbers for transect were log transformed and fitted against time
statistical analyses of survival (failure-time analysis, using linear regression, with goodness-of-fit assessed
Fox 2001). Variables in the failure-time analyses that using R-squared values and examination of residuals.
were included as factors were recruit type (seedlings or This tested the hypothesis that spotlight counts showed
suckers), species, treatment and site. However, treat- an initial rapid decline and then approached a zero
ment was also considered as a covariate, with the three asymptote.
different levels of grazing coded 0, 1 or 2 (All Access, Secondly, rabbit dung-pellets were counted in 10
Rabbit Access, No Access, respectively), while the 0.25 m2 quadrats within each plot at each census time
other factors were strictly nominal. Three other vari- (approximately every 3 months) between December
ables were considered to be covariates, two measures 1999 and November 2002, except for March 2002.
of size at the time of tagging (height and number of Pellets were removed at each sampling time and the
stems) and date of tagging. Date of tagging was used initial counts were not included in the analysis. These
as a covariate to allow for variations over time that data provided an independent and local index of rabbit
might alter the survival of each cohort. Time was abundance, and also tested the hypothesis that rabbit
in months since tagging, with censuses conducted use of the Rabbit Access treatment was not different
approximately every three months. Survival analysis from that in the All Access treatment. Counts of
followed a hierarchical sequence using Systat 8.0 for dung-pellets were analysed by repeated measures
Windows (SPSS Inc. 1998). Initial analyses were made ANOVA and MANOVA, with four between-subjects
using stratified non-parametric Kaplan-Meier life table factors (species, site, treatment and plot nested in
assessments, with recruit type, species, treatment and species) and one within-subject factor (time in
site included as stratified variables (factors). Log-rank approximately three month intervals). Only two
tests indicated whether stratified variables were likely to fencing treatments were included, Rabbit Access and
come from distributions different in either shape or All Access, and only those overstorey species that
location. Systat produces three log-rank tests, but we had the Rabbit Access treatment could be included
report results only for the Breslow-Gehan test, which (A. oleifolius, C. pauper, A. carneorum and A. ligulata).
in our analyses was the most conservative (i.e. reported Data were log-transformed (log(x + 1)) prior to
the highest P-values). Where levels of a factor did not analysis to reduce non-normality resulting from a
differ significantly (i.e. the log-rank tests were not large number of low or zero pellet counts.
significant), data were pooled across that factor, and in
the case of treatment, the pooling variable was then
entered as a covariate into the full model. Where levels
RESULTS
of a factor were significantly different, the data were
kept separate. Parametric survival models were then
obtained for each group of data using proportional Emergence of seedlings and suckers
hazards methods, initially including all suitable covari-
ates and sequentially removing those that were not Seedling emergence was sporadic, and few were tagged
significant at P = 0.1. Fit of the models was assessed until 2001. More than 50% of the total number of
using likelihood estimates. Exponential models were seedlings monitored in this study emerged between
REGENERATION OF ARID TREES AND SHRUBS AFTER RCD 589
August and October 2001, following above average Fig. 2a). Seedling mortality was extremely high initially
rainfall in September 2001, with follow-up rain in (median survival <3 months), while initial mortality
October (Fig. 1). Two study species reproduced only of suckers was lower and remained fairly constant
by seedlings (A. ligulata and D. viscosa), while only one (Fig. 2a). The two species reliant on seedlings,
seedling was observed in any of the other species. A. ligulata and D. viscosa, were not different in their
This was a seedling of C. pauper that survived less overall patterns of survival (log-rank test, (cid:5)2 = 0.28,
than three months. Sucker emergence was more d.f. = 1, P = 0.60). For all seedling species combined,
regular. In A. carneorum, new suckers typically all treatments were significantly different from each
emerged in autumn and spring, as noted by Auld other (log-rank test, (cid:5)2 = 43.0, d.f. = 2, P < 0.001).
