Cat Confusion: 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 false?

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 following definitions:

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 potential.

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.

Fig. 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 dashed.  

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 predation.

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.