REPORT
The framework and criteria used for the presentation of this report is based on the criteria in sections 5.1 to 5.8 inclusive of the "Proposal for the Importation of the Rabbit Calicivirus Disease Virus (RCD Virus) as a Biocontrol Agent. Matters to be Considered for Decision-Making."
1. The rabbit as a pest in New Zealand
The lack of quantified information about the damage rabbits cause to the New Zealand environment makes it difficult to assess the impact of the rabbit as a pest. High rabbit numbers are only one of a number of factors which have contributed to land degradation. It is likely that rabbits are, at least in part, a symptom of the pressures of grazing by domestic livestock and burning for pastoral purposes because rabbits prefer the more open landscape generated by these activities. Consequently, it is difficult to apportion the damage that can be directly attributed to rabbits in relation to the pressures caused by livestock grazing and burning.
Since the Rabbit and Land Management Programme (RLMP), which ran from 1989 to 1995, runholders have made some changes in farm management. They have become more actively involved with maintenance control of rabbits, monitoring of vegetation and land condition, more formal farm planning and diversification. However, there appears to have been less change in other farm practises such as grazing levels and stock management (Parkes, 1995).
| The RCD Applicant Group points out in their Response to review reports, part one: | |
| "Whatever the reason for the current condition of these lands, we must now look forward to the future; we must accept that the land is inherently prone to damage by rabbits; and we must accept that rabbit control will continue to be essential for many years, whatever the land use. Even if the land is abandoned, rabbit control will still be required." | |
Rabbits occupy approximately 56% of the New Zealand land area. Preferred habitats have free draining soils supporting dry open grassland at an altitude below 1000 m. The preferred climate is dry (less than 1000 mm), with long sunshine hours and mild winters, typical of the Mediterranean climate. Some preferred areas have severe winters, which can limit the length of the breeding season. However, this is outweighed by the rapid increase in rabbit numbers within the (shorter) breeding season. When high populations densities are established, competition for food may force them to migrate to less favourable environments where they will survive under a wide range of conditions, e.g. at an altitude of 1600 m.
Rabbits occupy approximately 56% of the New Zealand land area.
1.1 A measure of the severity and distribution of the impacts on pastoral agriculture
It is believed that all the land classified as moderate or above in rabbit proneness, totaling 8.9% of the New Zealand land area, has the potential to be severely damaged by rabbits.
The magnitude of the effects of rabbits on ecosystems depends on the rabbit population density. This is strongly related to the quality of the rabbit habitat, reflected in the ‘rabbit-proneness’ classes. ‘Rabbit proneness’ has been used to indicate the propensity of land to carry different densities of rabbits and the potential rate of population increase. The impacts of rabbits on ecosystems within each proneness class will depend on actual rabbit density and population stability which is attained, as well as the inherent susceptibility of the land to damage.
- High to extremely high rabbit proneness: rabbits are not effectively controlled by natural mechanisms. Populations may build up to high levels and the rate of increase can be rapid. Rabbit population densities may exceed the equivalent sustainable sheep carrying capacity of the land, even if no farm stock is carried. This land often has low resistance to damage and does not easily recover following damage. The effects of uncontrolled rabbit populations on these ecosystems may be severe. Land in this category is restricted to the South Island and is 3.7% of the New Zealand land area.
- Moderate rabbit proneness: these lands are in an intermediate situation. Although rabbit numbers are usually held in check by natural control mechanisms, variations in weather or management practices can induce conditions very favourable to rabbits. Populations may surge to high densities which may damage ecosystems. Some 5.2% of the New Zealand land area is classified in this category and about 57% of this is in the South Island.
- Negligible to low rabbit proneness: rabbit numbers are maintained at low levels through natural control mechanisms. The rate of increase in rabbit numbers is relatively slow. The land tends to have a relatively high resistance to damage and the adverse effects on these ecosystems are therefore likely to be small.
The Application notes too, that the proneness of land to infestation by rabbits is generally inversely related to the inherent productivity (financial returns) of the land.
The case for serious direct economic impact of rabbits on pastoral production on moderate to extreme rabbit-prone land (semi-arid regions and sandy soils) is well supported by submitters. However, it is certain that the majority of New Zealand sheep and cattle farmers do not have an economic problem with rabbits.
In comparison to the total human population and land area of New Zealand, relatively few people and a small area are adversely affected by rabbits.
The impacts of rabbits on pastoral agriculture are four-fold:
- competition with livestock for grazing,
- change in pasture composition,
- land erosion and reduction in soil fertility,
- rabbits support a guild of predators, some of which may be involved in the spread tuberculosis.
Competition between stock and rabbits is highest on the land classes categorised as highly rabbit-prone where somewhere between 7 and 15 rabbits are considered to be equivalent to one stock unit. On this basis the loss of grazing due to rabbits is estimated to be sufficient to support between 2 and 4.6 million sheep nation-wide. However, even when dry sheep equivalents are reliably known it is not reasonable to assume that rabbits can always be replaced by an equivalent grazing density of livestock.
As a general rule, grasses dominate the vegetation community of pastures. The grazing activity of rabbits can reduce this dominance and promote species diversity which is not compatible with pastoralism. Also, former grazing land may be denuded of vegetation or the remaining ground cover becomes sparse. This has often been attributed in whole or in part to the presence of rabbits. The result is that stock numbers have to be reduced and in some areas they have been reduced to zero.
Rabbits cause destruction of soil structure by their burrowing and scraping activities. If this leads to vegetation loss and soil erosion then these effects are not easily reversed.
There are also indirect effects of rabbits on overall economic return to the affected farmers and the wider New Zealand economy. The worst affected farmers are unable to invest in fertilisers and alternative land use regimes because of the recurring rabbit control costs. The true measure of the impact of rabbits must therefore include indirect effects and lost potential for economically and environmentally sustainable use of the land.
Though not proven with direct experiments, a growing body of circumstantial evidence suggests that bovine tuberculosis (Tb) can be spread to cattle and deer by ferrets. It is certain that the number of ferrets (and cats) is higher in areas with higher rabbit abundance, and that reduction in rabbits will cause a reduction in ferrets. Reduction of ferrets may also be necessary to remove the disease from Tb endemic regions. Accordingly, costs of ongoing Tb vector control, Tb testing and culling may be reduced along with the overall risk to our dairy and meat export trade from non-tariff trade barriers (Ecosystems).
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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