The impact of rabbit haemorrhagic disease on wild rabbit (Oryctolagus cuniculus) populations in Queensland
Abstract and Figures
Rabbit haemorrhagic disease virus (RHDV) escaped from quarantine facilities on Wardang Island in September 1995 and spread through South Australia to Queensland by December 1995. To determine the impact of this biological control agent on wild rabbit populations in Queensland, shot sample and spotlight count data were collected at six sites. RHDV spread across Queensland from the south-west to the east at a rate of at least 91 km month–1 between October 1995 and October 1996. The initial impact on rabbit density appeared highly variable, with an increase of 81% (255 ± 79 (s.e.) to 385 ± 73 rabbits km–2) at one site and a decrease of 83% (129 ± 27 to 22 ± 18 rabbits km–2) at another during the first outbreak. However, after 30 months of RHDV activity, counts were at least 90% below counts conducted before RHDV arrived. Using a population model to account for environmental conditions, the mean suppression of rabbit density caused by rabbit haemorrhagic disease (RHD) was estimated to be 74% (ranging from 43% to 94% between sites). No outbreaks were observed when the density of susceptible rabbits was lower than 12 km–2. Where rabbit density remains low for long periods RHDV may not persist. This is perhaps most likely to occur in the isolated populations towards the northern edge of the range of rabbits in Australia. RHDV may have to be reintroduced into these populations. Further south in areas more suitable for rabbits, RHDV is more likely to persist, resulting in a high density of immune rabbits. In such areas conventional control techniques may be more important to enhance the influence of RHD.
Figures - uploaded by Russell Palmer
Author content
All figure content in this area was uploaded by Russell Palmer
Content may be subject to copyright.
... In 1995, the biological control agent rabbit haemorrhagic disease virus (RHDV, otherwise known as rabbit calicivirus disease) spread across Australia, rapidly reducing rabbit populations to very low levels (Cooke 2002;Mutze et al. 2002;Story et al. 2004). The impact of rabbit haemorrhagic disease (RHD) was monitored (the Rabbit Calicivirus Monitoring Program) at sites selected throughout Australia (Cooke 2002;Henzell et al. 2002;Story et al. 2004). ...
... In 1995, the biological control agent rabbit haemorrhagic disease virus (RHDV, otherwise known as rabbit calicivirus disease) spread across Australia, rapidly reducing rabbit populations to very low levels (Cooke 2002;Mutze et al. 2002;Story et al. 2004). The impact of rabbit haemorrhagic disease (RHD) was monitored (the Rabbit Calicivirus Monitoring Program) at sites selected throughout Australia (Cooke 2002;Henzell et al. 2002;Story et al. 2004). One of these sites was established at Bulloo Downs cattle station in south-west Queensland. ...
... One of these sites was established at Bulloo Downs cattle station in south-west Queensland. Field surveys conducted between 1996 and 1998 (Story et al. 2004) showed Bulloo Downs to be the only arid site in the national and state monitoring programs that still had high rabbit numbers three years after RHDV arrived. The virus was killing rabbits at Bulloo Downs, and suppressing the population, but rabbit numbers were still unacceptably high (Story et al. 2004). ...
