Predator/Prey Interactions in New
Zealand: Possible Impacts of RCD
Predators such as ferrets and cats are an important part of rabbit control in New Zealand. They are responsible for the death of 80 to 100% of rabbits born. Unfortunately, rabbits are not the only thing they eat. These predators and others, such as stoats and weasels, have had a major impact on some of our native birds.
If rabbits are a major food source for predators, what happens when that food source is suddenly reduced by a disease such as RCD? Will this change in abundance of rabbits be different from that faced by predators when rabbits are poisoned on a large scale? Will they move to other areas and/or eat some of our endangered species? These are all questions that must be considered as part of assessing the potential benefits and risks of a new rabbit control such as RCD.
World-wide, there have been many studies on predator/prey relationships. New Zealand studies have recently been reviewed and considered at a workshop, to identify:
- potential impacts of predators on other prey species when rabbit numbers are reduced;
- potential interactions between predation and RCD in the control of rabbits;
- ways to minimise any potential conservation impacts caused by a widespread reduction in rabbit numbers due to RCD.
The main findings of this review were:
- The effect of RCD on rabbit predators (such as cats and ferrets) and what other animals they eat, will depend on how much and how often RCD reduces rabbit numbers.
- Overseas research and experience suggests that, in New Zealand, RCD would initially cause large reductions in rabbit numbers, but that it may not persist. Therefore, if RCD is to keep rabbit numbers consistently at low levels, it may need to be reintroduced.
- impacts that RCD-induced reductions in rabbit numbers may have on native animals will depend on a complex set of interactions between rabbits, their predators, the changes in plant communities and in the mix of predator species in an area (see diagram).
- major habitat types, containing 20 native species that are vulnerable to increased predation when rabbit numbers decline, were identified. These are:
- seem-arid tussock grassland/river bed habitats, where black stilts are at risk;
- coastal areas, bordering seem-improved pastures, that are home to yellow-eyed penguins;
- open forest floors in areas inhabited by yellow-heads;
- forest, scrub, pasture mosaics inhabited by kiwis;
- wetlands that are the home of the crested grebe.
- At present it is difficult to predict the impacts of any RCD-induced rabbit reduction on native fauna for any habitat type in New Zealand. However, research in some habitats indicates that predators do eat more native species (insects/skinks) when rabbit numbers decline. The impact is probably short-lived, because predator numbers also decline as a result of the loss of their major food source, rabbits.
- A permanent reduction in rabbit numbers and, as a result, ferrets and cats, may have an effect on the numbers of other predators. Research in some habitats:
- tussock grasslands;
- seem-improved pasture; and
- open forest valleys,
indicates that stoat or ship rat numbers are likely to increase as rabbit predator numbers decline. The overall effect on native animals of this change in what is called the Predator guild is unknown. However, stoats and rats are considered more serious conservation pests than ferrets or cats.
- Native fauna in areas of higher rabbit numbers have been exposed to intensive rabbit control since 1950, with major fluctuations in numbers. RCD may not pose any greater threat to native species in these areas than current control practices.
- The known importance of predators as rabbit controllers, and modelling of RCD impacts on rabbits, suggests that the overall impact of RCD on rabbit populations may be reduced if it is necessary to control predators in anticipation of their effect on wildlife. Control of cats and ferrets may also lead to the undesirable increase of stoats and rats.
The review, and subsequent workshop, concluded that it was important that the Department of Conservation identify all native animal populations most at risk to predators after rabbit numbers decline. This is being done, and plans are being developed to:
- monitor RCD outbreaks and changes in numbers/predator diets around endangered populations;
- instigate any necessary predator control;
- estimate the cost of any control which, in some cases, would only be additional to what is done now.
The need for better predator control methods was identified. Current methods need improving; new methods are required and are being researched.
This diagram summarises the main ecological issues surrounding RCD’s influence on predators and their prey in New Zealand.
The relationships are as follows:
RCD is expected to reduce rabbit numbers to varying extents across the New Zealand landscape (link A).
Depending on the effectiveness of the disease, the condition of the land, and regional land/climate factors, there will be a response in vegetative growth (link B).
The vegetative response may have benefits for a number of native animals (link C), particularly in the rabbit-prone tussock grasslands.
Declines in rabbit numbers are likely to affect the number and behaviour of "rabbit predators" (link D): namely, ferrets, cats, harrier hawks and, to some extent, stoats. Flow-on effects from link D may alter the predation pressure on native animals and exotic game birds (link E).
If predator numbers decline after RCD outbreaks, or if predator control is considered necessary to minimise prey switching effects, other predator guilds dominated by stoats and/or rats may increase in response to fewer rabbit predators (link F).
The effects of stoats and rats on native animals (link G), particularly birds and invertebrates, may then be more severe than the effects of rabbit predators.
Further reading:
Understanding the Implications of Rabbit Calicivirus
Disease for Predator-Prey Interactions in New Zealand: A
review*
Norbury G & Murphy E
1996
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