Greg Martin: Clark Island rat study brings conservation benefits

COMMENT

Fears expressed in the Herald that rats from Clark (Hauturu) Island will invade Whangamata following a Department of Conservation-sponsored research project are unsoundly based.

Whangamata is not about to become plagued with rampant rodents like some Hamelin town of old.

Clark Island has an existing rat population and there will be far fewer on the island during this project than at present. Eventually they will all be removed.

The Whangamata invasion would have happened already if it was going to, with no assistance from government departments, crown research institutes or PhD students.

Alan Foggie, of the Whangamata Ratepayers' Association, says he can already tell the university researcher that rats breed like rats, but that oversimplifies the issues involved and overlooks the conservation benefits to be gained.

The research on Clark Island is not a frivolous exercise dreamed up by academics. It is designed to protect huge investments on conservation islands.

New Zealand is a world leader in eradicating rats, having removed them permanently from 40 islands. The largest-ever rat eradication has been completed by Department of Conservation staff on 11,400ha Campbell Island at a cost of $2 million.

The problem is that all rat-free islands could be reinvaded, either accidentally from ship wrecks or naturally by rats swimming from other islands. Should rats reinvade large and remote islands like Campbell, treatment of the entire island for rats would be prohibitively expensive.

We therefore need to study rat invasion behaviour to discover how far rats move when they arrive and how quickly the populations increase. This will help to identify the size of the area that would require treatment after an invasion, and how soon after an invasion detection is likely to be possible.

The existing rats and mice on Clark Island will be removed, using traps and poison, most likely in winter when alternative foods for rodents are limited. The experiment will then begin with release of a pregnant female Norway rat, fitted with a radio transmitter so movements and survival after release can be monitored and detection methods tested.

The rat will be allowed to give birth and breed naturally for one season, at the end of which there may be 30 to 50 rats present. They will all then be trapped, using live traps, until no further rats are caught.

Norway rats are the most common species found around our harbours and waterways. They are also the most proficient swimmers, known to cross water gaps of at least 500m.

The release of a single, radio transmitter-carrying rat on Clark Island should, therefore, provide answers to some serious questions: how far a rat moves on arrival, how long it remains near its landing place, and whether it shows interest in the wax-tabs or gnaw-sticks now used to detect invasions.

Once the rats produce offspring, we will better understand the rate of increase and the speed at which rats spread across the island. Rat-free islands are not suitable for this research because of likely damage to their flora and fauna.

The project was fully discussed by the Waikato Conservation Board at a meeting and members expressed unanimous support for the research and for the conservation benefits that would flow from it. Members were told that three methods would be used to ensure rats were finally eradicated from Clark Island - traps, poisoning and rat-detecting dogs from the department's pest-finding team.

For a total cost to the department of about $25,000 spread over three or four years of study, the information to be gained from Clark Island will more than repay taxpayers' investment.

The information will be used to protect millions of dollars spent on our most important islands. As a bonus, Clark Island will be rat-free at the end of it, and the people of Whangamata will have a conservation asset on their doorstep.

* Greg Martin is the Department of Conservation's Waikato conservator.