Rabbits Without Predators: the likely consequences

Kate Guthrie

Do cats and ferrets control rabbits – or do rabbit numbers control their predators? It’s an important question. Farmers are understandably concerned that if introduced predators are eradicated from their land, rabbit numbers will increase dramatically. Some are asking who will pay for the additional rabbit control needed. It’s the same intuitive logic that led to ferrets, stoats and weasels being introduced in the first place.

When introduced rabbit numbers were booming in the 1870s-1880s, New Zealand introduced stoats, ferrets and weasels – their natural predators – to keep rabbits under control. It was an understandable response for the knowledge people had at the time. But do predators really control rabbit populations?

Scientists today would argue that it’s often the other way around; that predator numbers are limited by food availability. In other words, the number of prey controls the number of predators which can co-exist with and feed on that prey population.

So what’s the situation with rabbits in New Zealand and what will happen to rabbit numbers if their introduced predators – ferrets, stoats, weasels and feral cats – are removed?

Predation is only one of the factors that can potentially limit rabbit populations. Habitat and soil type (the soil’s suitability for burrowing) has an effect on how many rabbits live in a particular area. Disease outbreaks can also cause rabbit populations to crash and residual disease levels probably have an ongoing limiting effect.

Drought limits food supply and breeding success. High rainfall floods rabbit burrows and drowns their young. Sick or starving rabbits and those flooded from their burrows, are a lot easier for feral cats and ferrets to catch, but they may have died anyway. Competition for food from other herbivores (eg grazing stock) and within a rabbit population, also limits how high rabbit numbers can get before starvation causes a population crash.

There’s a lot going on and it’s all inter-related. So how important, within that network of factors, is the predator/prey relationship? If cats, ferrets and other predators are removed, will soil type, disease, climatic events and a limited food supply be sufficient to keep rabbit numbers at current levels?

In an effort to answer these questions, Landcare Research scientists, Grant Norbury and Chris Jones, supported by the Cape to City Project, reviewed current knowledge on rabbit population dynamics from research carried out in New Zealand, Australia and Europe. Their paper was recently published in the international scientific journal ‘Mammal Review’ (LINK to original article). So what can the latest research tell us about rabbit population dynamics and predator/prey relationships?

Research in New Zealand

“Reddiex et al. (2002) removed predators (cats, ferrets and stoats) from two sites in North Canterbury at the same time as Rabbit haemorrhagic disease (RHD) arrived. Rabbit abundance was measured there and at two other sites where predators were not removed. Rabbit numbers declined on all sites during the RHD outbreak, but the declines were only moderate where predators were removed and quite dramatic where predators were present. Mortality rates of juvenile rabbits were also higher where predators remained.”

Thus predation seemed to magnify the impact of the newly introduced disease. Sick rabbits make easy prey.

“Reddiex (2004) replicated the experiment at two other South Island sites in Central Otago, where conditions for rabbit survival and growth are more favourable. Again, the experiment coincided with an outbreak of RHD, and although the effect of the disease on rabbits was not quite as pronounced as in North Canterbury, no effects of predator removal were apparent on the rates of rabbit population decline.”

So where conditions (eg climate, soil) were more favourable, introduction of the new disease had less impact on rabbit numbers and predation had no discernible magnifying effect on that impact. In the Canterbury situation where a predator effect was detected, rabbits may have also been facing other survival challenges from other conditions (climate etc) which made them more vulnerable to the additional challenges of disease and predators (the ‘final straws’, so to speak).

What about diseases that are well established in a rabbit population? Does the presence of predators to pick off the sick have a significant effect?

“A more recent predator-removal experiment in central and eastern Otago showed no effects of predator removal on rabbit abundance in low-density rabbit populations that were suppressed by RHD.”

So in this situation, disease was suppressing rabbit numbers already and the presence or absence of predators had no discernible effect. The sick and dying are likely to die anyway, whether cats and ferrets eat them or not.

What about the relative importance of predators and climate on rabbit numbers? What can research tell us?

“While losses of young rabbits to predation can sometimes be high, mortality caused by other factors, such as disease, flooding of burrows, or burrow collapse, seems to be of equal or greater importance. High rainfall is generally associated with these other causes of mortality. In drier areas, seasonal pulses of high productivity and lower juvenile mortality are thought to allow rabbits to reach higher densities, but also to cause numbers to fluctuate widely according to conditions.”

The overall picture from New Zealand research seems to be that factors other than predators are controlling rabbits. So what about the reverse? Are rabbits controlling predator numbers?

“The evidence for rabbit numbers influencing predator abundance is more compelling than the evidence for predators influencing rabbit abundance, at least in the rabbit-prone areas of Central Otago and the Mackenzie Basin. Rabbit populations are also driven by favourable environmental conditions which enable them to maximize their reproductive output as does pasture development through the replacement of indigenous vegetation with productive pasture species and the application of fertilizers. Predator populations in rabbit-prone areas respond indirectly to this increase in primary productivity by responding to increases in rabbit productivity and hence the availability of young rabbits. Consequently, predators appear to have relatively little effect on rabbits.”

Australian research

In Australia, research results show a similar picture, although Australia’s predators, such as foxes, are more generalist hunters and less dependent on rabbits as a source of prey. Rabbit numbers do not totally control predator (fox) numbers in other words, since the fox can easily hunt something else.

“Most predator manipulation experiments in Australia show some effect of predation on rabbits, but this only appears to moderate the overwhelming effects of environmental conditions and food supply on rabbit population growth... drought conditions reduced rabbit populations dramatically in a semi-arid grass-shrubland in western New South Wales. Predation by foxes and cats held rabbit numbers at low levels for longer periods than where predators were removed. When the drought ended, rabbits recovered up to four times faster at predator-removal sites than at sites where predators remained.”

Thus cats and foxes in the Australian situation do not control rabbits, but they can slow rabbit population recovery from the effects of other rabbit control events.

One reason that predators fail to control rabbits, is the fact that cats, ferrets and other predators have a distinct breeding season, whereas in favourable conditions, rabbits can breed all year around. Predator populations simply can’t increase their numbers fast enough to have a controlling effect when conditions are favourable to rabbits.

In Europe

The situation in Europe is more complex, with a greater range of predators. In fact, in some areas both rabbit and predator numbers are considered, by conservationists, to be undesirably low, making the European situation less applicable to the issues faced by New Zealand and Australia. But an overall review of research shows that:

“...predators have their strongest regulatory effect during and after rabbit numbers have been reduced by other factors, and that predators have little effect on high-density populations of rabbits... Although predators influence rabbit abundance, predation is less important than the bottom-up effects of food and habitat.”

Conclusions

The reviewers conclude from the published research that: “Rabbit population dynamics are typically driven by processes other than predation, and there is good evidence that, in many circumstances, rabbit abundance drives the abundance of predators. When rabbit populations are in decline, or are regulated by climate, food availability or soil condition, predation may act to accelerate the decline or to limit the rate at which populations recover.”

Farmers, therefore, have little to worry about regarding proposals to remove feral cats, ferrets, stoats and weasels from their land. The research shows that removal of predators will, in most situations, have little effect on rabbit abundance, because other factors such as climate, food and disease are already limiting rabbit populations. Monitoring of rabbit numbers in association with the predator-removal operation offers the opportunity to both reassure farmers and to further increase our understanding of how rabbit population dynamics work.