1. Abstract
2. Rabbit Problem
3. RCD as a Biocontrol Agent
4. The Effects of
Introduction on New Zealand Flora and Fauna
Increased Predation Pressure on Native Fauna
Impacts on Fauna due to Changes in Flora
Impacts on Flora
5. The Need for Management Intervention to Mitigate Environmental Risks of an RCD Release
Strategic Predator Control
Monitoring Rabbit Density
Monitoring Spread of RCD
Monitoring of Native Fauna and Flora before and after Rabbit Declines
Identification of and Commitment to other Measures Necessary to Mitigate Risk
Effectiveness of Management Intervention in Mitigating Increased Predation Pressure
6. Programme to Release the Virus
PRF 0030
28 February 1997
Barry O'Neil
Chief Veterinary Officer
MAF Regulatory Authority
PO Box 2526
Wellington
Dear Barry
APPLICATION TO IMPORT RCD INTO NEW ZEALAND : ISSUES REPORT
This report follows the Department's preliminary report which identified issues from the public submissions round that it considered were sufficiently "new and significant" to require a further round of public consultation.
The purpose of this second report is to present the Department's opinion as requested on all issues (both new and previously identified) within its area of expertise.
This report deals with issues falling under the broad headings of; the rabbit problem, RCD as a biocontrol agent, the effects of introduction on New Zealand flora and fauna, the need for management intervention to mitigate environmental risks of an RCD release, the programme to release the virus, and information from the Australian experience. Public submissions were received on all these issues and the Department has endeavoured to provide its opinion on them and the associated issues that they raise.
A number of submissions questioned whether New Zealand's rabbit problem was widespread enough to warrant the introduction of a virus that would probably affect the whole country (eg 429, 436).
Much of the problem with rabbits relates to their interference with agriculture. As an agency responsible for conservation, the Department is not qualified to comment on the extent of the rabbit problem for agriculture in New Zealand. Rabbits do cause some problems for conservation in New Zealand (e.g. disturbance of some rare native plants in sandy soils) but these conservation problems on their own would probably not warrant the release of RCD.
If RCD behaved in New Zealand as it has behaved overseas, significant reductions in rabbit numbers could be expected initially. Whether RCD would be successful in the long term as a biocontrol agent is difficult to predict as the goal of introduction is not clear from the application and the behaviour of the virus once here is impossible to predict.
Many submissions stated that if the release of RCD was to realise its full potential as a biocontrol agent it would have to be accompanied by other measures in a long term integrated land management strategy (eg 99,167,175). These comments relate to maximising the agricultural potential of the land and, as such, are not for the Department to comment on at length.
4. The Effects of Introduction on New Zealand Flora and Fauna
Increased Predation Pressure on Native Fauna
Many submissions expressed concern that native fauna could suffer from increased predation pressure following RCD-induced rabbit declines. Such increases could be caused by predator prey-switching and predator guild changes.
Predator Prey-Switching
Predator prey-switching was raised as a concern in a large number of submissions (eg 99,175,257,266,305,383,546,796). It occurs when predators change their diet to include higher proportions of other prey to make up for the unavailability of a regular prey item.
There have been few New Zealand studies on the relationships between rabbit numbers, predators, and alternative prey. Those studies that have been carried out, however, have consistently shown prey switching by rabbit predators such as ferrets, cats and harriers during rabbit declines. The alternative prey that predators have switched to in the wake of rabbit declines have mainly been native fauna. Birds, lizards and invertebrates have all shown to have been targeted (Gibb et al. 1978; Pierce 1987; Pierce and Maloney 1989; Heyward and Norbury 1995).
Recent, as yet unpublished, results from Project River Recovery in Twizel provide more evidence for prey-switching following rabbit declines. Monitoring of banded dotterel nests revealed that chicks hatched from fewer nests in years when rabbit numbers had been dramatically reduced by aerial 1080 drops than in years when they had not (Sanders, M, pers. comm. 1997).
