Mythbusters – can predators control rabbits in New Zealand?
Monitoring predator numbers in the Macraes Flat area. Image – Grant Norbury.
Welcome to a new section of Kararehe Kino. Here, and in subsequent issues, we aim to scrutinise myths, beliefs and dogma surrounding the ecology and management of vertebrate pests. Our aim is to see how well some widely held views match up with available scientific evidence. In this issue, and in keeping with the ‘drylands’ theme, Grant Norbury and colleagues tackle the question ‘can predators control rabbits?’
A common refrain from farmers is that ‘predator control results in more b!@@%y rabbits’. That belief is often used as an argument against the control of ferrets to prevent the spread of TB and the control of cats and mustelids to protect native fauna. Such assertions need to be examined in the light of relevant ecological principles and evidence to find out whether they are soundly based, and if so, under what conditions.
Whether or not predators can help to control rabbit populations depends on the favourability of the habitat for rabbits and on causes of mortality other than predation. In moist, lowland areas, predators do appear to play a role in suppressing rabbit numbers. In trials in the Wairarapa and in North Canterbury, predator numbers were reduced and the subsequent change in rabbit numbers recorded. In North Canterbury, rabbit numbers declined during an outbreak of rabbit haemorrhagic disease (RHD), but declined least where predators were controlled. In the Wairarapa, rabbit numbers increased when predators were removed, although this study was confined to a single 8-ha enclosure. In both these trials, predators helped to suppress rabbit populations that already had been reduced by diseases such as RHD and coccidiosis or by adverse weather conditions (young rabbits often drowned in their nests). In both areas, rabbits breed almost year-round, and the continuous supply of baby rabbits helps sustain predators year-round. As neither study was replicated, the results must be interpreted cautiously. Nevertheless these studies show that in some circumstances predators and disease can provide substantially better control of rabbits than disease alone.
The situation is very different in drylands, as they provide ideal conditions for rabbits; namely, a dry climate and relatively lower incidence of diseases. Here, experiments have shown that predator control has little, if any, impact on rabbit abundance because in most years rabbits out-breed any off-take by predators. The number of rabbits determines the abundance of predators in this environment, rather than the other way around.
Predator–prey theory can help in understanding these differences. Grant and his colleagues show this schematically by graphing (1) the rate at which juvenile rabbits are recruited into the population in the absence of predators, and (2) the rate at which rabbits are killed by predators (Fig.). When the recruitment rate equals the kill rate, rabbit numbers are generally stable. Where recruitment exceeds predation, rabbits increase and where predation exceeds recruitment, rabbits decrease. In drylands, recruitment of rabbits is normally high (blue line in Fig.) and naturally declines with increasing population density as competition for food and shelter increases. Predation rates (red lines) tend to decrease at low densities because some rabbits always avoid predators, and at high densities because predators can eat only so many rabbits in a day. Importantly, in drylands, recruitment almost always exceeds predation, so populations increase until recruitment is balanced eventually by predation (density A in Fig.). Where predator numbers are controlled (lower red line), rabbit densities increase only marginally to B, i.e. there is no outbreak of rabbit numbers.
The situation is very different in moist environments. There, the recruitment rate is generally lower than the predation rate (because of other mortality factors), and populations tend to stabilise at lower densities around A. But note what happens now when predators are controlled in this environment – rabbit populations can increase markedly to point B.
Of course, the relationships shown in the figure apply only during the rabbit breeding season. During the non-breeding season, the size of the rabbit population is determined solely by survival rates. Data collected in all areas of New Zealand show that all mortality factors combined cause a density-dependent decline in rabbits over winter: high density populations at the start of winter decline rapidly, and low density populations decline slowly. The result is that rabbit populations start the breeding season in each area at roughly the same density each year. In ecological terms, this tends to stabilise rabbit populations ensuring their persistence in the long term.
These relationships help explain what is observed when predators are controlled in rabbit-occupied lands, but the question landholders will ask is ‘in which of these two environments does my property fall?’ It won’t always be one or the other, because there is a land gradient of favourability for rabbits. It’s really only at the environmental extremes where the response of rabbits to predator control becomes clear. So, is the myth busted? ‘Almost always yes’ for drylands, and ‘usually no’ for habitats that are less favourable for rabbits.
This work was funded by the Ministry of Science and Innovation (Programmes C09X0909 and C09X0910).