(1993). In C. pauper and S. acuminatum, there was Suckers overall had a median survival of 18 months
more or less continuous emergence of suckers. Only in (Fig. 2a), and there was no significant difference
A. oleifolius was sucker emergence rare. among suckering species with treatments pooled (log-
rank test, (cid:5)2 = 2.32, d.f. = 3, P = 0.51). For all sucker
species combined, the All Access and Rabbit Access
Survival of seedlings and suckers treatments were not significantly different from each
other (log-rank test, (cid:5)2 = 3.18, d.f. = 1, P = 0.07), but
Most seedlings and a large proportion of suckers were both had significantly poorer survival compared with
found to be ‘gone’ or ‘withered’, regardless of treatment the No Access treatment (log-rank tests, (cid:5)2 = 104.6,
(Table 1), reflecting the arid climatic conditions of the (cid:5)2 = 48.2 respectively, d.f. = 1, P < 0.001).
area. However, many of the plants classified as ‘gone’ Although survival was not significantly different
in the All Access and Rabbit Access treatments may among all species, we believed that it was instructive to
have been eaten and some of the ‘withered’ plants in examine each species separately. Within suckering
these treatments may have been damaged by herbivores species, the No Access treatment had greater survival
and thus more susceptible to desiccation. Plants that than the Rabbit Access or the All Access treatments,
were scored as ‘eaten’ in No Access treatment were while these latter two treatments were not significantly
where grazers temporarily breached fences. A kangaroo different. Site effects were significant for A. carneorum
jumped into an A. ligulata No Access plot and and C. pauper. Within the two seedling species, treat-
remained there for less than 3 months, while a rabbit ment effects were not significant for D. viscosa, while
remained inside a C. pauper No Access plot for a year for A. ligulata, there was a significant treatment-site
after the fence was erected. Most ‘missing’ plants interaction (see below).
occurred in All Access treatments due to the increased For the parametric models of survival, results of
exposure of the tags to inquisitive animals. likelihood ratio comparisons demonstrated that the fit
Seedling survival was significantly lower than that of of an exponential distribution (Weibull model with
suckers (log-rank test, (cid:5)2 = 542.6, d.f. = 1, P < 0.001; shape constrained to 1) was in no instance significantly
Table 1. Causes of seedling or vegetative sucker mortality. Data are pooled across sites and cohorts. All cohorts older than three
months have been included
Percentage of seedlings/suckers Number
Species Treatment alive eaten gone withered missing sampled
Seedlings
Acacia ligulata All Access 0.0 2.2 80.5 15.5 1.8 226
Rabbit Access 1.6 4.3 60.1 32.4 1.6 188
No Access 8.5 2.4 36.0 51.8 1.2 328
Dondonaea viscosa All Access 10.6 12.8 27.7 48.9 0.0 47
No Access 10.2 0.0 8.5 81.4 0.0 59
Vegetative Suckers
Acacia carneorum All Access 22.6 17.8 40.4 11.0 8.2 146
Rabbit Access 29.0 26.2 36.4 7.5 0.9 107
No Access 66.7 0.0 0.0 31.9 1.4 72
Alectryon oleifolius All Access 39.4 3.0 30.3 18.2 9.1 33
Rabbit Access 50.0 10.0 20.0 20.0 0.0 10
No Access 75.0 0.0 0.0 25.0 0.0 8
Casuarina pauper All Access 11.6 21.0 19.6 40.6 7.2 138
Rabbit Access 29.0 12.9 25.8 27.4 4.8 62
No Access 53.3 9.7 5.5 27.9 3.6 165
Santalum acuminatum All Access 8.6 14.1 50.9 3.1 23.3 163
No Access 74.3 0.0 5.7 20.0 0.0 70
590 A. J. DENHAM AND T. D. AULD
poorer than a two parameter Weibull. Thus, the risk consistency with the log-rank analyses, with lower
of death at any time was more or less constant. hazard values for No Access treatments compared with
Consequently, we present data only for the simpler, Rabbit Access or All Access treatments (Table 2).