Predator Guild Changes
Predator guild changes were also raised as a concern by some submitters (eg 383, 398, 674). They are changes in a community's relative proportions of predators. Such changes can affect the predation pressure exerted on prey. Rabbit declines may affect predator guilds in a number or ways: reduced competition for resources brought about by declines in some of the rabbit predators; reduced predation by rabbit predators on stoats and rodents; or improved vegetation condition and subsequent increases in suitable prey for newly invading predators.
It is not known for certain whether such predator guild changes would occur in the wake of RCD-induced rabbit declines, and whether they would lead to increased predation pressure on native fauna. The limited evidence collected so far suggests that increased predation pressure due to RCD-induced predator guild changes is a serious possibility. Studies have found evidence for changes in predator guilds due to rabbit declines in semi-arid tussock grasslands, semi-improved pastures, and open forest valleys (Gibb et al. 1978; Pierce 1987; Fitzgerald 1988). The main concern would be if declines in rabbit numbers led to increases in stoats and/or rodents. A study on the Tekapo riverbed in 1986 by Pierce (1987) showed the problems that can be caused when predator proportions change within guilds. Greatly reduced numbers of rabbits led to a decrease in feral cats, ferrets and harriers, but stoats increased in the riverbed area. The stoats (and other predators) exerted more pressure on ground-nesting birds and killed more adults than had ferrets, cats and harriers in the presence of rabbits (Pierce 1987).
Because stoats are mainly diurnal and nocturnal hunters, their increase may also be particularly serious for diurnal prey, such as grand skinks (Graeme Loh, pers. comm). Stoats are capable of living in areas that have become depleted of rabbits because of their small size (hence less energy requirement) and ability to exploit small prey. Furthermore, current methods to control stoats are insufficient to reduce their impact to an acceptable level over large areas.
Consequences of Increased Predation Pressure
Increased predation pressure is likely to lead to declines in the populations of some native fauna and, in particular, some threatened species. This is because in many cases it is their vulnerability to predation (due to life history patterns that include factors like poor anti-predator behaviour and unprotected nest sites) that caused them to be threatened in the first place.
An example of what could happen with increased predation pressure is provided by observations of black stilt populations during rabbit control operations. The huge reduction in rabbit numbers during the early 1950s was accompanied by a reduction in black stilt numbers from 500-1000 individuals to 100-200 individuals by 1960 (Pierce 1987). Pierce raised the possibility that predator diet shifts were a cause of bird declines, though other caused were also considered. Black stilt numbers declined further to about 30 birds by the late 1970s, and have since recovered to about 70 adults, now confined to the Mackenzie basin. During the early 1990s, rabbits were extensively poisoned in the Mackenzie basin under the Rabbit and Land Management Programme. This coincided with lower survival of fledged chicks (Christine Reed, pers. comm.), but again other factors may have contributed. Surveys did not show declines in adult numbers, but this is confounded by active management of the population during this period. It could be argued that without the active management, black stilts may well have become extinct during the latest phase of major rabbit poisonings. Black stilts are clearly one species that could suffer population declines following an RCD epidemic.
Other species thought to be particularly susceptible to increased predation include New Zealand dotterels (about 1350 remain; John Dowding, unpubl. data), New Zealand fairy terns (about nine pairs remain; Richard Parrish, unpubl. data), and caspian terns (about 3000 occur in New Zealand). Further examples include yellow-eyed penguins (for which predation is suggested to be the principal cause of chick mortality in the South Island; Darby and Seddon 1990) and sooty shearwater and little blue penguins (remains of which were found by Alterio (1994) to equal those of rabbits in diet analysis of ferrets in coastal Otago). Other vulnerable species include the wrybill, banded dotterel, black billed gull and black-fronted tern. Predators are also considered a threat to giant skinks in eastern Otago(Graeme Loh, pers. comm). Various species of invertebrates greater than 1 cm in length can also be expected to be threatened by any increased predation pressure (Brian Patrick pers. comm.).
The threat of decline posed to these species could be countered to some extent by management intervention.
Predation Pressure Over Time
The potential amount of damage that RCD induced prey switching could inflict on the native fauna of New Zealand would depend very much on the pattern that RCD outbreaks would take here.
