- 1.3 A measure of the severity and distribution of the impacts on other economic values.
- 1.4 A measure of the severity and distribution of the impacts on social values.
- 1.5 A measure of the severity and distribution of the of the impacts on Maori values.
1.2 A measure of the severity and distribution of the impacts on environmental values
In New Zealand the rabbit is the dominant wild herbivore grazing in modified pastoral ecosystems. Under favourable conditions they have the capacity to rapidly increase in numbers and contribute to plant and soil instability.
Associated with rabbits are several introduced mammal predators, notably ferrets, stoats and cats. 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. However, in regions where rabbit densities are high mustelids and cats have little or no regulatory effect on rabbit numbers (Fordham).
Ecosystem
There is a lack of well-quantified studies of rabbit impacts on "ecosystem values" [ This term is used to denote ecological processes of a broad scale. ] such as soil nutrients, soil structure, erosion, siltation of waterways and representativeness of natural community structures. Nevertheless, a mass of circumstantial evidence suggests such impacts are very likely to be occurring that such problems of degraded ecological systems are severe in rabbit prone areas. It is logical that they are exacerbated by overgrazing and there is abundant evidence that rabbits reduce the standing crop of plants to extremely low levels in many semi-arid regions. Prolonged suppression of rabbits is likely to ameliorate such effects in the long run provided that farm stocking rates are contained to prevent overgrazing in the absence of rabbits and fire is used sparingly (Ecosystems).
Rabbits damage the soils by disturbing the surface crust, contributing to a loss of carbon and soil fertility and changing the micro-environment of the soil surface. The decline in soil fertility is caused by the decrease in vegetation and removal of nutrients from the ecosystem. Rabbits contribute to soil erosion directly through disturbing the soil surface and through the decrease in vegetation. Erosion also depends on land form, wind and rain. Vegetation decline attributed to rabbits is thought to increase water loss from soils and adversely affect plant establishment and growth (Ministry of Research Science and Technology).
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 management (Fordham). Soil and biotic stability is so intrinsically fragile and degraded in these areas that more land may need to be retired from production.
Water quality may be affected by soil erosion and nutrient effects, however specific evidence of the adverse effects of rabbits on water quality is lacking (Allen et al.1995).
Flora
The most effective way to assess the impact of rabbits is to analyse their impact on individual species as well as on the communities in which they live. However, in the absence of a comprehensive species and community level impact assessment, comment on the impact of rabbits on natural flora can only be made at a fairly general level.
Rabbits have a high impact on native flora via:
- direct defoliation of plants
- modification of plant communities
- destruction of soil structure
Individual rabbits feed selectively and can reduce the abundance of preferred plants, particularly if they are intolerant to grazing. Conversely, plants which are not usually eaten by rabbits can become relatively abundant although they maybe eaten too when rabbits attain high densities (Macaulay).
Rabbit grazing can alter the competitive relationships between the communities' component plants. As a general rule, and particularly in nutrient rich areas, a few species, notably grasses, can dominate the vegetation community. The grazing activity of rabbits can reduce this dominance and promote species diversity (Macaulay).
The impacts of rabbits on indigenous flora and fauna are reviewed by Dale Norbury who notes that most of the published literature is devoted to short tussock grasslands where rabbits are most abundant. Where there is little published literature she is dependent on people’s observations.
In tall tussock grassland communities rabbit grazing reduces tussock cover and stature.
In short tussock grasslands rabbits may be beneficial to some conservation values. Grazing reduces the spread of exotic grasses and herbs thereby providing niches for less competitive native species such as Myosotis "glauca", Lepidium sisymbrioides subsp. sisymbrioides, and Myosurus minimus subsp. novae-zelandiae. However, of 34 species of native flora affected by rabbits only six species are identified by Norbury as deriving benefit from rabbit grazing. The others are adversely affected by rabbits.
Rabbits appear to limit the regeneration of many native shrubs by browsing seedlings. They also browse the foliage and ring-bark some species of native shrubs and trees. A few species of indigenous shrubs appear to increase under rabbit grazing.
On coastal sandy dunes rabbits are a serious conservation pest where they graze many native plants. Sandy soils are highly prone to erosion caused by rabbit activity.
Norbury suggests that in salt pan habitats rabbits are not a serious conservation pest although they do adversely affect some species.
In wetland margins rabbits graze many native wetland species, particularly orchids and sedges. They may play a positive role in suppressing weeds where wetlands border exotic pasture.
| In summarising the likely effects of a reduction in rabbit numbers on conservation values Norbury notes: | |
| "On a broader scale, it has been suggested by Wardle (1989) that New Zealand’s native grassland flora has evolved in the presence of significant grazing pressure from moas, therefore some grazing pressure may be appropriate for conserving native grassland flora. In addition, he argues that grazing is the cheapest tool available to control weed infestation over large areas of short tussock grasslands. This view is particularly relevant now that some areas of this vegetation type are being incorporated into the conservation estate under the pastoral lease tenure review process." | |
| Norbury concludes that, apart from the possibility of increased predation on native fauna, | |
| "rabbit control is likely to directly benefit native flora and fauna in most habitats in New Zealand. However, where exotic weeds are prevalent, such as in the short tussock grasslands, flora and fauna may be adversely affected by rabbit control." | |
Observed changes (following prolonged exposure to high rabbit densities) in highly and extremely rabbit prone land include a decline in biomass, the opening-up of the grass canopy, the loss of conspicuous tussocks and other grasses, the decline of small indigenous herbs (and possibly the loss of some species), the increase in flatweeds (especially hieracium), cushion vegetation and annuals and the increase in the extent of bare ground (Allen, J. et al. 1995).
