- 1. ABSTRACT.
- 2. PREFACE.
- 3. INTRODUCTION.
- 4. WHAT IS THE MAIN ISSUE?
- 5. SOME ECOLOGICAL MATTERS
FOR CONSIDERATION.
- 5.1 The Rabbit as a Pest in New Zealand.
- 5.2 Assessment of Current Control Measures.
- 5.3 RCD Virus as a Biocontrol Agent in New Zealand.
- 5.4 The Intended Programme in which RCD Virus is Proposed to be used.
- 5.5 The Effects, Positive and Negative of RCD Virus.
- 5.6 The Likely Success and Costs of Measures to Ameliorate Negative Impacts.
- 6. REFERENCES.
- 7. APPENDICES.
Application to Release Rabbit
Calicivirus Disease Virus (RCD Virus) in New
Zealand - 1996
Comment on some Ecological Issues surrounding the Application and the Submissions in response to the Application.
FEBRUARY 1997
This report is CONFIDENTIAL to the Chief Veterinary Officer,
MAF Regulatory Authority;
PO Box 2526
WELLINGTON
Robin A Fordham B.Sc (NZ) M.Sc PhD (Well.)
Associate Professor in Ecology, Massey University.
1.1 The predictability of ecological events in any system is limited (Section 3.1).
1.2 Central to the proposal to add RCD virus to New Zealand is the question can people ever manage the natural environment? Natural systems are not amenable to management by means of large scale manipulations. All manipulations create ecological ripples running on through time. The ecological boundaries of the rabbit problem are hard to define, and the expectation of managing rabbits by releasing RCD virus is likely to be unrealistic (Section 4.1).
1.3 The relative ecological impacts of releasing or conserving RCD virus can be assessed in terms of viral Aefficiency@, and changes in the relative levels of mammal predators and native prey over time. The need for national control of predators is outlined (Section 4.2).
1.4 Biocontrol of weed and invertebrate pests is common. The use for the first time in New Zealand of a disease to control a wild mammal would, however, be a major step because rabbits and their predators have a significant influence in many ecosystems. The complex long term ecological effects of such an action are incalculable at present. A similar argument would probably not apply to the possum (Section 4.3).
1.5 The ecological instability of soils and the biota in severely rabbit prone areas raises issues of wise long term land use that could now be reassessed (Section 5.1).
1.6 Existing measures for killing rabbits can affect native species directly or indirectly, but can be timed and localised with predetermined intensities. This sort of planning would, however, be largely unavailable with RCD virus after its first release (Section 5.2).
1.7 Any release of RCD virus would be an ecological invasion which should be accompanied by a rigorous monitoring programme; novel research (refer FfRST); and reference to the ISSG. Such monitoring could be instructive if introduction of any other biocontrol pathogen for a mammal was ever proposed (Section 5.3).
1.8 It is likely that appropriate existing rabbit control measures would need to complement RCD virus indefinitely (Section 5.4).
1.9 Significant reduction of rabbits would have mixed effects on important plants, but overall would be an ecological plus. Aside from predators (see 1.10) risks to important animals are unclear, mainly because many areas of viral performance are quite uncertain (Section 5.5).
1.10 Because of the large, nationally distributed load of mammal predators associated directly and indirectly with rabbits, the rabbit problem is unique. No other mammal herbivore in this country supports a like load. The serious, pervasive, lasting, ecological ripple effects of the predator guild attendant on rabbits following a release of RCD virus, would also be unique. Similar problems would not arise if, say, possums were in question as a target for a biocontrol pathogen because possums have no effective predators, and the ripples in populations of predators and other prey would probably be negligible (Section 5.6.1).
1.11 If rabbit number fell sharply predators could be expected to learn quickly, specialise in new prey, lose condition, breed less well, die, and disperse. Marked variation between areas in rabbits knocked down would affect all these variables (Section 5.6.2).
1.12 Predators have relatively large home ranges and disperse long distances, many kilometres in the stoat. Heavy rabbit mortality would be likely to induce early local loss of new prey, including native species. Later, through dispersal and inter-predator effects, there could be other losses in distant rabbit-free areas with different habitat and different native animals (Section 5.6.3).
1.13 The path for many native animals is ultimately downhill to extinction unless predators are controlled. The RCD virus proposal provides the opportunity, and justification, to institute a national predator control strategy. Establishment of appropriately resourced programmes would reflect the intrinsic value perceived by the Crown to reside in native animals and plants, and would produce better management of New Zealand ecosystems (Sections 5.6.4 & 5.6.5).
1.14 In summary there are many ecological uncertainties: (a) consequent on the short and long term performance of RCD virus in New Zealand situations and, (b) surrounding the direct and indirect effects of the national guild of mammal predators associated with rabbits. In the long run RCD virus would not control rabbits by itself because natural evolutionary processes in the virus and the rabbit would stop it. Complementary conventional measures would always be needed and these costs need assessing.
The rabbit problem is unique because of the predators associated with rabbits. Whether or not RCD virus is released, the proposal to release it highlights the urgency for nationally coordinated programmes aimed against mammal predators, and in protection of, native species.
Finally, serious questions need asking about whether severely rabbit prone land should remain in "production" or whether wiser use of such land, in terms of ecological stability, should be identified.
I am deeply indebted to Suzanne Bassett for energetic and insightful assistance with researching literature, and to Jodi Matenga for her patient and meticulous conversion of words to files.
The views in this report are those of a general ecologist with interests in populations and over 30 years experience in research and teaching in New Zealand, Scotland and Canada. The report reiterates useful points already raised by others, and attempts to emphasise issues that appear significant ecologically.
