Environmental impacts:
Historically, grazing and burrowing by rabbits has caused major damage to soils and vegetation and markedly altered landscapes (RCD Applicant Group, 1996) (Working Party on Sustainable Land Management, 1994). Early overgrazing by sheep and frequent burning were also significant factors but these practices are uncommon today.
Grazing by rabbits is still closely inter-linked with that of sheep and contributes to low, open vegetation, bare ground, an increasing abundance of unpalatable species such as thyme and a reduction or loss of palatable species such as blue wheat grass, plume grass, blue tussock, fescue tussock and inter-tussock species (Allen et al, 1995).
Those charged with making recommendations or policy decisions relating to rabbit management in the high country would find their roles much easier with a clear separation between the impacts of rabbits and those of farmed animals such as sheep, and a separation between the relative impacts of rabbits and sheep on productive versus environmental values. There is a view in some regional councils that further effort could be made to differentiate these impacts.
However the mix of herbivores (sheep, rabbits, hares, cattle, deer, invertebrates and even wallabies) and variability in climate, vegetation, stocking rates, farm management practices and in the vulnerability of the land, makes a clear separation of impacts a very difficult goal to attain. There would need to be compelling reasons for attempting it (see section 14.1); informed judgement is required.
It is difficult to directly link vegetation changes with declines in rabbit populations because other factors, such as climate, farm stocking rates, etc, are interrelated. (Working Party on Sustainable Land Management,1994).
It is difficult to relate vegetation change and land degradation to causes. The effects of grazing by rabbits and farmed stock (especially sheep) and burning are inextricably linked and are not simply additive, Hence we have been unable to clarify the particular role of rabbits in the overall decline of vegetation and soils. (Allen et al, 1995)
It is difficult to quantify the economic or environmental effects of changing rabbit populations. (RPMS for Marlborough: Operational Plan Report 2008/2009) The National Vegetation Survey Database, being developed by Landcare Research Ltd, holds a collection of vegetation monitoring done in New Zealand and includes the annual vegetation monitoring that was undertaken over the five years of the Rabbit and Land Management Programme. After the RLMP ended, some farmer based landcare groups continued to carry out vegetation surveys on semi-arid land properties.
If this data were available for analysis it might allow a better understanding of the changes that have occurred since the end of the RLMP and give a perspective on what will occur should rabbit populations increase again. However, it is unlikely that the monitoring would enable broad interpretations on the differential impacts of different grazing species on environmental and productive values.
Even though their impacts can not be clearly differentiated from the effects of other grazing factors, there is no doubt that rabbits pose a major threat to the health and integrity of ecosystems and habitats within the semi-arid lands, and they are a direct threat to the survival of numerous threatened plant species present. Maintaining low numbers of rabbits is necessary to prevent damage to ecosystems, threatened plants, their habitats and ultimately the soil, sometimes with subsequent effects on water values.
Plant communities with specific conservation values, and threatened species, can be vulnerable to rabbit (and sheep, deer and cattle) grazing. The Department of Conservation has recently reviewed the potential impacts of rabbits on ecological values in Canterbury.
Direct impacts of rabbits on threatened plants include selective browsing on palatable species. This causes premature mortality and/or reduces the vigour and reproductive output of individuals. In turn this leads to fragmented populations, fewer populations and smaller numbers of individuals. In addition to increased mortality and reduced plant vigour, rabbits browse flowers, seeds and seedlings. This prevents seedling recruitment and causes populations to become dominated by mature and ‘moribund’ individuals, further reducing resilience and vigour of threatened plant populations.
The Tekapo Scientific Reserve provides a good demonstration of the impact that grazing by rabbits and sheep can have on fragile semi-arid land. In 1992 the vegetation there comprised exotic weeds, bare ground and little tussock. Since then, the removal of sheep and a reduction in rabbit populations has led to a significant recovery – tussock cover has increased while the abundance of weeds and the amount of bare ground has decreased.
A high diversity of native inter-tussock species is now also present, including good populations of several dryland rare and threatened plant species. Shrub species are also returning as a result of the recovery of old ‘suppressed’ plants and from renewed seedling recruitment … this recovery emphasises the importance of controlling rabbits, not only to prevent ecosystem degradation, but to allow for recovery of indigenous biodiversity.
High rabbit numbers also pose an indirect risk to valued fauna such as birds, lizards and invertebrates that are vulnerable to rabbit predators (RCD Applicant Group, 1996). This impact is usually highest when breeding and nesting occurs, with both adult and young being put at risk. When rabbit populations are allowed to reach very high levels over time, and then removed by primary poisoning (e.g. 98% kills), there is a heightened risk that the predators previously supported by the high rabbit population will turn to native prey for sustenance.
Prey switching is less likely following RHD epizootics because of lower kills and the fact that young rabbits with natural resistance remain as prey (RCD Applicant Group, 1996). The risk of prey switching can be avoided by maintaining rabbit numbers at low and stable levels. The reduction in rabbit populations following the arrival of RHD in 1997 appears to have favoured the spread of wilding trees in some rabbit prone areas (Fastier, pers. comm.)
(Henning, 2005); this outcome was predicted (RCD Applicant Group, 1996) and illustrates the importance of maintaining a systems approach to pest management rather than focussing on a single pest. It is inconceivable however that rabbits should be regarded as a ‘desirable, biocontrol for wildings.
A survey found that most farmers considered that damage to the soil was the main impact of rabbits (Henning, 2003). Direct damage by burrowing can be very significant in some areas; it can damage machinery such as hay/baleage/winter feed making equipment, cause leakage in border dykes and possible damage to irrigation head-races. Widespread indirect damage occurs when overgrazing of vegetation by rabbits and sheep exposes soils to erosion by wind and water. Loss of soil in this manner effectively means the loss of a non-renewable resource in human time scales, especially in the semi-arid lands of New Zealand.
The Department of Conservation is often criticized for harbouring rabbits on conservation lands. Although the Crown can choose not to be bound by Regional Pest Management Strategies, the Department endeavours to meet the same requirements as apply to surrounding private land. In Otago there are no examples where the Department has not met council requirements (Robson, pers. comm.). Often rabbits are maintained at lower levels than required by councils - to minimise risks to particular ecological values (for example at Flat Top Hill Reserve near Alexandra). Total expenditure on rabbit control in conservancies for the 2008/2009 financial year was approximately $220,000 (MAF, 2009).
Rabbit populations on most of the land administered by the Department of Conservation in Canterbury are stable, with some local infestations. DOC considers that numbers can be held down as long as funding remains constant. In Otago, conservation areas are mostly below MMS 3 apart from some hotspots (Kennedy, pers. comm.). Land Information New Zealand (LINZ) conducted five successful control programmes on other Crown land in 2009 and has two other areas under notice in Canterbury. LINZ expenditure on rabbit control programmes in 2008/2009 was $210,000, with an additional 45 days of internal staff and external project management time spent supporting these programmes (MAF, 2009).
One hundred and fifty years under a pastoral regime, early fires and a history of repeated ‘explosions’ in rabbit populations have markedly modified the original vegetation of the semi-arid lands. In large areas then, the main environmental threat posed by rabbits is to soil and landscape values. By the time rabbit populations reach levels at which significant soil loss is occurring, it is very likely that farm business values (actual and potential grazing) have already been largely lost.
Subsequent soil loss then becomes a matter of public interest, especially on Crown owned land, because of externalities such as offsite threats to water values and, more importantly, the intergenerational impact of the loss of the soil resource. Regional councils have indicated that such overt environmental impacts are now uncommon.
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