exponential model type. The models showed Where they were significant, covariates had low values,
Fig. 2. Kaplan-Meier survival estimates with standard errors. (a) All new recruits – treatments, species and sites pooled, ((cid:2))
seedlings, ((cid:3)) suckers; (b) Acacia carneorum – ((cid:2)) All Access and Rabbit Access pooled, ((cid:4)) Site 1 No Access, ((cid:3)) Site 2 No
Access; (c) Casuarina pauper – ((cid:5)) All Access and Rabbit Access pooled at Site 3, ((cid:2)) All Access and Rabbit Access pooled at
Site 4, ((cid:3)) No Access sites pooled; (d) Alectryon oleifolius – Site 5, ((cid:2)) All Access, ((cid:3)) No Access, ((cid:5)) Rabbit Access; (e)
Santalum acuminatum – ((cid:2)) All Access sites pooled, ((cid:3)) No Access Site 7, No Access at Site 8 (not shown) had no losses; (f)
Dodonaea viscosa – ((cid:2)) All Access, ((cid:3)) No Access; (g) Acacia ligulata – Site 9, ((cid:2)) All Access, ((cid:3)) No Access, ((cid:5)) Rabbit Access;
(h) A. ligulata – Site 10, ((cid:2)) All Access, ((cid:3)) No Access, ((cid:5)) Rabbit Access.
REGENERATION OF ARID TREES AND SHRUBS AFTER RCD 591
indicating that they had only a slight influence on survival of cohorts was similar at all stages of the
survival. In all cases, the two size covariates were experiment.
positive, indicating that plants that were larger when
tagged had greater survival probabilities. Where it
Acacia carneorum
was a significant covariate, fencing treatment was
in one instance positive, indicating that plants with Survival of suckers of A. carneorum was much greater
greater protection had greater survival probabilities, in the No Access treatment than in either the All Access
and in the other instance negative, indicating the or Rabbit Access treatments (log-rank test, (cid:5)2 = 45.9,
reverse (see below). Date when tagged did not d.f. = 2, P < 0.001, Fig. 2b). For the No Access
contribute significantly to the models, indicating that treatment, there was a significant site effect (log-rank
Table 2. Survival parameters for exponential models of seedlings and vegetative suckers grouped according to stratified survival
analyses. Covariates are included where significant at P < 0.1
Species: treatment, site combinations Parameter Value (SE) t-ratio P
Acacia carneorum: No Access, Site 1 Location 64.58 (15.22) 4.24 < 0.001
Hazard 0.015
Maximized log-likelihood –93.02
A. carneorum: No Access, Site 2 Location 286.8 (128.3) 2.248 0.025
Hazard 0.003
Maximized log-likelihood –33.29
A. carneorum: Rabbit Access and Location 10.82 (1.41) 7.65 < 0.001
All Access pooled Height When Tagged 0.09 4.19 < 0.001
Hazard 0.055
Maximized log-likelihood –678.33
Casuarina pauper: No Access Location 37.24 (6.27) 5.94 < 0.001
Height When Tagged 0.06 (0.03) 2.02 0.04
Hazard 0.018
Maximized log-likelihood –348.85
C. pauper: Site 3, Rabbit Access and Location 7.92 (1.70) 4.66 < 0.001
All Access pooled Height When Tagged 0.10 (0.05) 1.91 0.06
Treatment 0.62 (0.21) 2.99 < 0.001
Stems When Tagged 0.05 (0.02) 3.55 < 0.001
Hazard 0.053
Maximized log-likelihood –401.58
C. pauper: Site 4, Rabbit Access and Location 6.23 (1.28) 4.90 < 0.001
All Access pooled Height When Tagged 0.05 (0.02) 1.98 0.05
Stems When Tagged 0.05 (0.02) 1.91 0.06
Hazard 0.10
Maximized log-likelihood –162.61
Alectryon oleifolius: Site 5, All Access Location 37.38 (7.63) 4.90 < 0.001
Hazard 0.03
Maximized log-likelihood –110.90
Santalum acuminatum: Site 7, No Access Location 85.83 (20.23) 4.24 < 0.001
Hazard 0.01
Maximized log-likelihood –98.14
S. acuminatum: Sites 8 and 7 pooled, Location 13.29 (1.26) 10.54 < 0.001
All Access Hazard 0.08
Maximized log-likelihood –398.22
Acacia ligulata: Site 9, No Access Location 9.91 (0.67) 14.73 < 0.001
Hazard 0.10
Maximized log-likelihood –714.75
