Predators have a price tag. According to a University of Auckland study, the cost of ridding New Zealand of pests over 50 years is about $9 billion.

The alternative cost - of continuing to defend the country against pests, which involves more poisons being spread, along with the application of new technologies - is almost $16 billion, the research concluded. The scientists produced another figure too - the economic and tourism benefits of a predator-free environment - which they put at $9.32 billion.

These big numbers helped persuade the Government to stump up $28 million to fund a joint venture, Predator Free New Zealand Ltd, to wipe out rats, stoats, possums and feral cats by 2050. The ambitious public-private project carries risks, and the financial commitment is a long way shy of the costs estimated by the Auckland study. Reaching the target rests partly on technology which does not yet exist, though Conservation Minister Maggie Barry believes that a "scientific breakthrough" will emerge to eradicate at least one small mammalian predator from New Zealand by 2025, less than a decade away.

Even this is a stretch, though it may come down to the definition of a "small mammalian predator"' if this initial goal is to be achieved. New Zealand covers 268,000sq km. An existing project on a chunk of land a fraction of that size - the removal of mice from the 24sq km Antipodes Island, 760km southeast of Dunedin - has cost about $4 million already, and the outcome will not be confirmed until 2018.


A lot of intellectual endeavour is being applied to find more effective and targeted ways of wiping out pests. Gene science offers promise, as do breeding programmes to produce females which bear sterile offspring. New generation self-setting traps seem encouraging, and poisons, which have done the heavy lifting for the pest control industry, are becoming more selective. This could not come too soon for opponents of the pesticide 1080, which has for years been the go-to poison for the control of possums and rats and which is being dropped over large areas of the South Island backcountry in the latest "Battle for our birds" campaign.

The controversial toxin was not mentioned in the statements, though for large-scale attacks on unwanted pests it remains the weapon of choice.

Destructive introduced mammals have been in New Zealand for centuries. Rats arrived as long ago as 700 years, and other unwelcome invasive species followed. As much as one third of native bird life has been lost. Kiwi are expected to vanish from the mainland within 50 years unless their decline is arrested.

For decades now, tens of millions of dollars have been invested in pest control programmes, yet the survival of many remaining species is uncertain and the presence of predators is still entrenched. The goal of wiping them out, even with more funding and the outlines of a co-ordinated plan, will be extremely hard to achieve. To keep what we've got, there is really no other option.