A philosophical note is relevant. Introductions of rabbits, and later predators intended to control them, have led to consequences far beyond those ever envisioned. If one thing about the proposal to release RCD virus can be foretold, it is this: history shows that the introduction of any exotic organism is, in ecological terms at least, largely a jump into the unknown.
This section expresses caution about making ecological predictions. The broad nature of the rabbit problem is recognised. A preliminary report (January 1997) on potentially significant ecological issues in the proposed importation of Rabbit Calicivirus Disease Virus (RCD Virus) to control rabbits comprises Appendix
3.1 Limitations to Ecological Prediction.
While physical sciences may often work with values that are near absolute or constant, or processes that have near certain outcomes, the biological sciences offer few certainties. Living systems are immensely variable, changing continuously over space and time. This is because the material from which they are constructed, and the way in which it behaves alters as living components interact with each other and their physical surroundings. Field ecology, at one end of the life science continuum, is eclectic. It deals in likelihoods and probabilities that often are impossible to estimate. There are no absolutes. The ecological processes that are likely in any field situation are generally known. But their actual occurrence, the final variations of the processes, their timing, and the magnitude of any effects are questions until they have been studied. Because of this the ecological outcome of either releasing or conserving RCD virus is ultimately uncertain. Formal acknowledgement of the lack of relevant information is provided by FfRST. In July 1996 the Foundation sought interest for research under the Public Good Science Fund (Output 15) on:
"The provision of a conceptual model that can be verified and will predict how ecosystems containing threatened or iconic species (plant and animal) may respond to species invasion or predator changes should rabbits be removed from ecosystems which are considered to be rabbit prone".
The proposal by FfRST under Output 15 implies an difficulty in predicting ecological outcomes in an important area, should RCD virus be released. The biology of the virus and the consequences of its behaviour constitute another important area where uncertainties have the potential to significantly affect ecological prediction.
3.2 Ecological Problems Associated with Wild Rabbits.
The rabbit was introduced to New Zealand in multiple releases for food and trade22. A herbivore (primary consumer), the wild rabbit generates serious problems of plant consumption and substrate damage in some localities. Aside from domestic stock sometimes occupying the same range, in no New Zealand locality is there a varied guild of differently sized mammal herbivores, as in African grasslands, which partition the plant food they evolved with. In New Zealand the rabbit is isolated as the dominant wild herbivore grazing in modified pastoral ecosystems. This, together with a capacity for marked fluctuation in demography, contributes to floral and edaphic instability.
Associated with rabbits are several introduced mammal predators, notably ferrets, stoats and cats (Appendix 2). These carnivores bring further problems by carrying bovine Tb, and/or preying not only on rabbits but on a wide range of native vertebrates and invertebrates that were never designed by evolution to deal with them23,31,85. Moreover, in regions where rabbit densities are high mustelids and cats have little or no regulatory effect on rabbit numbers15,22,54,86.
Trends in sea and land temperatures reflect hemispheric-wide climate warming57,58,81. Ecological problems associated directly and indirectly (through their predators) with wild rabbits could, therefore, intensify in the medium to long term.
In short, every ecological problem associated with wild rabbits is the result of the deliberate, assisted introduction of rabbits, their direct predators, and other predatory mammals. All these introductions may now be seen as the result of bad, ill-informed decisions that have led to huge environmental and economic losses for the nation. Release of RCD virus would be one more attempt, like the use of predators, poisons and shooting, to correct the original rabbit mistake.
Issues surrounding the conservative stance of no action on these problems, or the alternative of attempted mitigation by human intervention, are addressed below (Sections 4 and 5).
3.3 A Visual Model for Populations.
The rabbit, and each of the predatory species, comprise local populations linked via dispersal into larger (meta-) populations40. Numerical losses in one sub-population can therefore be compensated by increases elsewhere. Populations can start in new areas from dispersing individuals, or go extinct after a period. The initiation, interconnection, extinction, and possible re-starting of local populations in different areas over time is described by the visual model shown in Figure 1. The general notion of populations as entities in space and time is fundamental to understanding ecological processes affecting rabbits and the attendant predators. Put simply events at one place and time have consequences in other places later. If RCD virus is released and persists, it will also fit this model.
Conceptual model for population anatomy based on Rostownew=s53 drawing of the stelar structure of the adder=s tongue fern. The stele represents the space-time reticulum created by the distribution of an arbitrary density level through time.
Figure 1:From Taylor L R & Taylor R A J (1977). Aggregation, migration and population mechanics. Nature 265 : 415 - 421.
This section is a brief overview of ecological matters arising directly from the main issue at the heart of the problem.
The proposal to add RCD virus to the suite of methods already deployed in control of rabbits raises a fundamental question. Can people ever "manage" the natural environment? Here management means direct manipulation of some environmental component(s). Successful management would imply that after a manipulation the environment continued to operate as before, with relatively stable nutrient and energy budgets and insignificant changes to the biota and substrate. The answer to the question depends on the scale of the manipulation and the time over which management is planned. Clearly, however, most introductions (including the rabbit and its predators) fail to meet these criteria.
The underlying rationale for releasing RCD virus is to significantly reduce rabbit numbers. Ferrets and stoats (Appendix 2) were introduced to New Zealand for precisely the same reason, yet manifestly fail. Manipulation of (interference with) natural systems stretches from the application of biocides and nutrients to major changes in land use and extinction of species. Recovery periods during which ecological processes return to normality run from short to infinitely long times.
The release of RCD virus in New Zealand would be a manipulation of all the ecosystems supporting rabbits, together with the species and physical environmental factors which interact directly or indirectly with rabbits. That constitutes a wide net of ecological interactions.