A. ligulata: Site 9, Rabbit Access and Location 4.03 (0.21) 19.1 < 0.001
All Access; Site 10, all treatments pooled Height When Tagged 0.14 (0.06) 2.43 0.015
Hazard 0.23
Maximized log-likelihood –1187.47
Dodonaea viscosa: No Access and Location 4.47 (0.78) 5.73 < 0.001
All Access pooled Height When Tagged 0.13 (0.02) 5.40 < 0.001
Treatment –0.18 (0.10) –1.84 0.067
Hazard 0.22
Maximized log-likelihood –275.47
592 A. J. DENHAM AND T. D. AULD
test, (cid:5)2 = 7.24, d.f. = 1, P = 0.01), with survival at Site considerably greater at Site 3 than at Site 4 (hazard =
1 lower than that at Site 2 (median survival 36 vs 0.05 cf. 0.1). Some suckers were eaten in all treatments
>54 months, Table 3). The Rabbit Access and All of C. pauper, including the No Access treatment
Access treatments were not significantly different from (Table 1). This was due to our failure to eliminate what
each other (log-rank test, (cid:5)2 = 0.82, d.f. = 1, P = 0.37). appeared to be a single rabbit from the No Access plot
With these treatments pooled, sites were not signifi- at Site 4 for a year after starting the experiment. A
cantly different from each other (log-rank test, number of suckers were grazed as a result and a few
(cid:5)2 = 0.62, d.f. = 1, P = 0.43). For this group the of these subsequently died. No suckers were grazed
hazard function of the model was considerably following additional treatment of the warren in the plot.
higher, and median sucker survival was less than a
third that of suckers in the No Access treatment
Alectryon oleifolius
(Table 3).
For A. oleifolius, suckering was relatively rare, with 51
suckers tagged at Site 5 and only two suckers found at
Casuarina pauper
Site 6. At Site 5, treatments were not significantly
The No Access treatment in C. pauper had significantly different, possibly due to the limited number of
greater survival of suckers than the other two treat- replicates (log-rank test, (cid:5)2 = 3.96, d.f. = 2, P = 0.14,
ments (log-rank test, (cid:5)2 = 33.5, d.f. = 2, P < 0.001, Fig. 2d). There were sufficient data only to model the
Fig. 2c). For the No Access treatment, sites were not All Access treatment at Site 5. No covariates were
significantly different (log-rank test, (cid:5)2 = 0.004, retained in the model (Table 2), with the hazard
d.f. = 1, P = 0.95). Survival in the No Access plots was relatively low at 0.03.
high (hazard = 0.02), with median survival approxi-
mately 30 months (Table 3). There was no significant
Santalum acuminatum
difference in the survival of suckers in the Rabbit
Access and All Access treatments (log-rank test, For S. acuminatum there was a significant treatment
(cid:5)2 = 0.96, d.f. = 1, P = 0.33). With these data pooled, difference with sites pooled (log-rank test, (cid:5)2 = 39.0,
there was significantly different survival of suckers d.f. = 1, P < 0.001), but within treatments there were
across sites (log-rank test, (cid:5)2 = 14.7, d.f. = 1 no significant site differences (log-rank tests, (cid:5)2 = 3.60,
P < 0.001, Fig. 2c). At Site 3, for the All Access and d.f. = 1, P = 0.06, for No Access and (cid:5)2 = 0.031,
Rabbit Access treatments pooled, the model retained d.f. = 1, P = 0.86, for All Access, Fig. 2e). At Site 8 in
the covariates Treatment, Stems When Tagged and the No Access treatment, there were no losses what-
Height When Tagged (Table 2). Treatment was a soever. Consequently modelling was restricted to Site 7
positive covariate in this model, indicating that greater in the No Access treatment, where no covariates were
protection from grazing increased survival of suckers. retained in the survival model. Similarly, for the All
In contrast, Treatment was not a significant covariate Access treatment with sites pooled, no covariates were
at Site 4, while Stems When Tagged and Height retained in the survival model (Table 2). Survival was