The biggest concern would be if the disease were to perform the same way here as reports indicate it has in Europe, where Brian Cooke (CSIRO Division of Wildlife and Ecology, pers comm. 1996) has reported cyclic epidemics with fluctuations in rabbit numbers. The greater danger posed by cyclic epidemics was raised in some submissions (eg 383, 674). Cyclic epidemics that caused large fluctuations in rabbit numbers are the pattern with the most potential to inflict damage on native fauna. Under this scenario, rabbit declines and the increases in predation pressure (due to prey switching and predator guild changes) that are anticipated to follow such declines, would occur periodically and not just with the initial introduction of RCD. This would present a new problem in the management of threatened species as, although predation increases in the wake of rabbit declines are nothing new, the predation increases associated with cyclic RCD epidemic-induced rabbit declines could be less predictable, less conveniently timed, and more extensive than predation increases following past rabbit declines (as dealt with under the next heading "Relevance of Past Rabbit Control Operations"). The problem could be especially acute for slow breeding species that may have insufficient time to recover between periodic predation surges. All of this would make it very difficult to predict predator/prey scenarios in advance, making the job of protecting key threatened species that much more difficult.
If RCD did not cycle and instead resulted in a steady suppression of rabbit numbers (or if such a steady suppression were achieved by integrated conventional rabbit control) the problems associated with unpredictable swings in predation pressure would be less. However long term increases in predation pressure may still result even without cyclic RCD epidemics due to altered predator guilds (see earlier comments under the heading "Predator Guild Changes").
It the disease did not persist in the New Zealand environment once introduced, as modelling by Barlow (1995) suggests is possible, then cyclic RCD epidemics could not result and the potential damage of an RCD introduction to native fauna would be less.
Relevance of Past Rabbit Control Operations
Large rabbit declines have occurred in the past due to conventional rabbit control initiatives. The limited studies that have looked at the effects of such declines have found evidence that they have had negative impacts on native fauna (see earlier comments on Pierce's 1987 study on the Tekapo riverbed, and Sanders' 1997 results from Project River Recovery in Twizel). The Department has become increasingly aware in recent times of the problems rabbit control operations can cause for native fauna.
The relevance of these past operations is that it might be postulated that rabbit declines brought about by RCD may pose no more of a danger than the rabbit declines of the past. However, this would only be the case if rabbit declines due to RCD were no greater and no differently timed than the declines that occurred in the past. As pointed out in some submissions (eg 674), this would probably not be the case and RCD epidemics would probably cause more extensive and less conveniently timed declines in rabbit numbers that would lead to a greater risk of predation on native species.
RCD epidemics are likely to cause more extensive declines by achieving greater coverage than past control operations as they can reach the scrub covered, steep and mountainous lands inaccessible to conventional control. Also, the epidemics can, and most probably will, occur in geographically contiguous areas simultaneously, whereas the necessity of destocking before poisoning often meant that landowners could not poison all parts of their property at once and hence rabbit declines were less sudden.
In addition, past rabbit control operations have concentrated on areas with very high rabbit numbers. Accordingly, many areas with moderate to high rabbit densities have not experienced drastic rabbit declines in the past and therefore RCD induced declines could represent a new threat to any native fauna living there. Such areas include remnant populations of giant skinks, now confined to semi-improved tussock grassland habitats in the Lindis Pass and Macraes Flat areas (Graeme Loh, pers. comm).
With respect to timing, it is likely that there would be little or no control over the timing of RCD-induced rabbit declines. If epidemics were to cycle as they do in Europe, then declines could occur at any time of the year. This presents a threat, as some native fauna are particularly vulnerable to predation at certain times of the year. Traditionally, rabbit control operations are undertaken in autumn and early winter, when threatened species are less vulnerable (i.e. the breeding season is over, many of the braided river birds have migrated, and reptiles and invertebrates are less active and so not as available). The opportunity with RCD for such delicately timed control would only exist if RCD were not to persist in the environment and was able to be applied as a biocide.
Also, secondary poisoning of predators during past rabbit control operations was mentioned in some submissions (eg 383) as a reason why RCD induced rabbit declines could be more damaging for fauna. It was argued that without the predator by-kill that is suspected to have accompanied such past operations, RCD induced rabbit declines could see greater numbers of hungry predators than before. Studies by Norbury and Heyward (1996), however, indicate that secondary poisoning may not be very important in reducing the severity of prey switching as they only recorded 8% mortality among predators caused by secondary poisoning.