Many organisms have been deliberately introduced to New Zealand. Where those introductions have concerned non-laboratory systems few (if any) have ended up carrying out the precise ecological role expected of them. In other words all introductions initiate pronounced ecological ripples running on through time. Because native species have not evolved with the introduced species they usually have few behavioural or demographic mechanisms for coping with them85. So every wild or feral mammal continuously alters native systems, and all are pests to some degree. There is, therefore, no background of experience in New Zealand, or for that matter any other country to which a foreign species has been deliberately introduced that allows certain prediction of future ecological ripple effects of RCD virus, were it to be released59. The ecological boundaries of the rabbit problem are hard to define and the question remains: can we realistically expect to manage rabbits by releasing RCD virus? The answer is likely to be no. This problem is pursued in Section 4.2.
Arising from the central question in Section 4.1 is a related question of radical significance to the proposal to import RCD virus. Are the ecological ripple effects of releasing RCD virus likely to impact (in a deleterious sense) more, less, or no differently on native systems than the effects of maintaining the status quo? Again, the nature of living systems precludes absolute prediction. Several factors are, however, clearly important:
1 Viral "efficiency"
Aspects of viral biology that would, or could significantly affect ecological processes in native systems include:
(a) the stability of RCD virus,
(b) the host specificity of RCD virus,
(c) the level of viral virulence in New Zealand rabbits,
(d) viral epidemiology.
An ability of RCD virus to remain stable, be host specific, and cause high rabbit losses in successive epidemics would greatly improve predictions of the long term outcome. The natural evolutionary process in species would, however, indicate that both pathogen and host populations will alter over time i.e. they will co-evolve. The Myxoma virus provides a relevant example. Ecological ripple effects would then be seen in rabbit survival, viral epidemiology, and the demography of animals and plants interacting with rabbits. Since the timing or extent of any evolutionary changes cannot be foreseen, neither can the effects.
2 Relative levels of mammal predators and native prey species
Mammal predators already prey upon native vertebrates and invertebrates whether or not rabbits are also available15,28,30,44,45,47,56. Absolute and relative levels of predators and native prey species vary with local conditions and influential historic events (weather, food, production and the like). Nationally, the effects of mammal predators are increasing by:
(a) natural diffusion of predators into new areas and habitats. Repeat immigration to areas from which they have previously been removed also occurs,
(b) accidental and deliberate release of captive predators,
(c) widening exploitation of native prey species.
Whatever the regional stocks of native prey species are now, they are highly likely to be lower in the future from continuous depredation. In other words native prey species will continue to suffer losses (for some long-lived slow maturing species e.g. Kakapo, Kaka, Kiwi unsustainable) from predation whether or not rabbit numbers are reduced by RCD virus.
Release of RCD virus would be expected to reduce the availability of rabbits to mammal predators. It may also locally, but temporarily, cut predator numbers by influencing their dispersal and death. In addition, however, predator exploitation of non-rabbit prey (in particular native species) can be expected to continue in neighbouring, less affected areas.
The point is this. If RCD virus is released native prey species will almost certainly continue to decline through increasing exploitation by mammal predators - relative population levels being determined by local rabbit numbers and current and historic environmental factors. If RCD virus is not released native prey species will continue to decline by predation anyway. To quote Brockie6 "It must be assumed that rats, cats and mustelids continually threaten the survival of vulnerable native species". Thus aside from the field performance of the virus which has uncertain ecological consequences, the release of RCD virus would produce ecological problems more associated with rabbit predators than with rabbits themselves.
The main ecological "default" issue, then, becomes one of managing the mammal predators irrespective of release of the virus. Here, the first option is to do nothing and, over time, let all introduced predators and their native prey species slide to equilibrium. This option would unavoidably mean the extinction of may native species in a wide range of groups. The second option would involve establishing nationally coordinated anti-predator programmes. If this were achievable some significant ecological ripples consequent on any release of RCD virus would be smoothed.
The suite of introduced mammal predators in New Zealand includes whole species (all mustelids and rodents) and feral individuals (e.g. cats and dogs) that profoundly affect native species - snails, lizards, petrels, native pigeons and kiwi are examples. The present ripple effects of some predators are so serious that it is abundantly clear they should never have been introduced. This view, of course, begs the question that native animals have intrinsic values that should be retained at the expense of introduced animals.
One thing is clear. There is a fundamental difficulty in "managing" the native environment for the long term when foreign species are introduced.
As argued above (2) loss of native species - conceivably at an increasing rate - will continue whether or not RCD virus is released, because of the mammal predators already present. If native species are valued sufficiently highly nationally coordinated programmes dedicated to the reduction of mammal predators will be needed. Such programmes would need to be:
1) designed for each species,
2) appropriate to each locality,
3) coordinated with programmes for other species,
4) sufficiently resourced,
5) backed by legislation,
6) linked with public awareness and education.
Each of these elements presents signal difficulties. Suitable new methods and technology may need developing. The resources required, both human and financial, would be considerable, but (if RCD virus is released) might be balanced by lower long term costs for rabbit control and the reduced risk of spread of bovine Tb by mammal predators. The status of mammal predators and their commercial varieties would need revising. For instance the holding, sale1 and release of mammal predators could become illegal except under rigorous conditions. Importantly, public awareness, and acceptance of anti-predator programmes would have to be fostered. In ecological terms any view seeking protection of introduced predators together with the native species they prey on is illogical. Similarly, identification of some introduced predators as "harmful" but others as relatively "harmless" fails to recognise that all wild or feral species disrupt any native community they enter. There are no good mammal predators in New Zealand.