Context For over 100 years, control efforts have been unable to stop rabbits causing damage to cattle production and native plants and animals on large properties in arid parts of Australia. Warren destruction by ripping has shown promise, but doubts about long-term success and the perceived expense of treating vast areas have led to this technique not being commonly used. Aims This study measured the long-term reduction in rabbit activity and calculated the potential cost saving associated with treating just the areas where rabbits are believed to survive drought. We also considered whether ripping should be used in a full-scale rabbit control program on a property where rabbits have been exceptionally resilient to the influence of biological and other control measures. Methods Rabbits were counted along spotlight transects before warrens were ripped and during the two years after ripping, in treated and untreated plots. Rabbit activity was recorded to determine the immediate and long-term impact of ripping, up to seven years after treatment. The costs of ripping warrens within different distances from drought refuge areas were calculated. Key results Destroying rabbit warrens by ripping caused an immediate reduction in rabbit activity and there were still 98% fewer rabbits counted by spotlight in ripped plots five months after ripping. Seven years after ripping no active warrens were found in ripped plots, whereas 57% of warrens in unripped plots showed signs of rabbit activity. The cost of ripping only the areas where rabbits were likely to seek refuge from drought was calculated to be less than 4% of the cost of ripping all warrens on the property. Conclusions Destroying rabbit warrens by ripping is a very effective way of reducing rabbit numbers on large properties in arid Australia. Ripping should commence in areas used by rabbits to survive drought. It is possible that no further ripping will be required. Implications Strategic destruction of warrens in drought refuge areas could provide an alternative to biological control for managing rabbits on large properties in the Australian arid zone.
... However, the effects of RCD varied among the sites that were under observation for RCD outbreaks. In particular, it appeared that environmental conditions such as increased rainfall and high temperatures decreased the effectiveness of RCD as a control (Story et al. [2004]). Also reported in Story et al. [2004], there were no occurrence of RCD outbreaks in rabbit populations where the density of susceptibles were less than 12 rabbits km −2 and that the overall effect of RCD was least in sites that were most suitable for rabbits. ...
... In particular, it appeared that environmental conditions such as increased rainfall and high temperatures decreased the effectiveness of RCD as a control (Story et al. [2004]). Also reported in Story et al. [2004], there were no occurrence of RCD outbreaks in rabbit populations where the density of susceptibles were less than 12 rabbits km −2 and that the overall effect of RCD was least in sites that were most suitable for rabbits. ...
... It was suggested that it was because mosquitoes were the main vector for myxomatosis and thus it was more favourable over RCD in such areas. Also, it was postulated that for areas with low rabbit density (where RCD would be less effective), myxomatosis could prove to be a better control agent (Story et al. [2004]). Marchandeau et al. [2004] showed statistically that for most of their study areas, prior infection of myxomatosis was a significant factor in the prediction of contraction of RCD, and vice-versa. ...
Increasing resistance of rabbits to myxomatosis in Australia has led to the exploration of Rabbit Haemorrhagic Disease, also called Rabbit Calicivirus Disease (RCD) as a possible control agent. While the initial spread of RCD in Australia resulted in widespread rabbit mortality in affected areas, the possible population dynamic effects of RCD and myxomatosis operating within the same system have not been properly explored. Here we present early mathematical modelling examining the interaction between the two diseases. In this study we use a deterministic compartment model, based on the classical SIR model in infectious disease modelling. We consider, here, only a single strain of myxomatosis and RCD and neglect latent periods. We also include logistic population growth, with the inclusion of seasonal birth rates. We assume there is no cross-immunity due to either disease. The mathematical model allows for the possibility of both diseases to be simultaneously present in an individual, although results are also presented for the case where co-infection is not possible, since co-infection is thought to be rare and questions exist as to whether it can occur. The simulation results of this investigation show that it is a crucial issue and should be part of future field studies. A single simultaneous outbreak of RCD and myxomatosis was simulated, while ignoring natural births and deaths, appropriate for a short timescale of 20 days. Simultaneous outbreaks may be more common in Queensland. For the case where co-infection is not possible we find that the simultaneous presence of myxomatosis in the population suppresses the prevalence of RCD, compared to an outbreak of RCD with no outbreak of myxomatosis, and thus leads to a less effective control of the population. The reason for this is that infection with myxomatosis removes potentially susceptible rabbits from the possibility of infection with RCD (like a vaccination effect). We found that the reduction in the maximum prevalence of RCD was approximately 30% for an initial prevalence of 20% of myxomatosis, for the case where there was no simultaneous outbreak of myxomatosis, but the peak prevalence was only 15% when there was a simultaneous outbreak of myxomatosis. However, this maximum reduction will depend on other param-eter values chosen. When co-infection is allowed then this suppression effect does occur but to a lesser degree. This is because the rabbits infected with both diseases reduces the prevalence of myxomatosis. We also simulated multiple outbreaks over a longer timescale of 10 years, including natural population growth rates, with seasonal birth rates and density dependent (logistic) death rates. This shows how both diseases interact with each other and with population growth. Here we obtain sustained outbreaks occurring approximately every two years for the case of a simultaneous outbreak of both diseases but without simultaneous co-infection, with the prevalence varying from 0.1 to 0.5. Without myxomatosis present then the simulation predicts RCD dies out quickly without further introduction from elsewhere. With the possibility of simultaneous co-infection of rabbits, sustained outbreaks are possible but then the outbreaks are less severe and more frequent (approximately yearly). While further model development is needed, our work to date suggests that: 1) the diseases are likely to interact via their impacts on rabbit abundance levels, and 2) introduction of RCD can suppress myx-omatosis prevalence. We recommend that further modelling in conjunction with field studies be carried out to further investigate how these two diseases interact in the population.