Risk that Net Predation Implications of an RCD release for Native Fauna will be Negative
It is the Department of Conservation's assessment that there is a serious risk that the net predation implications of the rabbit declines likely to follow in the wake of an RCD introduction would be negative for native fauna. Although absolute predator numbers may decline over time (due to declines in rabbits as their primary food source (see Marshall 1963: Gibb et al. 1978; Pierce 1987; Pierce and Maloney 1989; Gibb and Williams 1994; Norbury and Heyward 1995; Norbury and McGlinchy 1996)), predation pressure is likely to increase due to prey switching, predator guild changes and predator movements. This increase in predation pressure is likely to be more damaging than the increases that followed the rabbit declines caused by the traditional control operations of the past. This is because RCD-induced declines may; (i) spread to areas not reached by past control, (ii) occur simultaneously in geographically contiguous areas and (iii) occur unpredictably and at times of the year when some native species are particularly vulnerable.
Impacts on Fauna due to Changes in Flora
The possible changes in flora wrought by the proposed RCD introduction are likely to have effects on native birds, reptiles, and invertebrates.
Birds
The Department of Conservation considers that changes in flora owing to RCD-induced rabbit declines could pose a risk to some native birds - though the magnitude of that risk is highly uncertain.
Increases in tall grasses and shrubs could reduce the nesting suitability of herbfields for birds such as banded dotterels (Bomford 1978)
There is a possibility that RCD-induced rabbit declines could release riverbed weeds such as broom, gorse, willow and lupins from grazing pressure that was limiting their growth and spread (Graeme Loh, Department of Conservation, pers. comm.). This would be a problem for riverbed nesting birds as the weeds would interfere with their habitat. For example wrybill tolerate little or no vegetation on their nesting islands or shinglebanks (Marchant & Higgins 1993).
The role of rabbit browsing in limiting the spread of these weeds is not known. The importance of other factors such as seed sources and spring flooding would also need to be investigated if the threat is to be correctly ascertained. It is suspected that the lowland rivers of Canterbury would suffer from greater weed invasion than the rivers of Otago due to their extensive weed seed sources (Bruce McKinlay, Department of Conservation, pers. comm.). Furthermore spring floods (depending on their frequency) could play a role in preventing the spread of weeds.
Reptiles
It is the Department of Conservation's assessment that some changes in flora caused by RCD-induced rabbit declines may be beneficial for some native reptiles.
If rabbits are significant browsers of woody shrubs then their decline may benefit skinks in conservation areas. A large part (30-40% from some preliminary studies of grand skinks and other diurnal skinks) of skink and gecko diet is fruit of native herbs and shrubs such as Melicytus, Gaultheria, Leucopogon and Meuhlenbeckia. These plants are now often limited to rock outcrop refuges. If reduction in rabbit browsing allows greater growth, fruiting and establishment of these native fruiting plants the skinks would benefit (Graeme Loh, pers. comm.).
Invertebrates
The Department of Conservation considers that the great majority of native invertebrates would probably not be further threatened by vegetation changes caused by RCD-induced rabbit declines.
If the RCD release is accompanied by measures that keep rabbit populations stable and low then the consequent increase in plant biomass in conservation areas could lead to a long-term increase in invertebrate abundance. Invertebrate biodiversity may increase over the longer term as species move out from refugia across the landscape. This will be slow for many invertebrate species, especially flightless species, and the vegetation recovery would have to be sustained for it to happen.
If the release was not accompanied by integrated conventional control then plant biomass is unlikely to recover to the degree necessary for the above benefits to eventuate in conservation areas
General Predation
A further concern was raised by submission 383, that reduced rabbit grazing might lead to increased stoat numbers, and hence increased predation, through a process similar to the mast seeding in beech forests. In beech forests huge production of seed occurs irregularly in a phenomenon known as mast seeding. This represents a large food source for a number of animals, especially mice, birds and invertebrates, which all increase in numbers. Stoats also increase in these years as they prey on the extra birds and mice available. Such a process was observed on Mana island in the early 1980s when grass seeded after cattle grazing ended and the mice population increased. However there were no real predators of mice on Mana island so it is uncertain whether such a process would be repeated on the mainland where cats, ferrets and stoats will all eat mice.