Due to a broad range of effects, it is likely that some native plant species and communities benefit from the presence of rabbits whilst others are negatively influenced. However, despite some of these complexities, rabbits have undoubtedly decreased native biodiversity and the conservation estate would be better off without them or with lower densities (Parkes, 1995).
Fauna
| Given the appropriate conditions, rabbits can reach high densities and have a marked affect on other fauna: | ||
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Rabbits maintain a short growth form of grass, which does not suit native fauna preferring a habitat with longer vegetation. Conversely their presence probably has a positive influence on those requiring short grassland habitat. Although in some circumstances the latter may occur naturally in New Zealand, in the main, the evolution of New Zealand native fauna has been in the absence of high impact ruminant and lagomorph herbivores such as the rabbit.
| 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. | |
| 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. 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." | |
| It is likely that most native fauna, with the exception of the few that prefer specialist short grassland species, is negatively affected by the type of habitat modification effected by rabbits. | |
| The Department of Conservation (DoC) acknowledges: | |
| "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." | |
DoC undertakes little direct expenditure on rabbit control ($397,000 in the 1996/97 year) compared to other species conservation threats and control is often in their ‘good neighbour’ role rather than for conservation gain. However, their awareness of the potential for predators to switch to native fauna as a consequence of rabbit declines is causing a re-evaluation of this position. Most of the funds were spent on unoccupied crown land and the remainder was for DoC land ($91,000).
The distribution of expenditure on rabbit control per region:
| Nelson/Marlborough | $20,000 |
| Canterbury | $284,000 |
| Otago | $50,000 |
| Southland | $43,000 |
In many areas rabbits form the main dietary item of several predators including the introduced ferrets, stoats, and cats as well as for the raptors such as the New Zealand Harrier. There are several examples in the scientific literature, in which the densities of a key prey species influence the numbers of a predator species (and vice versa) and it is likely that the populations of the above predators are linked in some way to the rabbit populations in New Zealand. Causality in this linkage is difficult to prove, but it is likely that the large numbers of rabbits sustain greater population sizes of the predators than would occur in the absence of rabbits. Rabbits have an important role in shaping the population sizes and the composition of guilds of predator species (Macaulay).
DoC acknowledges that 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 been targeted.
1.3 A measure of the severity and distribution of the impacts on other economic values.
Tourism is important in the semi-arid land of the South Island and rabbits are degrading parts of the landscape in what is commonly considered to be a "national heritage" region. The image portrayed in badly rabbit-affected areas is counter to the New Zealand "clean green" image in our advertising.
The case for adverse rabbit impacts on intensive agriculture, establishment of trees, horticulture (grapevines, vegetable cultivation and floriculture) and gardens in urban areas outside the rabbit prone areas has been supported by several submitters.
The Upper Clutha Fruitgrowers Association claims that rabbits are a problem for all orchards but particularly those surrounded by rabbit infested land. They claim that costs for rabbit control on a 30 hectare orchard with 16 hectares of trees in 1995 were $6,339 and tree losses cost $17,000.
The Southern High Country Branch of the NZ Farm Forestry Association considers that production forestry has a big future in Central Otago, the Upper Clutha and the Mackenzie Country. With increasing subdivision and lifestyle blocks there is an opportunity for a wide range of tree varieties to be planted including hazelnuts, walnuts, chestnuts, olives and others. Rabbits are seen as a significant inhibitor to the establishment of new plantations of trees especially Douglas fir. This experience has been reported in other areas too.
1.4 A measure of the severity and distribution of the impacts on social values.
| Many New Zealanders are drawn to the high country of the South Island. | |
| "It is impossible to explain precisely the attraction the high country holds for so many people, yet it seems to embody our need for some understanding of and kinship with the land that slopes up towards the high hills and mountains which are the nearest things we have to shrines in this country." (Brian Turner) | |
People recognised the value of the natural heritage and biodiversity of the high country and utilize the rural and backcountry areas for recreation. The loss of vegetation, the degradation and erosion of this land due to the action of rabbits is unacceptable to many people.
Farmers are also distressed that the land in their care is being degraded by rabbits. Holes created by rabbits are also recognised as a potential cause for injury to animals and people.
1.5 A measure of the severity and distribution of the of the impacts on Maori values.
For a review of all Maori issues see section 5.7.
Conclusion
About 9% of the land area in New Zealand is classed as moderate to extremely high in rabbit proneness.
Rabbits can reach high numbers on this land and cause serious ecological damage which is costly in environmental, cultural and financial terms.
It is likely that most native fauna, with few exceptions, is negatively affected by the type of habitat modification effected by rabbits. Some native fauna is even more directly affected by rabbit predators.
Rabbits have a high impact on flora via the direct defoliation of plants, modification of plant communities and destruction of the soil structure.
In addition to these ecological effects rabbits also compete with livestock for feed and damage crops and trees.
The rabbit is a pest in New Zealand. It is clear that the beneficiaries of rabbit control extend beyond the pastoral farmer, forester, orchardist, horticulturist, gardener, etc. The preservation of our environment with its associated conservation values is of potential benefit to all New Zealanders.
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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