... Previous genomic analyses for the Australian viruses only included viruses sampled from 1950 to 1999, with most samples being obtained between 1990 and 1996. However, the ecological environment facing rabbits in Australia changed dramatically in the mid-1990s with the release of a second biocontrol agent, rabbit hemorrhagic disease virus (RHDV), that greatly reduced the size of the rabbit population (32)(33)(34)(35). To determine whether MYXV evolution differed markedly before and after the emergence of RHDV, we sequenced a large set MYXV isolates sampled between 2008 and 2016, extending previous studies by nearly 20 years and considerably expanding the geographic coverage. ...
... A change in selection pressures during the period from 1996 to 2012 may be the most likely explanation for the emergence of the divergent viruses associated with lineage C. Two major changes in rabbit populations occurred during this period, and these could conceivably have influenced the transmission of MYXV and, hence, altered selection pressures. From 1995, a novel viral pathogen, RHDV, spread through Australia, causing rabbit population crashes in many areas and reducing the number of MYXVsusceptible hosts (32)(33)(34)(35)(66)(67)(68)(69). In addition, a major, prolonged drought occurred over southeastern Australia between 1996 and 2009, putting additional pressure on rabbit populations. ...
The coevolution of myxoma virus (MYXV) and European rabbits in Australia is one of the most important natural experiments in evolutionary biology, providing insights into virus adaptation to new hosts and the evolution of virulence. Previous studies of MYXV evolution have also shown that the virus evolves both relatively rapidly and in a strongly clock-like manner. Using newly acquired MYXV genome sequences from Australia, we show that the virus has experienced a dramatic change in evolutionary behavior over the last 20 years, with a breakdown in clock-like structure, the appearance of a rapidly evolving virus lineage, and the accumulation of multiple nonsynonymous and indel mutations. We suggest that this punctuated evolutionary event may reflect a change in selection pressures as rabbit numbers declined following the introduction of rabbit hemorrhagic disease virus and drought in the geographic regions inhabited by rabbits.
... Previous genomic analyses for the Australian viruses only included viruses sampled from 1950 to 1999, with most samples being obtained between 1990 and 1996. However, the ecological environment facing rabbits in Australia changed dramatically in the mid-1990s with the release of a second biocontrol agent, rabbit hemorrhagic disease virus (RHDV), that greatly reduced the size of the rabbit population (32)(33)(34)(35). To determine whether MYXV evolution differed markedly before and after the emergence of RHDV, we sequenced a large set MYXV isolates sampled between 2008 and 2016, extending previous studies by nearly 20 years and considerably expanding the geographic coverage. ...