When Tagged were significant (Table 2). Survival was considerably poorer than for the No Access treatment,
Table 3. Median estimates of survival of suckers and seedlings grouped according to log-rank tests. Medians (time at which
survival probability = 0.5) for the non-parametric Kaplan–Meier values interpolated from life-table data. Median survival from
exponential model calculated using mean values for significant covariates (see Table 2)
Median survival from
Median Kaplan-Meier exponential model
Species/treatment/site group survival (months) (months)
Acacia carneorum: No Access Site 1 36 44.8
A. carneorum: No Access Site 2 > 54 198.8
A. carneorum: Rabbit Access and All Access pooled 11 12.6
Casuarina pauper: No Access 30 39.0
C. pauper: Site 3, Rabbit Access and All Access pooled 8 13.1
C. pauper: Site 4, Rabbit Access and All Access pooled 3 7.0
Alectryon oleifolius: Site 5 All Access 19 25.9
Santalum acuminatum: Site 7, No Access > 27 59.5
S. acuminatum: Sites pooled, All Access 8 9.2
Acacia ligulata: Site 9, No Access 4 6.9
A. ligulata: Site 9, Rabbit Access, All Access; Site 10, all treatments < 3 3.0
Dodonaea viscosa: No Access and All Access pooled < 3 3.2
REGENERATION OF ARID TREES AND SHRUBS AFTER RCD 593
with median survival of 8 months compared with more Fig. 2g), while at Site 10 survival in the No Access
than 27 months (Table 3). treatment was significantly greater than in either the
Rabbit Access or All Access treatments (log-rank test,
(cid:5)2 = 65.2, d.f. = 2, P < 0.001, Fig. 2h). Survival over-
Dodonaea viscosa
all was greater at Site 10 than at Site 9 (Kaplan-Meier
For D. viscosa, the treatment effect was not significant survival probability at 3 months of 0.41 cf. 0.28), but
in the log-rank test ((cid:5)2 = 0.08, d.f. = 1, P = 0.78, was low at both sites, and few recruits are expected to
Fig. 2f), but was retained, along with Height When persist to maturity (at least 3 years, Auld 1995a). We
Tagged as a covariate in the parametric model modelled the No Access treatment at Site 10 separately
(Table 2). Treatment had a negative value in the model, from all the others, as the log-rank tests indicated that
indicating that survival tended to be higher for plants survival in this group was significantly greater than in
with less protection from grazing. The hazard was very all other groups, and we pooled data for the other
high at 0.16 with overall survival lower than for the combinations of site and treatment. For the No Access
sucker producing species (median from Kaplan-
Meier estimates of less than 3 months), and similar
to the other seedling producing species, A. ligulata
(Table 3).
Acacia ligulata
For A. ligulata, while over 700 seedlings were tagged,
few survived beyond 9 months (Table 1, Figs 2g,h).
However, there were significant treatment (log-rank
test, (cid:5)2 = 54.3, d.f. = 2, P < 0.001) and site effects
(log-rank test, (cid:5)2 = 17.6, d.f. = 1, P < 0.001) and a
site–treatment interaction. This resulted from greater
survival of seedlings in the Rabbit Access treatment
than in either the All Access or No Access treatments at
Site 9 (log-rank test, (cid:5)2 = 8.96, d.f. = 2, P = 0.01,
Fig. 3. Rabbit spotlight counts per 5 km transect over
time. Filled symbols for transect closest to our study plots,
open symbols for the other two transects. Regression lines, Fig. 4. Mean rabbit dung-pellets per m2 over time for ((cid:2))
all data (dashed line), y = e(36.9–0.00097 (cid:4) Julian day), R2 = 0.48; Acacia ligulata, ((cid:5)) A. carneorum, ((cid:6)) Alectryon oleifolius and
transect closest to study plots only (solid line), ((cid:4)) Casuarina pauper: (a) All Access treatment; (b) Rabbit
y = e(42.6–0.00111 (cid:4) Julian day), R2 = 0.57. Access treatment.