4.3 The Performance of RCD virus.
Biocontrol agents (diseases, parasites, predators) are widely used in this country against weeds and pests, particularly in horticulture. Vespulid wasps are widespread pests for which parasitic biocontrol is being attempted. Biocontrol of a vertebrate by means of a disease agent has, however, not been formally attempted in New Zealand and is a major step. An important feature of rabbits is that they initially helped to generate, and now partly support, nation-wide populations of predators. This does not apply significantly to vespulid wasps. Neither would it apply to the possum if that animal were a candidate for pathogenic biocontrol (See Section 5.6.1).
Viral performance following a controlled introduction has been modelled5 although pertinent information that could influence the models was lacking. In simplest terms a "successful" introduction would cause local rabbit populations to initially follow a path towards extinction, including a severe drop in numbers and structural simplification7. After that many variables - demographic, behavioural, genetic, environmental, and socio-economic7 could affect rabbit numbers. The survival of young rabbits, one pregnant female, or other individuals could eventually lead to the re-establishment of new local populations.
Although introductions of species to new areas can usefully be conducted as experiments4 from which much can be learned, for RCD virus that approach is unavailable because containment of the virus once it is released outside the laboratory is impossible67. Thus calls for trial releases in the field are scientifically, and ecologically appropriate, but cannot be met. Best-estimate comparisons of New Zealand with other countries already carrying RCD virus remain the only option.
5. SOME ECOLOGICAL MATTERS FOR CONSIDERATION.
This section reviews ecological effects appearing to be likely consequences of an RCD virus release. No attempt is made to detail all ecological processes since the parameters are unknown. Rather, attention is drawn to events likely to alter the stability of communities or ecosystems.
The ecological issues discussed here concern only wild rabbits and their surroundings in natural and modified habitats. There are no significant ecological problems directly associated with rabbits held captive or in controlled surroundings providing:
(a) there are no accidental or deliberate releases of these animals. Such events could interfere adversely with any controlled release of RCD virus.
(b) captive animals are appropriately vaccinated against RCD virus and do not serve as reservoirs for the disease.
The numbering and headings below follow those given by MAF RA.
5.1 The Rabbit as a Pest in New Zealand.
The ecologically destructive actions of wild rabbits have a long history in New Zealand22,23,33. Severe local or regional effects53,72 include damaged soils, over grazing and lost production52. There is no doubt that, taken overall, removal of the rabbit from all New Zealand ecosystems would be an ecological advantage to plant communities even though localised conservation issues may arise79. Rabbit predators indirectly increase the pest significance of the rabbit, and the ecological problems they generate are serious (Section 5.6).
In ecological terms most significance attaches to those dry upland areas severely affected by rabbits, with loss of plant cover and eroding broken substrate, where production would be low to marginally economic even if rabbits were absent. Such areas have been the repeated focus of comment about their sensible long term management43,50,62,63,65. Soil and biotic stability is so intrinsically fragile and degraded in these areas that some should be retired from production21,50,62. They could then be managed by the Crown towards restoration so far as that were possible. Rabbit control would become a responsibility of the Crown rather than individuals. The short point is that the use of land for production in some severely rabbit prone areas is ecologically unsound. The RCD virus proposal creates another opportunity to stop this and reassess wise land use strategies.
Existing control measures for rabbits such as shooting and poisoning can have severe effects on rabbits, inducing predators to focus more on available native species79. Such operations can, however, be timed and localised with predetermined intensities. To a degree, then, predatory outcomes can be predicted and mitigating actions on behalf of native species can be planned and executed. This is not the case with RCD virus where the timing, location, and intensity of outbreaks would be hard to foresee74,79.
Of the existing control measures for rabbits poisoning has the greatest potential for affecting native animals. Direct losses of birds and invertebrates can occur3, as well as secondary losses through the consumption of animals already poisoned. But if this involved predators of rabbits dying from eating rabbits killed by poison, the overall result could be positive for the native biota.
The rabbit, and the carnivores brought in to control it are ecological invaders. Each species has thrust its way into New Zealand communities with results that are still unfolding. If RCD virus is released it, too, will be an invading organism, contributing to what Soulé60 called "the onslaught of alien species". The fates of invading species are influenced by combinations of factors10 peculiar to each case. These fall into three groups. (1) Specific features of the invading organism e.g. high rates of growth and reproduction, and a strong dispersal ability84. (2) Features of the invaded region e.g. suitable climate, habitat and food (in this case suitable hosts), and lack of significant competitors. Finally (3) the process by which the invasion occurs or, in this case, is engineered, e.g. how, when, and where the invasion happens. Rigorous analysis of successful invasions is still quite rare, but can suggest key elements in the process. For instance birds released simultaneously at a number of different sites were more likely to be successful than those restricted in their release sites87. An approach such as that probably contributed to the successful introduction of rabbits and their predators32.
For RCD virus several fundamental features likely to promote successful invasion appear to exist. The virus can proliferate rapidly, the rabbit host is here, and the planned process includes simultaneous releases at multiple sites55. Other significant features are, however, less certain67,71 and await demonstration in New Zealand e.g. vector-assisted dispersal of the virus at levels sufficient to induce rapid widespread epidemics; stability in the virus; resistance in the rabbit host; and the risk of cross-infecting other host species. These features could assist or retard invasion by RCD virus.
Simberloff59 points out that pathogens can have controlling ("keystone") roles in the field. Thus RCD virus, if very successful as an invader, would become a keystone organism by controlling a dominant herbivore (the rabbit) which physically shapes whole communities. If RCD virus is released careful monitoring of the process and its repercussions will be essential instruction for all concerned5,53,66,67,77,79. The experience gained by such monitoring could be invaluable if introductions of other biocontrol pathogens for mammal pests were ever proposed.