... A change in selection pressures during the period from 1996 to 2012 may be the most likely explanation for the emergence of the divergent viruses associated with lineage C. Two major changes in rabbit populations occurred during this period, and these could conceivably have influenced the transmission of MYXV and, hence, altered selection pressures. From 1995, a novel viral pathogen, RHDV, spread through Australia, causing rabbit population crashes in many areas and reducing the number of MYXVsusceptible hosts (32)(33)(34)(35)(66)(67)(68)(69). In addition, a major, prolonged drought occurred over southeastern Australia between 1996 and 2009, putting additional pressure on rabbit populations. ...
Myxoma virus (MYXV) has been evolving in a novel host species – European rabbits – in Australia since 1950. Previous studies of viruses sampled from 1950 to 1999 revealed a remarkably clock-like evolutionary process across all Australian lineages of MYXV. Through an analysis of 49 newly generated MYXV genome sequences isolated in Australia between 2008 and 2017 we show that MYXV evolution in Australia can be characterized by three lineages, one of which exhibited a greatly elevated rate of evolutionary change and a dramatic break-down of temporal structure. Phylogenetic analysis revealed that this apparently punctuated evolutionary event occurred between 1996 and 2012. The branch leading to the rapidly evolving lineage contained a relatively high number of non-synonymous substitutions, and viruses in this lineage reversed a mutation found in the progenitor standard laboratory strain (SLS) and all previous sequences that disrupts the reading frame of the M005L/R gene. Analysis of genes encoding proteins involved in DNA synthesis or RNA transcription did not reveal any mutations likely to cause rapid evolution. Although there was some evidence for recombination across the MYXV phylogeny, this was not associated with the increase in evolutionary rate. The period from 1996 to 2012 saw significant declines in wild rabbit numbers, due to the introduction of rabbit hemorrhagic disease and prolonged drought in south-eastern Australia, followed by the partial recovery of populations. We therefore suggest that a rapidly changing environment for virus transmission changed the selection pressures faced by MYXV and altered the course of virus evolution.
IMPORTANCE
The co-evolution of myxoma virus (MYXV) and European rabbits in Australia is one of the most important natural ‘experiments’ in evolutionary biology, providing insights into virus adaptation to new hosts and the evolution of virulence. Previous studies of MYXV evolution have also shown that the virus evolves both relatively rapidly and in a strongly clock-like manner. Using newly acquired MYXV genome sequences from Australia we show that the virus has experienced a dramatic change in evolutionary behavior over the last 20 years, with a break-down in clock-like structure, the appearance of a rapidly evolving virus lineage, and the accumulation of multiple non-synonymous and indel mutations. We suggest that this punctuated evolutionary event likely reflects a change in selection pressures as rabbit numbers declined following the introduction of rabbit hemorrhagic disease virus and drought in the geographic regions inhabited by rabbits.
... One of these sites was established at Bulloo Downs cattle station in south-western Queensland. This site was the only arid site to maintain high rabbit numbers three years after the release of RHDV, even though the virus was killing rabbits (Story et al. 2004). Bulloo Downs has a long history of rabbits in high numbers, being able to support 102 commercial rabbit hunters in the 1960s, following the introduction of myxoma virus (Bowen 1987). ...
Context European rabbits have a great impact on native vegetation and small vertebrates in Australia. Rabbits consume vegetation and promote invasive plants and invasive predators, and compete directly and indirectly with native animals suppressing those populations. Aims We explored the changes in small native vertebrates and invertebrates following the removal of rabbits. Methods Warren ripping was undertaken on a property in south-western Queensland at four sites and the results of pitfall trapping were compared with four nearby paired control sites. Invertebrates and small mammals were counted in pitfall traps, and bird surveys were conducted in all treatment and control sites. Key results Following a rabbit-control program, we observed a four-fold increase in the number of dunnarts trapped in treatment plots, whereas no change was observed in control plots. The spring following the rabbit-control program also saw an increase in some lizards in treatment plots. Conclusions The presence of rabbits in arid-zone Australia can suppress native animal populations. Implications Many species of small native mammals and lizards rely on food sources that fluctuate greatly with environmental conditions. The presence of rabbits altering the landscape, supporting introduced predators, reducing vegetation and, therefore, insects, adds increased pressure for insectivorous species. Rabbit control through warren ripping in arid-zone Australia is an effective method to reduce rabbit numbers, and allowed for an increase in small vertebrates in treated areas.