Rare Plants
Only a few rare plants are known to be affected by rabbits. For example:
- The Castle Hill buttercup is browsed by rabbits and has to be given netting protection. It therefore could theoretically benefit from reduced grazing pressure. However, it is unlikely to benefit much as netting protection would still be needed unless virtual eradication of rabbits was achieved.
- Native brooms, many of which are rare or threatened and are eaten by rabbits, would probably benefit from reduced rabbit browsing. Rabbits are currently thought to be severely reducing the recruitment of the rarest native broom in New Zealand, Carmichaelia hollowayi, on Otekaike limestone in the Waitaki valley (Geoff Rogers, Department of Conservation, pers. comm.).
- Some rare native plants in dune hollows or in sandy soils would probably benefit from relief from the disturbance of rabbit burrowing and scraping activity. These benefits could, however, be outweighed by releasing competitor plants that are normally held in check by rabbits. Rabbit exclusion experiments with the habitat of the tiny gentian Sebaea ovata growing on the Wanganui coast have shown that it can be smothered by introduced grasses as a result of reduced grazing (Colin Ogle, Department of Conservation. pers. comm.). Such benefits could also be outweighed for some plants (e.g. Sebaea ovata ) that utilise rabbit scrapes for some of their seed germination (Colin Ogle, pers. comm.).
Uncommon Ecosystems
Rabbits have significant adverse effects on certain types of remnant lowland plant communities. For example, they are believed to intensively crop the palatable native plants and foster the invasion of introduced grasses in dune slacks, a rare coastal ecosystem in New Zealand. They crop silver tussock in the inland low dunes near Cromwell, and following rabbit control on Flat Top Hill Conservation Area there has been a proliferation of Elymus and Festuca, native grasses believed to have been formerly structurally dominant in these very dry tussock grasslands. Such plant communities would probably benefit from reductions in rabbit numbers.
Tussock Grasslands
Tussock grasslands in the conservation estate may benefit from an RCD introduction. Such benefits would depend on the extent of ongoing conventional rabbit control to minimise fluctuations but, even with minimal control, benefits may accrue.
Reduction of rabbit activity would probably lead to a gradual recovery of tussock grasslands and reduction of bare ground. Intense disturbance to soil structure by rabbit burrowing would reduce, with obvious benefits for soil stability, surface soil erosion, increased plant cover, and the diversity of soil biotas.
Rabbits are often a critical factor in the maintenance of mat or scabweed vegetation in degraded tussock grassland. Rabbit reduction might result in structural change, with an eventual return of short tussocks and then tall tussocks. This would lead to an increase in grass biomass which would benefit soil health, soil microbial activity, moisture holding capacity and fertility. There could also be an increase in woody species, which would benefit conservation by allowing expansion of native scrub.
However, as the role of rabbits in containing the spread and density of woody weeds such as broom, gorse, brambles, briar, lupin and boxthorn is largely unknown, the proliferation of such weeds is a risk that cannot be discounted. The spread of such weeds once released from rabbit grazing pressure was a concern among some of the public submissions (eg 30, 416, 674, 710).
The possible increase in woody species due to rabbit decreases may be less likely when one considers that most shrubs require a minimum of 2-3 years to grow above rabbit browse height. Peaks in fluctuating rabbit populations may occur more often than this, preventing such woody species from maturing past their vulnerable stage.
5. The Need for Management Intervention to Mitigate Environmental Risks of an RCD Release
Potential exists for the environmental risks of an RCD release to be mitigated by management intervention
A large number of submissions (eg 383, 546, 674, 687, 790) stressed the necessity for intervention to take place, if a release were to go ahead. The Department of Conservation agrees and considers that, before any approval of the current application is given, appropriate institutional arrangements should be in place to ensure the release of RCD in New Zealand is complemented by risk management measures that are appropriately funded and supported by appropriate research.
