Upsetting the Balance  


Dr MIKE BAMFORD’S principle area of work is in population ecology of vertebrates (amphibians, reptiles, birds and mammals) and he has been an environmental consultant since 1987. Dr Bamford was chairman of Birdlife Australia WA and on its national council.

IN Australia, there are only three domestic animals that owners are not required by law to keep on their own property, or under control. 

These are the honey bee, homing pigeon and domestic cat.

The reason for this is obvious for the first two species, but why does the domestic cat get free rein? 

Perhaps it was once considered to be useful in the control of rats and mice (it usually isn’t), and it certainly is an animal that can be hard to restrain, but the tradition of ‘sharing’ your cat with the neighbourhood is increasingly coming under question. 

One of the key reasons for this is the impact your cat has on wildlife.

It is against the law for you togo out and kill wildlife, as with very few exceptions native wildlife are protected. You can, however, at least presently, allow your cat to wander uncontrolled. 

No matter how domestic, the domestic cat is a predator and it effectively can’t help itself. 

The law doesn’t hold cat owners responsible for the protected wildlife the cat may kill; that would be hard to prosecute. The question of legal liability, however, is academic, as in a conservation sense the real concern is the consequences of what cats get up to. 

They are only doing what they do; predation is part of nature. Untold animals get eaten, naturally, by other animals every day.


Ecosystems wouldn’t function without predation. Without predation, we would soon be knee-deep in insects, spiders, lizards, birds and all sorts of wildlife. 

In a sense, the balance of nature relies on predation happening. 

So while many of us may not like the idea of cats eating native wildlife, it is really a question of what that consumption is doing, rather than the consumption itself, that is important. 

We talk of the balance of nature, and part of that balance involves the ability of the population of a species being eaten to survive; for predation to be balanced by reproduction and immigration. 

For a bird like the splendid fairy-wren, it doesn’t really matterif some of its number are eaten bydomestic cats or the native collared sparrowhawk; what matters is whether the wren population produce enough young to replace those that are lost. 

And if it can’t produce enough young, can other wrens move in from nearby to replace them. 

We need to think like that when examining the risk that domestic cats pose to wildlife in the suburbs. 

Scientist talk of the ‘predator-prey’ relationship, and when the subject is the domestic cat and urban wildlife, there are some important things to consider in order to understand why domestic cats can ‘tip the balance’. 

There are a lot of domestic cats. 

Predators are usually uncommon as they need a lot of prey to support them. A predator that eats all its prey is itself doomed. Domestic cats are not uncommon. They occur at impossibly high densities.

They also do not rely on native wildlife for the bulk of their food as they are fed by their humans. 

They are over-abundant as a predator, and even if only some wandering domestic cats eat native wildlife, that should be cause for concern. 

In urban landscapes, some species are especially vulnerable to predation. 

Very few native species flourish in the suburbs, and those that survive at all are often confined to small bushland areas. 

In such areas, they may be in small numbers and are likely to be at least partly isolated from other groups of their own species.

This makes them vulnerable to local extinction. 

Such local extinction can be the result of many factors, but predation is one of them. 

Predation by domestic cats has been linked to local extinctions on several occasions. 


This pattern of local extinction has already happened across much of Perth, even in the larger bushland reserves. 

Birds like the robins, fairy-wrens, scrubwrens, thornbills and shrike-thrushes have disappeared from many urban reserves, and may still be declining. 

Small mammals like the honey possum, western pygmy-possum, grey-bellied dunnart and noodji (a fluffy, pale-grey native mouse) have gone. 

It is not until you get to large tracts of woodland north of Muchea before you can encounter any of these.

The absence of these species is striking for people who are aware they should be present, but of most concern is that for many people there is no awareness of what has been lost. Or why.

A keen bird-watcher recently commented that domestic cats could have no impact, because species sensitive to predation by cats were not present in suburban reserves. 

The irony of this statement appeared to be lost on him.

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