The Okiwi Basin has been under the conservation spotlight for some years now as D.o.C seek to arrest the decline in the brown teal duck (Anas aucklandica) population. Their tactics have included feral cat control, pukeko control and intensive monitoring to seek answers to the problem of low recruitment in the teal population.

A. In a lot that we have done over the years, the results have been entirely dependent on continuity – that’s the nature of control. Now we have reached the point where a lot more sanctuaries are setting up aiming for eradication – more in the last ten years than were ever dreamed of. This movement is based on the understanding that the cheaper option is to spend the big bucks and eradicate, and then keep it that way. However, in the last five years the on-going lesson has been that eradication is not the end point, that unless you have funds in reserve to deal with contingency response, you are back in a control situation fast. There is still a lot of learning to be done.

It’s interesting that 20 years ago we stopped seeing rabbits as a conservation pest. We focussed our efforts at getting very good at predator control and to my mind we have overlooked the significance of large mammals like rabbits in the equation. We are only now, in the last couple of years (through a lot of the work that myself and others have been doing) seeing just how critical they are. We are yet to put rabbits back on that (conservation pest) list – in my view they need to be there, pronto! You get out at night here and see rabbit numbers which far exceed the worst densities of rabbits I have worked on in the North Island. The numbers are only comparable to Motuhie Island (Hauraki Gulf) where we took 25,000 rabbits off in two and half years.

Q. Do think rabbit control is a relevant issue for pateke conservation in the Okiwi and other areas on GBI?

A. I think it is fair to say that anywhere you have rabbits and cats living on the same estate that one is going to be supported by the other; that rabbit densities will determine your cat densities. Large numbers of prey equals large numbers of predators: but large numbers of predators do not have a controlling effect on large numbers of prey because of the harvest nature – the predators are taking the youngest and easiest of the animals. The successful breeders that have been around a bit longer remain. If you attempt to control cats in isolation you are only ever going to take a portion of the cat population each year.

Q. Why is that?

A. In all fairness to staff (DOC) who do an awful lot of good work here, they are really up against it. Not only are the species we are trying to protect easy prey for any predator but they are also highly susceptible to being caught or killed by the devices put out in the field to protect them. We know that ground based semi-buried leg hold traps with big visual baits are the best way to trap cats. But you can’t use those sets in any number around pateke or banded rail. What we use is a mix of conybears (kill traps) baited with minced rabbit meat and cages with variety of baits. The cages only account for about 60% of the population – about 40% of cats will not enter a cage. These are the savy cats, the ones that are very good hunters, who have been around longer — and they are the breeders. So if you are only catching 60% of your cat population in the cages and when you look at the data from Okiwi that shows cages are outperforming coneybears, it suggests you are not catching the cats that really matter.

Q. It is an often repeated argument that cats only eat rabbits, the easy prey. In the Okiwi where we have plentiful rabbits (and cats) do you think the cats are exclusive in their diets?

A. What we know is that the highest densities of rabbits found anywhere in NZ are on offshore islands with large numbers of seabirds. Where cats and rabbits occur together diet analysis has shown that rabbits make up 70-80% of the cat’s diet. Rats, mice, invertebrates, lizards, and birds all make up a portion of a cat’s diet but it is a relatively small portion. We know that a percentage of cats, for whatever reason, will get a taste for something else. If that ‘something else’ is a threatened species those ‘rogue’ cats can have a really big impact on their target population. As the overall cat population rises in response to increased rabbit numbers the number of those ‘rogue’ cats increases correspondingly — It’s a numbers game.

On GBI the populations of threatened species that those cats are preying on are not increasing – and unfortunately, as we see in the Okiwi Basin, the number of cats captured per annum is not going down. We are capturing all age ranges of cats, which suggests that we are not taking out those important breeders, the mature animals that have successfully reared (or fathered) litter after litter. We know they are the most important predators, and if any animals are going to develop a taste for food other than rabbits then it is going to be those animals.

Q. Is there a way of controlling the ‘rogues’?

A. What I would advocate for the Okiwi Basin would be meaningful seasonal rabbit control. Get the entire basin to a point where you have removed a huge portion of the abundant prey (rabbits). You immediately follow up with intensified cat control using a variety of methods, taking full advantage of a window of opportunity to reduce the prey. The hungry cats are easier to catch and the remaining breeders are less fecund after that because they are not breeding in accordance with this abundance of prey. Within 2 to 3 years the impact of reduced rabbits and subsequent more effective cat control will allow some of those ‘control’ dollars to be then spent in other areas of pateke recovery – namely habitat creation. Habitat improvement has got to be undertaken along with the other measures we have discussed – it’s a vital step.

Q. What about the prey switching argument – does the cat trapping need to follow rabbit reduction?

A. Yes it does – and the cats also become easier to catch. The fact that this equation has applied to many sites around NZ shows how important rabbits are.

Q. The other predator species high on the profile in Okiwi is kahu (Australasian harrier hawk). How does it figure in the food web?

A. Harrier densities are entirely prey driven. They are (obviously) a lot more mobile and can respond relatively quickly to increased food availability. We are only partially through identifying the levels of predation on pateke by harriers. What we have to assume is that the more of these predators you have the more predation is going to occur. I know there is Iwi resistance to harrier control, but I would still say go ahead with rabbit control on the grounds that harriers are highly mobile. Predation by harriers following rabbit control is likely to be a first season effect. They will not hang around an area in a high number where there is no longer any food for them. However they will just as easily move back in if food availability increases. Therefore any control you do is going to have to be continuous.

An argument I have heard put forward is that for a long time harriers and pateke did OK. It is the increase in rabbit numbers that has altered the balance, just as it has done with the cats, to increase the numbers of rogue predators and thus the effect on the pateke.

Q. What has been your impression of rat densities here?

A. With regard to rats on the island, not just Okiwi but all of it, the only other site where I have been nightworking and seen the sheer number of black (ship) rats is Kawau Island, which is also an island with only black rats (no Norway rats).

Q. The GBICT’s mission statement is to explore the potential for eradication of introduced predators from Great Barrier Island.

A. That’s a pretty big first step!
I see a rat and mouse free Great Barrier as a huge goal. In terms of sheer ability to do it, we are still some years away. Great Barrier is complex and to my mind there are easier and more achievable goals than total eradication – addressing predator prey interactions on GBI could be initiated within twelve months and could be achieved, down to the point where the ongoing maintenance costs are very small, inside 3-4 years. I am specifically talking about rabbits. We are good at culling them, they are visual and leave a lot more field sign, and you have far more identifiable measures of success. You have very high densities of both rats and rabbits. Rabbit control is far more achievable. If you had a long dry summer followed by a long dry winter everybody would be shocked at rabbit densities. The integrity of all of the conservation initiatives on the island are threatened by this one key element.

Comment from John Ogden

Ditch raises some very important points about predator control in the Okiwi basin, and, by inference, elsewhere on Great Barrier. These points need open discussion, but they also need sound data to back them. As Ditch points out there are plenty of studies to back up his points about the importance of rabbits in the control of feral cats. The links to harriers and pateke are a bit more tenuous, but I wouldn’t disagree with them. I think it is important to recognize that D.o.C has been working with a very restricted budget – the economic uncertainties interact with the ecological, and so progress has been cautious – checking one aspect at a time.

There may be more danger in starting programmes that cannot be carried through, than in doing nothing. In fact the Trust’s latest analysis of the GBI pateke numbers shows that they have stabilized since 2000. So, although we’d all like to see a bigger pateke population on Great Barrier, D.o.C’s efforts at Okiwi, and those of the community elsewhere, do seem to be paying off.