... The recorded reductions in the damage by rabbits and costs to control them are huge (Saunders et al. 2002). Also in the case of the RHD, the effects of the introduced infections are strongly affected by other factors, such as aridity of the environment (Cooke and Fenner 2002;Mutze et al. 2002), virulence of the RHD strains or general condition of the rabbits (Bruce et al. 2004;Bruce and Twigg 2005;Story et al. 2004), and interactions with predator effects (Reddiex et al. 2002) or myxomatosis . ...
Concluding remarks: The presence of macroparasites can affect the pest status of small mammals and the damage they cause. Pest management of small mammal populations can also affect the macroparasite populations, in a positive as well as a negative way. Despite the effects of macroparasites on small mammal fitness, there is little hope for the near future that they can be used for biological control of small mammals, except perhaps for some bio-pesticides. Small mammals and macroparasites interact in complex ways, and the implications for pest management are equally complex.
... The recorded reductions in the damage by rabbits and costs to control them are huge (Saunders et al. 2002). Also in the case of the RHD, the effects of the introduced infections are strongly affected by other factors, such as aridity of the environment (Cooke and Fenner 2002;Mutze et al. 2002), virulence of the RHD strains or general condition of the rabbits (Bruce et al. 2004;Bruce and Twigg 2005;Story et al. 2004), and interactions with predator effects (Reddiex et al. 2002) or myxomatosis . ...
The idea for a book reviewing current knowledge on mammals and their parasites emerged during a visit by one of us (SM) to the laboratory of another of us (BK) at the Jacob Blaustein Institutes for Desert Research, Ben-
Gurion University of the Negev (Israel) in December 2004, with RP becoming associated with the project from its very beginning.
Frankly, we decided to restrict our focus to macroparasites, i.e., metazoan parasites such as helminths and arthropods. A second volume at least would be necessary to cover microparasites, i.e., viruses, bacteria, and protozoans. We also decided to restrict our scope to small (=micro) mammals, because they are the most abundant and diversified species in the order Mammalia. Moreover, most of our knowledge on the interactions between mammals and their macroparasites concerns small mammalian species, mainly rodents, but also insectivores, lagomorphs, and bats.
Our idea was to associate disciplinary fields (taxonomy, phylogenetics, physiology, genetics, ecology, evolution, conservation biology, mathematical epidemiology) that may not have enough opportunities to exchange and debate ideas. What better opportunity can there be than a book on the evolution and ecology of host–parasite interactions, and moreover, a book that focuses on and emphasizes a particular group of hosts and their parasites? A symposium on “Parasites and mammals: A macroecological perspective” (organized by BK and SM) at the 9th International Mammalogical Congress in Sapporo (Japan), held in August 2005, allowed us to
finalize the project with Springer Japan.
The book is conceived for a broad audience. Students will find up-todate reviews and state-of-the-art syntheses in several domains. We hope that they will find ideas and opportunities for new research and new applications. Senior researchers, who try to maintain themselves at the forefront of their discipline, will also be interested readers. They are forced to specialize, leaving them little time for exploring other fields, even those
closely related to their interest. This volume is organized in order that they will easily find reviews, summaries, data and references. Environmental managers, veterinarians, and conservationists have to use the results of
fundamental science for their daily tasks: evaluating different options to manage natural populations and habitats. They have to deal with and/or know that parasitism and diseases are important emerging problems. They need to have a clear picture of current knowledge, and the contributions in this book will prove invaluable.
