Reddiex and Norbury (2005) concluded that ‘there is no way at present to assess the marginal costs and benefits of rabbit control’, this is confounded by the inability to differentiate the impacts between the combined effects of grazing by sheep and rabbits.
Furthermore, the inability of such analyses to adequately account for the environmental impacts of rabbits means that direct comparisons between production and environmental costs and benefits are not available to inform decisions on rabbit management or funding.
The impacts of rabbits on land-based businesses include:
- Short term direct grazing losses (otherwise available to livestock)
- Longer term grazing losses through modification of vegetation cover and composition
- Financial costs of rabbit control
- The major disruption to grazing management associated with the need to spell from grazing any land treated with 1080 until sufficient rain has fallen to make it safe to livestock
- Loss of soil (at high densities)
- High rabbit populations assist in maintaining high predator numbers. ‘This can lead to significant costs being incurred in situations where predators carry bovine tuberculosis’ (ECan RPMS).
The RCD Applicant Group noted that one stock unit equated to about 12 rabbits. In the Mackenzie, counting of poisoned rabbits above and below ground (excluding those taken by hawks), on numerous half and one-hectare plots, found up to 62 rabbits per hectare (Robertshaw and Robson, 1990).
However rabbit densities are not usually measured in numbers per hectare but by indirect estimates such as night-counts and the Modified McLean Scale.
Furthermore, a simple substitution does not account for their effects on vegetation composition and soils (Allen, et al, 1995). Brown Copeland and Co (cited in RCD Applicant Group, 1996) estimated the annual costs of rabbit control to landholders and regional councils to be ‘a minimum’ of $12.6 million.
The Applicant Group suggested that $22 million was a more realistic assessment. Nimmo-Bell (2009) estimated the current annual production losses due to rabbits at $50 million, citing an earlier report by Bertram (1999) in which it had been estimated that 2.0 million sheep were being displaced by rabbits (at a 1999 value of $25 per head).
In reaching this estimate, Nimmo- Bell assumed that by 2009 rabbit populations had halved and that livestock values had doubled. It is simplistic to assume that sheep could replace rabbits - uncontrolled grazing by rabbits often leads to a level of overgrazing that a prudent farmer would never accept.
Following the arrival of RHD, the cost would have fallen dramatically, especially with many properties undertaking little if any secondary control. One large Mackenzie property reported a three to five-fold reduction in control costs after the arrival of RHD but has recently undertaken several primary poisonings; further poisonings are not anticipated there for several years.
Others have continued with more stable annual expenditure. For example $2 to $4 per stock unit (Tekapo), $3 and $7 per stock unit (Central Otago). Consistent annual expenditure is often easier to manage in a farming business. Direct economic comparisons between a policy of periodic primary poisons or ongoing stable secondary control should not overlook the risks associated with allowing survivors of RHD to remain and breed.
Assessments of the control costs per hectare of rabbit prone land might enable more informed comparisons between properties of similar rabbit proneness.
Between-farm comparisons are complicated by differing proportions and levels of rabbit prone land, and by local factors such aspect, terrain and ground cover. Some high-cost primary poison operations have been cited by farmers - $90,000, $60,000, $34,000 in 2008 on 3 different Mackenzie properties ranging from 9,500 to 28,000 stock units (the first of these was a failure, almost certainly due to deficiencies in the operation).
However, such costs do not necessarily reflect annual averages over the 12 years since the arrival of RHD, especially where little or expenditure has been incurred in the meantime.
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