If RCD is released in New Zealand, institutional arrangements should be in place so that strategic predator control for vulnerable species is carried out as required at appropriate sites. Strategic predator control would consist of trapping (and other measures) in order to lessen the risk of predation increases due to prey switching, predator guild changes and predator movements.
To lessen the effect of prey switching and predator movements, this control would have to be carried out whenever RCD epidemics caused rabbit numbers to drop. These drops could occur after the first RCD epidemic and then cyclically, depending on how RCD behaved in New Zealand.
Mitigation of predator guild effects may require continual predator control to manipulate the relative and absolute numbers of predator species within the guilds.
Predator control would have to take place at sites where fauna would be vulnerable to the predation increases that rabbit declines could bring. The Department is currently in the process of identifying sites where action would be required if species are to be protected.
Before any approval of the proposal is given, all vulnerable species (and the sites that need to be managed to protect these species) should be identified and agreements reached on responsibility for predator protection. Furthermore, predator protection funding issues should be resolved with decisions made as to how costs of control should be borne.
There do not appear to be any mechanisms currently in place to ensure that the monitoring of rabbit densities at sites identified as containing at-risk fauna is funded and carried out. With the release of RCD, such monitoring would be desirable in order to ensure that predator control is carried out at the appropriate times.
Before any approval of the proposal is given, responsibility for the above rabbit monitoring should be agreed and funding issues sorted out.
There does not appear to be a mechanism currently in place to ensure that monitoring of the spread and reoccurrence of RCD in rabbits throughout New Zealand is funded and carried out. Such monitoring would be desirable, because it would allow predator control actions to be instigated in time to mitigate the anticipated increases in predation pressure.
Before any approval of the proposal is given, agreements should be reached as to who will do the monitoring, how it will be done and how it will be funded.
Monitoring of Native Fauna and Flora before and after Rabbit Declines
Before RCD is released, an overall programme should be set up for the monitoring of native fauna and flora before and after rabbit declines. Such monitoring, for all species identified as vulnerable would be desirable to ascertain the effectiveness of management interventions. It would also provide data for predator/prey models that could be used to refine management interventions in the future.
Before any approval of the proposal is given, there needs to be a mechanism in place to ensure that this monitoring is funded and carried out.
Identification of and Commitment to other Measures Necessary to Mitigate Risk
Before any approval of the proposal is given there need to be arrangements in place to ensure any other measures identified (by suitable research - see below) as necessary to mitigate the risks of an RCD introduction are implemented. These measures might include suitable weed control measures to protect species identified as vulnerable from any deleterious growth in weeds resulting from decreased rabbit grazing. Other measures could include the training of staff necessary to carry out the numerous actions considered necessary to mitigate the risk of an RCD introduction.
Research into modelling of predator/prey systems is being undertaken by Landcare Research. It is expected that the results of this research will help refine management interventions and identify other measures necessary to mitigate the risk of an RCD introduction - such as complementary rabbit control to minimise rabbit fluctuations and hence (possibly) prey switching.
Before any approval of the proposal is, it needs to be ascertained whether it would be appropriate or possible to put more resources into this research in order to get results earlier rather than later.
Research into new predator control techniques is underway and funded, with results
expected over the next 2 two years. This research is proceeding as
quickly as practicable.
Effectiveness of Management Intervention in Mitigating Increased Predation Pressure
The management interventions outlined above are unlikely to counter all the non-intended effects that could flow from an RCD introduction.
It is unlikely that all the areas at risk from increased predation would be identified, and inaccessible sites would not be subject to management intervention. Local extinction of constituent native fauna is therefore a possibility. Likewise, attempted mitigation of any negative effects of weed growth is unlikely to counter all potential problems.
6. Programme to Release the Virus
RCD-induced rabbit declines would be less likely to be harmful to native fauna if they occurred in late summer-early autumn (February, March, April). Predators would (hopefully) starve (or be trapped or poisoned) over mid-winter before the arrival and nesting of birds in August-January.