The volume is divided into six parts, including a brief opening introduction explaining what micromammals and macroparasites are. The second part presents the major taxa that parasitize small mammals: helminths
(trematodes, cestodes, nematodes, acanthocephalans) and arthropods (ticks, mites, lice, fleas and bat flies). We did not include dipterans that are not normally considered as parasites but as blood feeders. Besides, the
main victims of dipterans are large rather than small mammals. In addition, we did not consider the chewing lice (Ischnocera, Amblycera) because they are generally understood to be commensals rather than parasites. A
review of the diversity of species, life traits and life cycles, and also of the known effects of these parasites on their hosts, is provided for each of these taxa. The third part deals with some ecological and evolutionary patterns of parasite associations: parasite species diversity, host specificity, co-speciation and co-phylogeography. The fourth part explores the processes that operate in parasite associations at both higher (populations and communities) and lower (individuals) levels of biological organization. Mathematical epidemiology, community ecology, physiology (with endocrinology, metabolism and immunology) and genetics are explored. The fifth part provides practical examples or applications of ecological concepts to management purposes: conservation biology, and the ecology of human and animal health. The volume ends with a conclusion that explores the future of host–parasite interactions in the face of global change.
... A second viral pathogen, rabbit haemorrhagic disease virus (RHDV, also known as rabbit calicivirus) escaped from a quarantine facility in Australia in 1995 and was introduced illegally to New Zealand in 1997. In both countries, the efficacy of the disease has been somewhat variable by region (Story et al., 2004;Parkes et al., 2002) but overall has been successful in substantially reducing rabbit populations. There are, however, signs of disease resistance developing in rabbit populations in both Australia and New Zealand, calling into question the long-term efficacy of RHDV as a control agent (Elsworth et al., 2012;Parkes et al., 2008). ...
Invasive vertebrate species have had a dramatic impact on the unique native ecosystems of both Australia and New Zealand. Some of these species were accidentally introduced, though many were introduced deliberately for a number of reasons: as a food resource, for hunting and trade, as a mode of transportation, as a control tool for other pests, and by acclimatisation societies to remind colonists of home. Regardless of the method of introduction, these invasive species have had major negative impacts on the native flora and fauna, including herbivory, predation, competition, disease, hybridisation and habitat change, and have also affected human health and industry. In both countries the aim is now to prevent establishment of new invasive species and preserve key areas of high biodiversity value through the control or eradication of invasive species. Introduction Invasions of vertebrate species into habitats outside their natural range have had major impacts across the globe and particularly in Australia and New Zealand (Simberloff & Rejmánek, 2011). Preventing the arrival of these species is the best protection for native ecosystems, but once introduction and spread have taken place, effective and efficient management of entrenched species is the goal. Sound management decisions rely on detailed information on the invasive population, the type and degree of impacts, and the strategic, science-based application of control.
... Common mynas (Acridotheres tristis) damage horticultural crops, compete with native bird species for nesting sites and defecate profusely, fouling public places (Olsen 1998). Perhaps our best recognised vertebrate pest, the European rabbit (Oryctolagus cuniculus) continues to flourish and degrade the environment despite the relative success of myxomatosis and rabbit haemorrhagic disease (RHD) in reducing its numbers (Coman 1999;Story et al. 2004). However, relatively recent and increasing knowledge of the impacts of the European red fox has seen it become recognised as one of Australia's most significant vertebrate pests (McLeod 2004). ...
Regardless of their sex and age, the persistence of 76 rabbits
(Oryctolagus cuniculus) translocated onto 5 different
sites with relatively low rabbit densities was identical to that of resident
rabbits. Emigration and exploratory movements by rabbits from 12 discrete
populations were positively correlated with rabbit density and mainly
undertaken by adult rabbits. Adult males moved significantly more often and
further than adult females. Two peaks in immigration were observed; a large
peak (usually in January) immediately following the breeding season, and a
second but smaller peak in March which preceded the start of the next breeding
season. Again, significantly more adult males than females immigrated. The
proportion of rabbits seen in spotlight counts was positively correlated with
rabbit density, which suggests that biases in population estimates could
result in some situations. No clear patterns on the effects of a variety of
weather variables on spotlight counts could be established, but increasing
rainfall, wind speed and moonlight may have reduced rabbit activity.