The role of additional measures to control rabbits depends on the realised pattern of RCD epidemics following a planned release. Epidemics that hit rabbits hard and in addition are complemented by existing control measures between epidemics, are likely to reduce recovery of rabbit populations5. Accordingly in any situation where the virus fails to establish or does so unevenly in space and time74, complementary control measures will be even more essential. A similar view of the planned introduction of RCD to Australia was taken by Studdert61 who supported its release but acknowledged that epidemics may not result and that multiple control strategies would be needed. Events in Australia have now moved on following the unplanned escape and spread of RCD virus to all mainland states2.
The FfRST sponsored work in Output 15 is clearly relevant to following up a viral release. More generally, the Invasive Species Specialist Group (ISSG) of the IUCN8, comprising a world wide network of those concerned with conservation impacts of invading species, is a resource for MAF planners and the Decision Maker.
5.4 The Intended Programme in which RCD Virus is Proposed to be Used.
Brief comment on this is made in Section 5.2. If released, the RCD virus may or may not become established with profound negative effects on rabbits. An intermediate possibility is that outbreaks are highly variable in effect. Because of this it seems likely that it would be necessary to plan for a control regime where appropriate existing measures complemented RCD virus indefinitely. The cost of such a regime requires estimation (See also Section 5.5).
5.5 The Effects, Positive and Negative of RCD Virus.
The effect of RCD virus on wild rabbit populations will be known only after a release. There are too many variables untested in the New Zealand setting to allow confident prediction before a release. What is almost certain, however, is that the virus and rabbits will co-evolve, with changes over time to viral identity and rabbit immunity67,68. Down stream effects of such changes are obscure, but the likelihood that the virus will stay specific to one host for ever is low. In the meantime it should be assumed that some native bird and mammal species, particularly those also occupying rabbit habitat and invariably, or sometimes using ground burrows (e.g. kiwi, penguins, petrels, shearwaters, and bats) will remain potentially at risk from the virus unless tests show otherwise. The risk of direct infection to native animals not occupying rabbit habitat is probably slight.
Significant exposure to viral vectors may constitute another risk factor for native animals. But since it is unclear which, if any, vectors would be likely to operate in New Zealand, the ecological repercussions cannot be assessed.
The indirect effects of RCD virus on native animals via the extended guild of predators associated with rabbits is addressed below in 5.6.
The mix of positive and negative effects of RCD virus on the flora are summarised by several authorities64,69,70,75,78,79. Significant reduction of rabbit grazing would be beneficial overall53,69,72 by slowing direct physical degradation of soils, and allowing plant regrowth52. In Australia removal of rabbits is seen as assisting in the conservation of native plants11 and animals12. Here, the balance of herbaceous and woody weeds, pasture plants, and native plants following an RCD virus release, would vary with locality and land use51. In some cases populations of native animals (e.g. lizards, invertebrates) could be resuscitated by the vegetational changes79. If animal production was to be retained as the principal land use weed eradication programmes might be necessary. What ever way these were achieved, animal diversity would decline once more. If, however, marginal land already severely affected by rabbits was retired from production long term ecosystem stability would almost certainly be enhanced. This point was taken up in Section 5.1.
5.6 The Likely Success and Costs of Measures to Ameliorate Negative Impacts.
5.6.1 The Ecological Uniqueness of the Rabbit "Problem", and the RCD Virus Proposal.
The set of introduced mammals with predatory roles in New Zealand (the guild of predators) is listed in Appendix 2. The species range from obligatory carnivores (e.g. cat, stoat) to opportunist or facultative predators such as the possum.
Discounting domestic stock in controlled settings the rabbit is one of some 22 species of wild or feral herbivorous mammals living in New Zealand32. These species differ widely in overall abundance and include marsupials, the hare, feral stock (horses, cattle, pigs, goats, sheep) and other wild ungulates. All damage native flora or substrates to some degree and some (e.g. possum, goat, red deer) have the status of significant, widely distributed pests.
The rabbit is unique among these herbivores in carrying a load of predators, some of which (the mustelids - ferret, stoat, weasel) were introduced specifically to deal with rabbits. Weasels may now be a minor predator of rabbits, taking mainly other species, including natives35, but cats also prey extensively on rabbits16,18. No other medium to large mammal herbivore in this country supports such a predatory load. Hares (also a lagomorph like the rabbit) are generally at low densities and suffer but little predation from cats in some habitats16,20. Similarly, young possums experience limited cat predation although nationally possums are present in huge numbers18,36. Rabbit populations therefore are directly associated with several numerous widespread, active, carnivores while all other herbivores (excepting hares and possums to a small extent) are essentially predator-free. Moreover, although rabbits are not the direct prey of rats, rodent populations are themselves preyed on by mustelids and cats17,30,34. There is then, a suite of predators directly and indirectly associated with rabbits.
Populations of rabbits, mustelids, cats, rodents, and in turn rodent prey species interact demographically29,30. Essentially predators follow (respond to) changes in their prey22. Accordingly marked changes in rabbit numbers are likely to influence shifts in diet among predators including enhanced negative effects on native species. Changes in the patterns of prey consumed would be expected in any ecosystem undergoing significant adjustment to the relative frequency of species in different feeding (trophic) levels. The changes could include some predators (e.g. stoats) killing more or fewer of other predators (e.g. rats)18,24. A similar response would be generated between the rats and their prey9,26,27 Communities of species in adjacent ecosystems interact continuously by way of dispersal, so these interactions are not confined to locations where rabbits and predators co-exist (see Section 5.6.3).
Sudden, severe reduction in rabbit numbers following, say, release of RCD virus, would almost certainly produce in each community a ripple of effects through the guilds of predators and their actual and potential prey.
