Cats' impact on native wildlife

Posted by Big Rat on Campus on Jan 24, 2013 in Rat Answers | Subscribe

Cats’ impact on native wildlife – Experts

23 January

A campaign launched by
economist Gareth Morgan to raise awareness about the impact
of cats on native wildlife has elicited a range of opinions,
but what does the evidence say?

Yesterday, Dr
Morgan launched the Cats-to-go campaign,
calling for further efforts to prevent cats killing native
birds. Recommendations included: affixing bells to collars,
keeping cats indoors, neutering, requiring registration and
microchipping for cats and — more controversially
–.calling on cat owners not to replace their pets when they
The Science Media Centre gathered the
following comments from experts on predator control and

Dr Yolanda van Heezik,
Senior Lecturer, Zoology, University of Otago,

“There have been three published
studies quantifying the prey caught by domestic cats in New
Zealand (in Auckland, Christchurch and my study in Dunedin)
and they all agree in reporting that pet cats catch birds,
including native species. Cats appear to catch species in
proportion to their abundance in the environment, so in
Dunedin, the two most abundant bird species (silvereyes and
house sparrows) were the two most common bird species caught
by cats. Cats in the Dunedin study also caught native
species such as fantails and bellbirds. Fantails seem to be
particularly vulnerable. My study identified that about one
third of cats did not bring any prey home, about a half
brought back prey infrequently, but that about 20% were
frequent hunters. The average number of prey brought back
per year was 13, but that included rats, mice, lizards and
invertebrates. It should be borne in mind though, that a
recent study using Kittycams in the US reported that cats
brought back only one third of the prey that they actually
caught and killed, so the NZ studies probably
under-estimated total numbers killed.

“I support Gareth
Morgan in his campaign to raise awareness about the impacts
that pet, stray, and feral cats have on our native wildlife.
I suspect that most people have never given the issue much
thought, or they think that the one or two birds caught by
their own cat makes no difference. People need to consider
that cats exists across cities at a density of about 225 per
sq km, and that even though individual cats may catch few
birds, cumulatively the total of birds killed is large.
Other countries such as Australia have regulations in place
around cat ownership and cat movements, and we need to start
thinking along the same lines, if we value our native
wildlife, and want to live in towns and cities where we can
encounter native wildlife as part of our everyday lives.

“Feral cats are known to be a problem. The impact of pet
cats is less well known. Because pet cats are fed, they are
referred to as subsidized predators. They are more likely to
hunt wildlife populations to extinction, as they do not rely
on wild prey for food, so they are less likely to switch to
alternative sources of prey when their prey population
becomes so depleted it costs too much energy to hunt for it.

“Stray cats are an important issue. Stray cat
colonies that are fed by the public still kill wild species.
Well-fed cats hunt. TNR policies (trap-neuter-return) have
been shown in other countries to be ineffective at
controlling stray cat populations.

“Cats do prey on rats,
which are also significant predators of wildlife. This is an
argument for carrying out simultaneous rat control in areas
where cats are absent.

“The recommendations given by
Gareth Morgan are reasonable: consider using a collar with a
bell: our research has shown they reduced catch by 50%.
There are other devices to curb cat predation, such as the
catBib, and I suspect more in development. Consider keeping
your cat inside at all times. This ruling is in effect
already in some parts of Australia. And consider not
replacing your cat when it eventually dies.

“I’m currently
involved in a study coordinated by Assoc. Prof Mike Calver
at Murdoch University in Australia, surveying the public in
NZ, Australia, the UK and the USA to ascertain the opinions
of the community on cat ownership and regulations around
ownership. The results from that study should be available
later this year.”
Mr John Innes, Scientist –
Biodiversity and Conservation, Landcare Research,

“On his website, Gareth
Morgan paints a broad picture of cats as major predators of
native birds in New Zealand (‘cats are wiping out our native
birds’) and suggests that New Zealand without cats is ‘a
New Zealand teeming with native wildlife, penguins on the
beach, kiwis roaming about in your garden. Imagine hearing
birdsong in our cities’. However, careful reading of the
site does show that its authors are aware that there are
other predator species to consider besides cats. They refer
to cats being ‘one step’ towards a Predator Free New

“The impact of cats – whether feral or pet – on
valued wildlife remains controversial because it is
site-dependent and ecologically complex, and because key
impact questions are frequently unresearched. The website
cites Medina et al. 2011
regarding birds threatened or made extinct by cats, but this
is a worldwide study of islands and not just New Zealand,
and effects of cats are frequently additive to those of
other predators, especially rodents, and habitat
modification. In New Zealand also, cats alone cannot be
blamed for the loss of any species. However, they are
undoubtedly key contributors to declines of some birds (and
other fauna) in some places, for example black stilts,
black-fronted terns and wrybills in braided rivers and other
shorebirds trying to nest on beaches, but so potentially are
hedgehogs, ferrets, stoats, four wheel drive vehicles,
people walking dogs and fishermen. When cats, ferrets and
hedgehogs were targeted in Mackenzie Basin braided rivers,
possums and Norway rats then ate the black-fronted

“Cat diet studies in New Zealand urban areas (eg Gillies Clout
; van Heezik et al.
) show that cats eat small mammals,
birds, lizards and invertebrates. The birds are mostly
common introduced species or widespread natives, especially
silvereyes. The research to clarify whether the negative
effects of cats on these fauna outweighs the positive
effects of their predation on ship rats, Norway rats and
mice has not been done. However, keeping cats inside,
especially at night, will negate these possible positive
effects. Urban sanctuaries like Zealandia in Wellington
clearly change this context because highly threatened
species like saddlebacks may be taken by cats, but it is
likely that they would be taken by ship rats or possums
anyway if cats were removed.

“In New Zealand native
forests, ship rats are the major prey, and this little-seen
predator eats many more birds than cats do. The Gareth
Morgan website refers to kaka, kokako, weka, mohua, tīeke
and robins as endangered, perhaps implying that cat control
might help them, but cats are not significant predators of
any of these species, except possibly weka.

“I agree with
the website that ‘we need to control cats and rats
together’, and in fact recent thinking around a Predator
Free New Zealand has always focused on several species,
usually stoats, ship rats and possums. Whether mice,
hedgehogs, Norway rats, ferrets, weasels and cats may yet be
included is still unfolding.

“In the meantime, pet cat
owners should educate themselves about the possible threats
of their cats to local wildlife, and they should plant trees
and shrubs suitable for native wildlife.”

Dr David Winter and Dr Wayne Linklater have also written
about the issue on science blogging network


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