Quarterly spotlight counts of rabbits were conducted at three sites in
central-western New South Wales. These counts commenced two years before the
arrival of rabbit haemorrhagic disease (RHD) in the winter of 1996. The
existing data on quarterly rates of change in rabbit abundance for the three
populations provided a unique opportunity to study the effects of RHD on
rabbit demography. Prior to the arrival of RHD, all three populations
underwent phases of sequential increase and decrease in each year. On the
basis of these patterns, RHD had a variable influence on the demography of the
three rabbit populations. In 1996–97, the density of two populations
declined over an expected period of increase, while at the third site the
density increased as expected from pre-RHD patterns. Twelve months after their
failure to generate expected positive rates of increase the two affected
populations had returned to the normal sequence of increases and decreases in
density although still at comparatively low numbers.
I report on the second part of a 2-phase experimental test of the predator regulation hypothesis. I examined the effect of predation by red foxes (Vulpes vulpes) on the population dynamics of European rabbits (Oryctolagus cuniculus) in montane Australia. Foxes were permitted to reinvade 2 sites where indices of rabbit numbers had increased 10.3- and 23.3-fold after 20 months of fox removal, and compared trajectories of these rabbit populations with those at 2 other sites where fox populations were not controlled. Over 16 months, foxes returned to both removal sites, reaching levels comparable to those on nonremoval sites, but lower than preremoval densities. Rabbit populations declined immediately after foxes reinvaded and remained low for 16 months on one site, suggesting that fox predation was effective at regulating numbers. However, the rabbit population on another higher-density site recovered and increased another 23% over the following 16 months, suggesting they were not regulated by predation. Rabbit numbers at nonremoval sites continued to be suppressed. These results did not provide consistent support for the predator regulation hypothesis, but provided evidence that rabbit populations may escape predator regulation once they exceed a critical density.
Our paper examines recent developments in climatology, systems analysis and decision support which are relevant to the management of northern Australian savanas. The structure, function and use of these communities have been well described in previous reviews which show the importance of pastoralism as the major economic activity. Annual variability of rainfall is high, resulting in uncertainty in management decisions. Systems analysis models of pastoral enterprises are being constructed which predict the response of savannas to management alternatives against a background of annual climatic variation as well as expected long-term global climate change. In northern Australia, El Nino/Southern Oscillation events account for over half the major ecologically significant droughts. The seasonal persistence of the Southern Oscillation phase allows forecasts to be made before the onset of summer rains. The potential of such forecasts is examined with respect to savanna management in northern Australia. Models of soil water budgeting, grass production, pasture utilization, animal production and financial analysis are being developed for each savanna community in northern Australia. The key processes in these models are plant growth as a function of climate inputs and the effect of grazing on plant survival and production. We describe a general experimental methodology to apply existing models to specific grassland/soil combinations. Examples of the application of these models show that: (1) periods of overgrazing can be identified when model output is combined with regional animal number statistics; and (2) management decisions such as burning can be improved when ENSO based forecasts are used. The challenge of future savanna studies is to influence individual managers to make better management decisions based on reliable models of savanna processes. The uncertainty of future climate change suggests more flexible strategies will be required for the evolution of sustainable and economic savanna use.
The age of the European rabbit (Oryctolagus cuniculus) in Australia can be estimated from the formula $\text{log}_{e}\left(\frac{314}{\text{lens}\ \text{weight}\ (\text{mg})}\right)$ , with standard deviation of 11 percent. The formula is based on the growth rates of lenses from 924 rabbits from field populations in which the approximate ages were known, and is calibrated for lens weights up to 250 mg, when rabbits are 24 months old. Rabbits exposed to stressful experimental conditions do not show the same age/lens weight relationships.
