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
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
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).
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.
Having said that, I agree that rabbit control should be given much
higher priority on GBI. The ARC offered to help with RHD (Colesi virus)
a couple of years ago, and the community rejected them out of hand.
Since then the virus has arrived anyway, and as expected, partial
immunity has developed. Rabbits are moving into the bush too, where
their diet includes tree seedlings and tree bark. As well as providing
food for feral cats, they are a serious problem in forest restoration.
Ditch’s suggestion of a blitz on rabbits, followed immediately by a
blitz on cats, has a lot to recommend it. For maximum effect it would
need to be island wide, not just confined to the Okiwi Basin.