Basically, the rabbit "problem" is unique because control of rabbits by measures like poisoning and shooting have, and probably would continue to have, repercussions in predator guilds. This would not arise if, say, possums were in question as a target for a novel biological pathogen. Why? Because possums have no effective predators and even heroic reduction of possums would be likely to produce negligible ripple effects in populations of predators and their prey. The same would apply substantially to hares, and to all the other wild or feral herbivores.
If RCD virus is released predators can be expected to respond individually and demographically. The timing and intensity of any response would be strongly influenced by the level of rabbit mortality in local and adjoining areas. Marked variation between areas in the proportion of rabbits knocked down would affect the speed with which predators reacted42,80, and the nature of the reaction, for instance whether and to what extent they hunted other prey in their own or neighbouring areas.
Local, high intensity, anti-predator control operations prior to a release of RCD virus would plainly dampen early responses by predators to sudden, high losses of rabbits. Current logistical limits to such operations would lead eventually to immigration from uncontrolled adjacent areas. Nevertheless intensive spot control of predators before a release of the virus would be a helpful, if minimal action, with short term benefit. A more far-sighted approach to predator control is addressed below (Section 5.6.4). Given the uncertainties surrounding likely rabbit kills prediction of predator responses is not straightforward.
The responses of predators to their prey fall into two main categories: those to do with an individual hunting, killing and eating a prey animal, and those at a population level concerned with how many predators are generated by exploiting the prey. These categories - respectively the functional and numerical responses of predator theory - are interactive, not discrete25,30,54.
First the individual level. Where rabbits composed a significant element of a predator's diet, relatively rapid loss of rabbits could be expected initially to increase hunger, stimulate hunting39,47, and later affect condition. Hungry predators would change to available alternative prey.
Mammals learn quickly and can specialise44 in taking new prey species while they are available. Here a wide range of native invertebrates and vertebrates are potential targets6,38,73. If alternative food was in short supply for a long time predators would be likely to lose condition39,47.
At the population level, predators able to supplement a rabbit-deficient diet with other foods could be expected to breed normally and eventually recruit young as reproducing adults. Predators short of food and low in condition may not breed successfully, or young survival may be lowered. Dispersal is another demographic response that would probably be stimulated if rabbit numbers fell sharply. This is now discussed.
5.6.3 Predator Dispersal - its Significance to Native Prey Species.
Dispersal into new areas is a normal feature of populations. In carnivores independent males and/or females usually emigrate, avoiding direct competition with relatives and neighbours. By this means populations increase their total range. Characteristically mammal predators maintain home ranges in which they hunt and reproduce. Home ranges can overlap, and vary in size with species, age, sex, and social status of individuals. In cats average male and female home ranges extend over 6 km and 3 km respectively and can be more than 13 km and 6 km6,19. Ferret ranges are over 31 ha for males and 12 ha for females46. In the stoat home ranges average 206 ha for males and 124 ha for females, and can extend 4.0 km and 2.3 km respectively48,49. All of these are significant areas.
An important determinant of predator home range size is the local mix of habitats and the animals they support. In general where prey are uncommon or patchy home ranges tend to be larger than where prey are abundant13,14,41. The species composition, age structure, and overall abundance of prey may vary between seasons and can influence predator foraging and dispersal.
Release of RCD virus causing high rabbit mortality over a short time would, unless alternative prey of an equal biomass were available, convert predator home ranges from resource-rich to resource-poor14,30. A likely consequence is that resident predators would progressively forage more widely to survive. Home ranges would therefore increase in size or shift. This would affect predator guilds in a complex ripple effect. Another consequence is that dispersal would be stimulated. Predators of rabbits in New Zealand ecosystems are highly mobile. Cats can move many kilometres in a few hours6. Male ferrets have been tracked over 1.8 km46, and stoat dispersal is astonishingly far and rapid. A marked male moved more than 20 km37, and a female travelled 65 km in 4 weeks49. These figures imply that mobile predators can inter-link distant areas.
Four main points arise:
1. Dispersal may not be confined to carnivores living in the rabbit area but progressively extend to other predatory species whose non-rabbit prey are in turn affected by dispersing animals (the ripple effect).
2. Arrival of a predator in a new area necessarily means prey will be exploited if the predator is to survive.
3. New areas receiving emigrant predators may be forest, wetland or coastal rather than classic dry inland rabbit habitat. Native species (birds, lizards, invertebrates) living in these habitats would therefore be at increased risk64,73,75,76,79.
4. Mass mortality of rabbits would be likely to induce predator effects in local habitats at first, and more distant habitats later. This effect is shown diagramatically in Figure 2.
The likelihood of ecological effects rolling on over areas and time has not received much attention but is a reason why a national predator control strategy should be in place if RCD virus is released (see Section 5.6.4).
The risks to native species emphasised in this section would, as outlined in Section 5.6.1, be:
(a) influenced by levels of rabbit kills realised by RCD virus in local and adjoining areas. If kills were uniformly high the risk could increase. If kills were very uneven or low the risk could increase, reduce, or be unaffected.
(b) reduced by intensive control of predators in local and adjoining areas prior to any release of RCD virus.
Figure 2. Potential predator ripple effects following a release of RCD virus.
5.6.4 A National Predator Control Strategy - Opportunity and Justification.
The importance of dealing with mamal predators was outlined in Section 4.2. In order to hold the line against the extinction of charismatic native species such as Kiwi, Kokako, Kakapo, stern control of certain mammal predators - especially mustelids, cats and rodents - is already essential. Currently these efforts are constrained by resources and implemented locally (e.g. Lake Rotoiti, Maud I, Lake Waikaremoana, Mapara) whether or not rabbits occupy the same system. If rabbit numbers were greatly reduced by RCD virus these and other measures would become even more important because predation on non-rabbit prey would almost certainly increase.