Rabbit haemorrhagic disease virus (RHDV) is foreign to Australia, and first entered populations of Australian wild rabbits (Oryctolagus cuniculus L.) in Australia in late 1995. Rabbits are serious environmental and agricultural pests in Australia, and RHDV, a major new pathogen, was introduced as a biological control agent to reduce their numbers. Our study evaluated some of the factors affecting survival of wild rabbits exposed to rabbit haemorrhagic disease (RHD) at 78 sites across Australia.Our data on rabbit numbers consist of the number of rabbits per spotlight kilometre present shortly before and shortly after an RHD outbreak at each site. They are a direct measure of survival rather than mortality. By reducing the interval between the pre- and post-RHD counts to the minimum possible, we sought to minimise the influence on the analysis of other causes of change in rabbit numbers. We calculated proportional survival as the ratio (number of rabbits present after RHD)/(number present before RHD), and used regression analysis to relate it to environmental and other variables. Proportional survival was lower at higher densities of rabbits; was lower if RHDV arrived naturally at the site rather than if it was deliberately released; was lower in areas with hot, dry climates than in areas with cold, wet climates; was lower in southern, inland areas than in warm, coastal areas; and, if the outbreak occurred during summer, was lower in areas of winter rainfall than in areas of summer rainfall. Rainfall seasonality was not correlated with survival at other times of the year. Only in the last effect was there a significant interaction with the time of the year that the outbreak occurred.Our statistical model describes correlations among the data, but does not in itself establish cause and effect. We interpret the properties of our statistical model to draw the following conclusions. First, the effectiveness of RHD is reduced in cold, wet areas and warm, coastal areas, because of the prevalence in these areas of one or more pre-existing caliciviruses in rabbits that impart year-round resistance to RHD. Second, we conclude that the poor summertime performance of RHD in areas that are wet in summer could result from poor survival of RHDV exposed to the combination of high temperature and high relative humidity, although it is also possible that during summer the effectiveness of vectors declines.
Statistical models have been developed to explain the influence of age and maternal antibody on the outcome of rabbit haemorrhagic disease virus (RHDV) infection of Australian wild rabbit kittens in terms of survival, survival time in those that failed to survive, and pyrexia (temperature response, or fever). Similar models describing survival and survival time were derived by substituting mass for age, owing to their high correlation. The models were developed from data obtained following the inoculation of 78 kittens 5–11 weeks old born to does with varying levels of α-RHDV immunoglobulin (IgG) antibody as measured by enzyme-linked immunosorbent assay (ELISA). A significant correlation was found between survival and doe titre but not between survival and kitten titre. It was deduced that maternal antibody in kittens can fall below the level of detection in the ELISA but still be protective. The model describing the influence of age and doe titre had the form logit(SURVIVAL) = 5.98 – 0.944(AGE) + 0.000473(DOE TITRE), and showed, for example, that kittens born to seronegative does had a 50% probability of survival at about 6 weeks old and, with a doe titre of 10 240, a 50% probability of survival at about 11 weeks old. There was a significant influence of kitten titre on pyrexia, and the model developed had the form logit(TEMPERATURE RESPONSE) = 1.436 – 0.0531(KITTEN TITRE). The influence of age and maternal antibody on survival time was fitted using Cox proportional hazard models. A parametric regression gave rise to a final model of the form S(t | AGE, DOE TITRE) = exp[–(t/ηAGE)1.7979], where ηAGE = exp[7.1838 – 0.2976(AGE) + 0.000227(DOE TITRE)]. The model showed that survival time decreased with age, but for each age category there was an increase in survival time with increasing doe titre. Kitten titre had only a marginally significant effect on seroconversion; there was no effect of kitten age or mass, or of doe titre. No kitten with a titre of 60 or more seroconverted.