Another consideration for release dates is the number of young rabbits (which are resistant to RCD) around at any one time. RCD can be expected to be more effective, with less chance of rabbit resurgence, if it hits at a time when there are less resistant young rabbits. These times vary, but March-June is understood to be the general period when young rabbits are least numerous. In New South Wales some rabbit populations appear little affected by the introduction of RCD and this has been attributed to the disease arriving at a time when resistant young rabbits were numerous (though there may be other factors).
A further possible factor contributing toward a desired release date is temperature and its effect on insect activity. In the Australian state of Victoria only 40% of rabbits succumbed to the virus and they have now almost reached plague proportions again. It is suspected that this poor showing is due to cool conditions that led to reduced activity of the insects that normally spread the disease. This perhaps indicates that a release during a warm period would be more appropriate, although one must be careful in applying information from the Australian experience to New Zealand. (Other reports from NSW have suggested that the virus performs less well during the hotter months - so the information from Australia on this aspect is not consistent.)
Therefore, if an RCD release were to occur, the best time would probably be March to April. A co-ordinated nationwide release would be desirable to prevent epidemics reaching some areas at later dates. Any declines that occurred in June or later would be problematic as they would leave predators alive during the vulnerable season for native fauna.
Integration with other control work
This report has already highlighted the management intervention considered necessary to mitigate the environmental risks of an RCD release.
A number of submissions (eg 27,257,454,476,479) wanted the decision to be delayed until the situation in Australia had been properly observed and yielded useful information.
Any information gained from the Australian experience with RCD must be applied with care to New Zealand, owing to the dissimilarity of the two environments. Nevertheless, the Department considers useful information is likely to come out of Australia. A later decision will always be a more informed decision and MAF needs to weigh this up against any perceived urgency for an introduction.
The Department has obtained information on the Australian experience with RCD via the ANZRCDP Rabbit Calicivirus Update Newsletter, through media reports and through discussions between its staff and people involved in RCD research in Australia.
As stated above, the spread and effectiveness of RCD in Australia has been patchy. In some cases it has been extremely patchy - in NSW warrens just a few hundred metres apart have shown totally different responses. Terry Korn, NSW State Co-ordinator for RCD, has been reported as attributing this variability to environmental conditions. He is quoted as saying "What this tells me is that we don't yet know how best to use the virus and how it spreads most effectively, but we'll learn all this in the next couple of years."
Studies in the Flinders Ranges in Australia have revealed declines in cat predators following RCD induced rabbit declines, with no increased predation on small mammals or birds. Predation increases on reptiles and invertebrates were observed, but as the area contained no endangered reptiles or invertebrates this was not a concern. Fox numbers have not yet declined and observers hope their numbers will drop off as a result of breeding and recruitment failure (caused by a lack of rabbit prey).
Monitoring and surveillance in southern regions of the Northern Territory have found increasing populations of small native mammals where rabbit numbers have been reduced by a combination of RCD and warren-ripping.
Though the incidence of increasing small native mammals is good news for the southern Northern Territory environment it is dangerous to assume RCD will have similar effects for native fauna in New Zealand. Unlike Australia, New Zealand does not have any small native mammals that would benefit from a reduction in rabbit competitors. New Zealand does have a number of endangered reptiles and invertebrates that would be at risk if the predation increases that have been observed for those classes in southern Northern Territory were repeated here. Furthermore, the desert environment of southern Northern Territory is very different from any New Zealand environment and contains much lower densities of predators than New Zealand environments such as the MacKenzie Basin and coastal areas.
Possible evidence of prey-switching can be found in the increased mortality of brush-tailed possums due to cat predation as observed in New South Wales (an environment more similar to New Zealand environments) following RCD epidemics.
The information so far is somewhat anecdotal and difficult to draw useful conclusions from. This situation should be improved with the release of a series of reports by Mary Bomford of the Bureau of Resource Sciences in Australia. She is compiling the results from 10 Australian RCD intensive site monitoring and surveillance projects and will provide detailed reports every 4 months for the RCD science sub-committee, which are then given to the RCD Management Group. The first of these reports is due in February.
Contact for Enquiries
Manager, Strategic Science Team
MAF Biosecurity New Zealand
PO Box 2526
Wellington
NEW ZEALAND
Phone: +64 4 894 0115
Fax: +64 4 894 0731
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