Playing God on the Barrier
by John Ogden
Feral cats are good because they control rats, mice and rabbits.
Feral cats are bad because they kill birds and skinks. True or
Frequently we hear that cats help to maintain ‘the balance of nature’.
Leaving aside the philosophical question of whether or not there is, or
ever was, such a balance in New Zealand we can be sure that any original
hypothetical balance did not include cats, or rats, or any other mammals
(except a couple of bats).
Balance there may have been, in the sense that the fauna and flora
changed little over thousands of years, but that was upset first by the
arrival of Maori people and kiore, and then again by Europeans with
their associated pests, pets and domestic animals. Since then, at least
forty species of birds have become extinct, and many others are reduced
to critically low numbers. This is partly due to bush clearance for
agriculture, towns etc, but predatory mammals, especially rats, cats and
stoats have played, and are still playing, a key role. Cats have been
directly responsible for at least one extinction – the lighthouse man’s
cat single-handedly finished-off the last flightless wren present on
Stephens Island in 1893. My point is that the natural balance was upset
long ago by many interacting introduced animals, birds and plants. We
cannot go back to this idyllic state of nature, but neither should we
expect that a few benign feral cats are somehow keeping it all in
balance for us. In my experience, cats just aren’t like that.
The Okiwi Basin is a good place to start, as the long-suffering staff of
the Department of Conservation are often told how to manage it by all
and sundry, so they must be used to it by now. My first point is that
the estuarine basin is changing rapidly through siltation and mangrove
spread. This is not a ‘natural’ process, as it was initiated by erosion
following burning of the forest c. 750 years ago. European forest
clearance for farming added another dose of silt, and higher nutrient
levels(1). The basin’s paddocks have also changed, mainly by spread of
kanuka. The details don’t matter — the point is that the habitats in the
basin are predominantly man-made, and the birds, especially brown teal,
have learned to use them. As these habits change, so too will the bird
populations which utilise them.
Figure 1 shows a simplified version of the ‘food web’ of which brown
teal are a part. Also indicated by arrows is the link between rabbits,
rats and ducks, and the vegetation. The intrinsically unstable dunes are
particularly vulnerable to rabbit grazing and/or seed predation by
rodents. It is fairly clear that vehicles, harriers, pukeko, rats and
cats all kill some teal. The question is, which is the worst, and what
can be done about it?
The SPCA information sheet suggests the
1. Domestic owned cats: These cats live
entirely with humans as companion cats. They are completely dependent on
humans to provide their food, water and shelter, as well as their social
structure. Humans are expected to control disease and reproduction
2. Domestic, unowned cats (can also be
known as stray or ‘wild’): These cats have many of their needs
indirectly supplied by human activities, acquiring much of their food
from scraps through carers who attend to their colonies. They are likely
to live in and around human habitation and interbreed with the
unneutered / unspayed domestic cat population.
3. Feral cats: These cats have none of
their needs provided by humans. By definition, feral cats do not live
around centres of human habitation. Their population fluctuates
independently of humans and domestic cats.
Harrier hawks (kahu) are found throughout S.E. Asia and Australia, and
one or two species have been present in New Zealand since before Maori
settlement(2). The current species is thus native, not introduced by man
as is often supposed. While in some parts of the country (eg. North
Canterbury) it is regarded as beneficial, scaring small birds from
vineyards and cleaning up rabbit carcasses etc., on Great Barrier it is
often seen as a pest because it undoubtedly kills some brown teal. But
it has done so for centuries — so if it is worse now, why? Presumably,
because there are more of them because they are no longer shot. But
then, there are more rabbits too. Note that harriers are not controlled
by any predator (except man), so their numbers are determined largely by
the abundance of their prey (”bottom-up” control).
Rabbits are almost certainly much commoner on kahu’s menu than teal, and
these have flourished despite high predation from cats and harriers.
Several studies elsewhere in New Zealand suggest that rabbits are high
on the preferred food list of many feral cats, so that removing cats may
favour rabbits. However, the short-grass farming habitat in the basin is
highly suitable for rabbits, just as it is elsewhere in New Zealand
where these same predators (and others) are also present. Top-dressing
presumably feeds rabbits just as it feeds stock.
Feral cats are the ‘top predator’ in the system, and they are
notoriously difficult to study due to their intelligence and
semi-nocturnal habits. Rabbits may be their preferred prey, but rodents
probably come next. Although pet cats often drag in dead birds(3), birds
are much harder to catch than rabbits and probably rarely constitute the
main food source of feral cats.
1. Some of the main food-web interactions between mammalian pests,
brown teal and other species and the possible effects of rat eradication
using poison. Supposedly minor interactions and effects are shown
The diagram suggests that, if we were to eliminate cats only, rabbits
and rats could both increase. There are cases where this has happened.
This is the ‘balance of nature’ often referred to. On the other hand, if
we were to eliminate rats, then the cats might devote themselves to baby
rabbits. They might also take more birds, and skinks. If we eliminate
rabbits (only) then both cats and harriers might prey more on ground
birds. Although my example is simplified, it is still complex.
Introducing a poison into this system specifically to eliminate rats
poses another set of problems – namely the effects on ‘non-target
species’. Fortunately the most affected are species we’d like to see
reduced anyway. Small indigenous native birds, even morepork, rapidly
regain their numbers following rat elimination by aerial poison
drops(4). Surviving cats are more likely to prey on rabbits than on
teal, but this may allow them to regain their numbers even in the
absence of rats, and teal cannot withstand even a small increase in
What should be evident from these considerations is that we, not feral
cats, control the ‘balance of nature’. Pretending to understand the
effects of removal, or reduction in, any one species, is tantamount to
playing God. We need good research data to support an integrated
approach. Targeting all the pest species in sequence over a short period
seems to be the safest approach. A variety of methods — not just poison
— will be required and nothing will be achieved without community
support. Many members of the community have years, even generations, of
observations on these topics, and these are highly valuable in helping
to predict what might happen when particular control or elimination
procedures are proposed. The answers to all the questions are not known,
and there are indeed risks — but the economic and biodiversity benefits,
which could accrue from a rat-free island, surely make this a topic
worthy of careful consideration by us all.
(1) See: Ogden, J., Deng, Y., Horrocks, S., Nichol, S. and Anderson, S.
2006. Sequential impacts of Polynesian and European settlement on
vegetation and environmental processes recorded in sediments at
Whangapoua Estuary, Great Barrier Island, New Zealand. Regional
Environmental Change 6: 25-40.
(2) Worthy, T. H. & Holdaway, R. N. 2002. The Lost World of the Moa.
Cambridge University Press. (pp 336–335).
(3) For a readable account of predation by pet cats, and what you can do
about it, see: Ann Graeme. 2007. The Fowl and the Pussy Cat. Forest and
Bird 326: 40-41.
(4) For example, see item by E. Fraser in the latest edition of NZ
Journal of Zoology.