594 A. J. DENHAM AND T. D. AULD
treatment at Site 10, no covariates were retained in the lesser degree (Fig. 4a vs Fig. 4b). Dung-pellet counts
model (Table 2). The hazard was high at 0.10. For the were generally higher in the A. ligulata plots for both
other treatment and site combinations pooled, Height treatments (Fig. 4), consistent with our casual observ-
When Tagged was retained as a covariate. The hazard ations of higher rabbit densities around these plots.
was very high at 0.23 (Table 2). With the exception of A. ligulata, higher dung-pellet
numbers in the Rabbit Access plots supported the
hypothesis that rabbits were undeterred by the fence
Rabbit abundance used to prevent access to larger mammalian grazers.
Within subjects, there were a number of significant
factors (Table 4) indicative of the high variability over
Transect counts
time for different combinations of the factors. The
The number of rabbits sighted declined rapidly over highly significant time factor reflected the overall
the first 2 years of the project, to about 25% of their decline in dung-pellets over the study period, from an
initial values (Fig. 3). The logarithmic curve fitted initial mean of 14 per m2 in March 2000 to 2.4 per m2
reasonably well, and was consistent with our expect- by October 2002.
ation that numbers would remain relatively stable
following the initial decline. The transect closest to the
study plots generally had higher rabbit numbers than DISCUSSION
elsewhere in the park (Fig. 3), but this transect showed
the same pattern of decline in rabbit numbers over time Grazing by mammals, particularly rabbits, reduced the
that was evident with all transects pooled. probability of new suckers surviving to become
juveniles. For the species that relied solely on the
establishment of seedlings, a combination of low rabbit
Dung-pellet counts
abundance would need to coincide with favourable
Dung-pellet counts were variable but declined in a climatic conditions to allow recruitment. While desicc-
fashion consistent with data from the spotlight surveys ation accounted for most mortality overall, unprotected
(Fig. 4). A number of significant effects were indicated plants that survived the initial impacts of desiccation
by the repeated measures analysis (Table 4), while the remained vulnerable to grazers and were likely to be
assumption of sphericity of the data appeared to be eliminated before reaching a grazing resistant size.
satisfied (Greenhouse-Geisser (cid:6) = 0.80 and Huynh- Below we discuss the results in detail.
Feldt (cid:6) = 0.93). The MANOVA found similar results.
Between subjects, there was no significant treatment
effect overall. However, there was a significant species– Emergence of seedlings and suckers
treatment interaction. This was because for A. ligulata,
the All Access plots initially had many more dung- There was variation in the pattern of emergence of
pellets than the respective Rabbit Access plots, while seedlings and suckers with a peak of seedlings following
for all other species the reverse was true, although to a large rainfall events. In addition to this sporadic
Table 4. Statistical results of repeated measures analysis of counts of rabbit dung-pellets
Source of variation SS d.f. MS F P G-G H-F
Between Subjects
Treatment 1.41 1 1.41 0.597 0.441
Species 111.00 3 37.000 15.600 < 0.001
Treatment (cid:4) species 41.8 3 13.900 5.88 0.001
Plot(species) 22.1 4 5.53 2.33 0.059
Treatment (cid:4) plot(species) 20.0 4 4.99 2.11 0.083
Error 334.00 141 2.37
Within Subjects
Time 129.90 9 14.400 40.600 < 0.001 < 0.001 < 0.001
Time (cid:4) treatment 5.64 9 0.626 1.76 0.071 0.089 0.076
Time (cid:4) species 38.2 27 1.41 3.98 < 0.001 < 0.001 < 0.001
Time (cid:4) species (cid:4) treatment 21.9 27 0.810 2.28 < 0.001 0.001 < 0.001
Time (cid:4) plot(species) 36.8 36 1.02 2.88 < 0.001 < 0.001 < 0.001
Time (cid:4) treatment (cid:4) plot (species) 12.5 36 0.347 0.976 0.510 0.502 0.507
Error 450.90 1269 0.355
F, F-test; G-G, Greenhouse-Geisser; H-F, Huynh-Feldt; SS, sum of squares.
Description:Survival and recruitment of seedlings and suckers of trees impact of rabbit control on vegetation, we examined whether recruitment of seedlings or
