Predator control benefits native species but not rabbits
Image – Jan Kelly
Landcare Research scientists have been working with Hawke’s Bay Regional Council (HBRC) to measure the effectiveness of the council’s predator control operations in protecting native biodiversity, and to review the evidence around a potentially contentious pest control issue concerning landowners in the region.
Since November 2011, HBRC has been controlling invasive predators on two pastoral properties adjacent to Boundary Stream Reserve – a large conservation area containing populations of rare native animals. The wide-scale trapping programme, which covers 8000 ha on Rangiora and Opouahi stations, is part of the larger Poutiri Ao ō Tāne project, a collaboration between HBRC, community groups and the Department of Conservation. The aim of the programme is to test whether predator control can benefit native biodiversity in a pastoral landscape with fragments of native bush. It is also hoped that trapping adjacent to Boundary Stream will reduce predators reinvading the reserve.
Before trapping began in 2011, Landcare Research scientist Al Glen and colleagues began monitoring predators and native species in the predator control area (see Glen & Dickson). This work is ongoing. For comparison, monitoring is also done on neighbouring Toronui Station, where no predator control has been or is being undertaken. Pest mammals are monitored using footprint tracking tunnels and motion-activated cameras (camera traps). The tracking tunnels also detect native skinks. Invertebrates are monitored using weta houses, while the diversity and abundance of birds are monitored using electronic bird song recorders.
Tracking rates of skinks have increased in the predator control area from zero in 2011 to 50% in February 2014. No skink tracks have been detected in the non-treatment area. Populations of native invertebrates are also more abundant where predators are controlled; the weta houses in the predator removal area consistently have around 30% more invertebrates than those in the non-treatment area. Further monitoring and analysis will determine whether pest control has also benefited native birds.
As programmes to enhance indigenous biodiversity expand, some landholders with overabundant rabbits believe that predator control on adjacent lands has exacerbated their rabbit problems. Because control is costing more, they are seeking subsidies from regional councils or the Department of Conservation. This issue will become increasingly complex and contentious as new technologies and ambitious visions of pest eradication over very large areas gain traction. Public perceptions are critical: many landowners and members of the general public believe intuitively that if predators eat rabbits they must be regulating rabbit numbers. However, predator–prey population dynamics are rarely that simple. As a first step, Grant Norbury and Chris Jones reviewed the published scientific literature covering predator effects on rabbit populations, both in New Zealand and overseas.
The review found that, in New Zealand, there is no compelling experimental evidence that predator removal increases rabbit abundance. Predators have relatively little effect on rabbit numbers compared with other forms of mortality, such as disease or flooding and collapse of burrows. In Australia, rabbit numbers are driven primarily by climate and its effects on food abundance and quality, and disease. However, where rabbit numbers are low, predation can limit population recovery. Similar patterns have also been described in parts of Europe. Overall, predation appears to be less important than the effects of climate, food, disease and habitat on rabbit numbers. Predator abundance (especially for species that specialise on rabbits) can usually be predicted by rabbit abundance, but the reverse is not necessarily true.
For areas of New Zealand that are highly favourable for rabbits (e.g. dry central South Island) predator control is less likely to result in higher rabbit numbers than in areas of higher rainfall where other forms of rabbit mortality prevail. Any increases in rabbit numbers subsequent to predator control, however, are likely to be small compared with the effects of climate, food, disease and habitat.
In circumstances where predator control might lead to more rabbits, we do not know whether the factors involved can be identified with enough certainty to predict where and when rabbit population increases are likely to occur, and if so, by how much? The researchers suggest a robust and consistent rabbit, predator and disease monitoring programme be instigated at sites adjacent to or overlapping predator control operations so that data on changes in rabbit populations can be collected alongside data on changes in disease prevalence and predator abundance. These data could be combined with local data on climate and other conditions to investigate the most likely conditions for rabbit populations to increase.
For the success of the council’s predator control programme, it is crucial for the community to understand that predators are ‘passengers’, not drivers, in predator–rabbit population dynamics, and Landcare Research efforts are helping generate this appreciation. The common misconception that predators drive rabbit abundance has the potential to damage the community uptake of voluntary predator control programmes in the future.
This work was funded by an Envirolink Medium Advice Grant (#1435 – HBRC196), Core funding from the Ministry of Business, Innovation and Employment, and leverage funding from HBRC.