Assuming that the "do nothing" option is abandoned and some control of predators is carried out two options are available:
(a) Intensive control of predators in local sensitive sites.
(b) Institution of a national predator control strategy.
Option (a) the status quo, would reiterate current piece-meal uncoordinated approaches, certain to leave many unprestigious native species at continuous risk. Option (b) is more than presently exists and would require a political decision involving several government agencies, and the allocation of substantial resources.
Eradication of all predators may be unattainable, but the establishment of an over-arching strategy for predator control is ecologically very sensible. It could coordinate national and regional programmes rigorously targeted at appropriate species, places and times (again, as foreshadowed in Section 4.2). The option taken will reflect the intrinsic values perceived by the Crown to reside in native animals and plants. Objectively, the path for many native species is ultimately downhill towards early extinction unless predators are controlled comprehensively6. Results from Output 15 sponsored by FfRST (see 3.1) should be useful in fine-tuning anti-predator programmes.
5.6.5 Summary of Predator Control.
In terms of the health and overall stability of indigenous ecosystems, including the diversity of animal species, effective, coordinated control of the national predator guild is needed urgently. The rabbit problem is unique because of the mammal predators directly and indirectly associated with it. The same situation does not arise with any other problem herbivore. Use of RCD virus would certainly have an ecological flow on. Enhanced plant cover and soil stability would be very positive. Predation on native animals is likely to be negative, and a strategy for the national control of predators would be sensible. The proposal to release RCD virus provides the opportunity. It also provides the justification for developing one.
A national predator control strategy could, of course, be developed whether or not RCD virus is released. But the links between rabbits and predators suggest that coordination of the two events would produce more effective anti-predator planning and, consequently, better management of New Zealand ecosystems.
Submissions made in response to the Import Impact Assessment and Application (IIA)55 to import RCD virus (June 1996) appear by number and author (e.g. 478 Ministry of Research Science and Technology). Many of the submissions were analysed by Taylor Baines and Associates 82 in December 1996.
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30 King, C.M. (1983). The relationship between beech (Nothofagus sp.) seedfall and populations of mice (Mus musculus), and the demographic and dietary responses of stoats (Mustela erminea) in three New Zealand forests. Journal of Animal Ecology 52: 141-166.
31 King, C.M. (1984). Immigrant Killers - Introduced predators and the conservation of birds in New Zealand. Auckland, Oxford University Press. 224 pp.
32 King, C.M. (Ed). (1990). The handbook of New Zealand mammals. Oxford University Press, Auckland. 600 pp.
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37 King, C.M. and McMillan, C.D. (1982). Population structure and dispersal of peak-year cohorts of stoats (Mustela ermina) in two New Zealand forests with especial reference to control. New Zealand Journal of Ecology 5: 59-66.
38 King, C.M. and Moody, J.E. (1982). The biology of the stoat (Mustela erminea) in the National Parks of New Zealand. II: Food habits. The New Zealand Journal of Zoology 9: 57-80.
39 King, C.M. and Moody, J.E. (1982). The biology of the stoat (Mustela erminea) in the National Parks of New Zealand. III: Morphometric variation in relation to growth, geographical distribution, and colonisation. New Zealand Journal of Zoology 9: 81-102.
40 Krebs, C.J. (1994). Ecology. The experimental analysis of distribution and abundance. Ed 4. Harper Collins, New York. 801 pp.
41 Lavers, R.B. and Clapperton, B.K. (1990). Ferret. In: King, C.M. (Ed). The handbook of New Zealand mammals. Oxford University Press, Auckland. 320-330.
42 Mills, R.G. (1994). Rabbit predators in the semi-arid high country of the South Island of New Zealand. Unpub. MSc Thesis, Lincoln University, Canterbury. 94 pp.
43 Molloy, L.F. (1980). Land alone endures: land use and the role of research. New Zealand DSIR Discussion Paper No. 3. 288 pp.
44 Moors, P.J. (1983). Predation by stoats (Mustela erminea) and weasels (M. nivalis) on nests of New Zealand native birds. Acta Zoologica. Fennica 174: 193-196.
45 Moors, P.J. (1990). Norway rat. In: King, C.M. (Ed). The handbook of New Zealand mammals. Oxford University Press, Auckland. 192-206.
46 Moors, P.J. and Lavers, R.B. (1981). Movements and home range of ferrets (Mustela furo) at Pukepuke lagoon, New Zealand. New Zealand Journal of Zoology 8: 413-423.
47 Murphy, E. and Bradfield, P. (1992). Change in diet of stoats following poisoning of rats in a New Zealand forest. New Zealand Journal of Ecology 16: 137-140.
48 Murphy, E.C. and Dowding, J.E. (1994). Range and diet of stoats (Mustela erminea) in a New Zealand beech forest. New Zealand Journal of Ecology 18: 11-18.
49 Murphy, E.C. and Dowding, J.E. (1995). Ecology of the stoat in Nothofagus forest: home range, habitat use and diet at different stages of the beach mast cycle. New Zealand Journal of Ecology 19: 97-109.
50 New Zealand Ecological Society and New Zealand Society of Soil Science. (1994). Review of South Island high country land management issues. New Zealand Journal of Ecology 18: 69-81.
51 Norbury, D.C. (1996) The effects of rabbits on conservation values. Draft report to the Department of Conservation. 33 pp.
52 Norbury, D.C. and Norbury, G.L. (1996). Short term effects of rabbit grazing on a degraded short-tussock grassland in Central Otago. New Zealand Journal of Ecology 20: 285-288.
53 Parkes, J.P. (1995). Rabbits as pests in New Zealand: a summary of the issues and critical information. Unpub Landcare Research Contract Report (LC9495/141). 39 pp.
