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Austral 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.

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Survival and recruitment of seedlings and suckers of trees impact of rabbit control on vegetation, we examined whether recruitment of seedlings or
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