54 Pech, R.D., Sinclair, A.R.E., Newsome, A.E. and Catling, P.C. (1992). Limits to predator regulation of rabbits in Australia: evidence from predator-removal experiments. Oecologia 89: 102-112.
55 RCD Applicant Group. (1996) Import Impact Assessment and Application to the Director General of Agriculture to approve the importation of Rabbit Calicivirus. 229 pp.
56 Roser, R.J. and. Lavers, R.B. (1976). Food habits of the ferret (Mustela putorius furo L.) at Pukepuke Lagoon, New Zealand. New Zealand Journal of Ecology 3: 269-275.
57 Salinger, M.J., Allan, R., Bindoff, N., Hannah, J., Lavery, B., Lin, Z., Lindesay, J., Nicholls, N., Plummer, N. and Torok, S. (1996). Observed variability and change in climate and sea level in Australia, New Zealand and the South Pacific. In: Bouma, W.J., Perman, G.I. and Manning, M.R. (Eds). Greenhouse. Coping with climate change. CSIRO Publishing, Collingwood. 100-126.
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59 Simberloff, D. (1990). Community effects of biological introduction and their implications for restoration. In: Daugherty, C.H. and Atkinson, I.A.E. (Eds). Ecological restoration of New Zealand Islands. Cons. Sci. Publ. No. 2. Department of Conservation, Wellington. 128-136.
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67 Submission 478 Ministry of Research, Science and Technology.
68 Submission 493 New Zealand Association of Scientists.
69 Submission 532 West Coast Tai Poutini Conservation Board.
70 Submission 548 Mark, A.F.
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Robin Fordham
Fax: (06) 350 5623
Elizabeth Stoddart
MAF Regulatory Authority
P O Box 2526
WELLINGTON
Fax: (04) 474 4133
Dear Elizabeth
Application to import RCD into New Zealand
PRELIMINARY REPORT
There are two main areas in which ecological issues are particularly important. It is a matter of perspective and judgement whether they are viewed as:
(a) "new significant issues which would fundamentally alter the approach (of the Application)" or
(b) consequences of a decision to import RCD.
1. PREDATORS
1.1 Although they are not new, there are serious ecological concerns embracing the predators supported largely, or partly, by wild rabbits. Demonstrably these predators impact negatively on other animals, including native species.
1.2 The predation load associated with rabbits is, barring hares (which are less common), absent for all other medium - large introduced mammal herbivores in New Zealand. These include the marsupials and ungulates - over 20 species in all - and of course domestic stock. To that extent the rabbit problem in this country is unique. The issue addressed here would not arise if (say) possums were in question as a target for a novel biological pathogen because possums have no effective predators. Thus severe reduction of possums would produce no flow on effects through predator chains.
1.3 Control measures aimed at certain mammal predators are already essential, and implemented locally whether or not rabbits occupy the same ecosystem. These measures would become even more important, if rabbit numbers were greatly reduced by RCD, because predation pressure on non-rabbit prey would increase.
1.4 Intensive control of predators will definitely be needed in local sensitive sites if RCD is released.
1.5 More sensibly, however, a rigorous over-arching strategy for control of predators is required. The establishment of predator control programmes, co-ordinated nationally and regionally, is more than simply a desirable ecological goal. The predators associated with rabbits threaten many native animal species. Objectively, the path for these species is ultimately downhill towards early extinction unless predators are curbed.
1.6 The institution of effective predator control programmes would be a costly political decision. For indigenous ecosystem stability in its broad sense (including animal species diversity), such a step is, however, needed urgently.
1.7 The association between rabbits and the major mammal predators is unique among mammal herbivores in New Zealand. It is therefore relevant to ask: should such programmes that would be directly stimulated by the release of RCD be developed now as germane and requisite parts of the Application?
2 LAND USE/LAND MANAGEMENT
2.1 The possibility of significant reduction of rabbit numbers refocuses attention on sensible use and management of dry/upland country, currently farmed, but marginally productive.
2.2 Strictly in ecological terms (e.g. substrate and biotic stability) there are arguments, long advanced, for no longer attempting to manage such land for production. Instead, land assessed as sufficiently fragile or unstable ecologically could be retired from production and managed by the Crown with a view to restoration, so far as that were possible.
2.3 Retirement of ecologically unstable land with high rabbit numbers and marginal production in favour of Crown management would shift the responsibility of rabbit control away from individuals. It could also, as a consequence, alter the argument for introducing RCD.
2.4 Large scale changes to land use practices, involving severely rabbit prone country, require potentially expensive political decisions. Such decisions could alter the nature of the Application.
3 OTHER ISSUES
3.1 Uncertainty surrounds many aspects of the RCD Application. Predation and land use have been emphasised for raising significant ecological issues.
3.2 Other factors which ultimately would, or potentially could affect ecological processes significantly, include: RCD stability, host specificity, virulence and epidemiology. These are not addressed here.
Robin Fordham
Introduced mammal predators*
In this document "introduced mammal predators" or "mammal predators" refers to:
Kiore (Polynesian rat) Rattus exulans
Norway rat R. norvegicus Rodents
Ship rat R. rattus
House mouse Mus musculus
Dog Canis familiaris
Stoat Mustela erminea
Weasel M. nivalis vulgaris Mustelids
Ferret M. furo
Cat Felis catus
Hedgehogs Erinaceus europaeus occidentalis and Feral pigs Sus scrofa are also notable predators in some communities, but are not included here, unless specifically mentioned. The Brushtail possum Trichosurus vulpecula is an opportunist predator in some situations.
* Nomenclature follows The Handbook of New Zealand Mammals, C.M. King (Ed) (1990)32.
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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