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  • Economist Gareth Morgan made headlines around the world in January 2013 with his public Cats to Go campaign. Debates have raged since then as cat lovers and conservationists have gone at it tooth and claw, and there has been more research into cat’s lives in New Zealand.

    The impact of cats on native wildlife

    The issue is that cats – whether feral, stray or domestic – can and do have an impact on native wildlife to varying degrees, particularly on bird, insect and lizard populations. Highlighted as part of Dr Morgan’s campaign has been the policy of the SPCA to trap, desex and release feral cats back into the environment. He suggested that these feral animals be euthanised (humanely killed) to protect the environment – he went as far as offering the SPCA $5 for each euthanised cat. The issue for the SPCA is that their mission is to help animals, not kill them – so whose responsibility should it be?

    Dr Morgan also encouraged cat owners to take some initiatives with their domestic cats such as placing bells on cats’ collars (so it is harder for them to stalk silently) and not replacing pet cats when they die.

    Rights: Rachel Douglas

    Domestic cat

    University of Otago research in 2010 showed that the average number of prey brought back per year by cats was 13.

    Media coverage of the cat fight has resulted in the worst round of cat-related puns ever, with fur flying on both sides of the debate as protagonists engage in rhetoric that loses sight of the original issues:

    • What impact do cats have on the native environment?
    • Are better control measures called for?
    • If feral cats are removed from the environment, what are the possible outcomes?

    In January 2013, two environmental experts gave their research-based perspective on these issues. While they generally supported Dr Morgan’s campaign, they both had a word of caution.

    More research needed

    Wildlife ecologist John Innes from Manaaki Whenua – Landcare Research said the research has not yet been done to clarify whether the negative effects of cats on the environment (mostly bird killing) outweighs the positive effects of their predation on rats and mice. He argues that cats are not the only introduced species killing our native wildlife, and if we remove cats, we need to consider the potential impact on our pest predators, such as rats. Will we see an increase in rat and mice numbers if we only concentrate on decreasing cat numbers?

    John Innes stated that, in New Zealand, 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 terns.” The last-ever Stephens Island wren was eaten by a pet cat.

    Ship rats major predator in native forests

    In New Zealand’s native forests, ship rats are a major predator, and these little-seen predators eat far more birds than cats do. The Gareth Morgan website refers to kākā, kōkako, 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, with the possible exception of weka. John Innes stated that he agreed with the website that we need to control cats and rats together.

    The Predator Free 2050 vision for New Zealand is focused on three species that have been identified as the most damaging introduced predators; rats, stoats and possums. Whether mice, hedgehogs, ferrets, weasels and cats may yet be included is still unfolding. In some areas, it is dogs that do the most damage, for example, killing kiwis.

    Feral cats can cover long distances, and while they are not as long-living as domestic cats, they are usually larger. According to the Department of Conservation, they have a major impact on native birds, bats, lizards, mice, wētā and other insects.

    Rights: Image courtesy of Nga Manu Images

    Rats eating eggs

    Rats are adapted to climbing trees and can locate nests easily due to their strong sense of smell.

    Density of cat population

    Senior Lecturer in Zoology at the University of Otago Dr Yolanda van Heezik supports Dr Morgan’s campaign to raise awareness about the environmental impact of cats. “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 exist across cities at a density of about 225 per square kilometre 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.”

    In 2010, Dr van Heezik and her team published a research project that showed about one-third of cats did not bring any prey home, about half brought back prey infrequently and 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 New Zealand studies probably underestimated total numbers killed.”

    Feral cats versus pet cats

    While it’s well known that feral cats are a problem in the wild, the impact of pet domestic cats is less well known. Due to pet cats also being fed by their owners, they are referred to as subsidised predators. This means domestic cats are more likely to hunt wildlife populations to extinction, as they do not rely on wild prey for food and are less likely to switch to another source of prey when their prey population becomes so depleted it requires too much energy to hunt for it.

    As Dr van Heezik says, “Stray cats are an important issue. Stray cat colonies that are fed by the public still kill wild species. Well-fed cats hunt. TNR (trap-neuter-return) policies 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.”

    More research

    Researchers from Victoria University of Wellington set up a study in 2018 to investigate the number of invasive mammals in Wellington – see the article Invasive animals in cities. Findings from this research showing the large number of sites occupied by cats and hedgehogs led the research to urge a review of current policies for the management of free-ranging cat populations and hedgehogs, as both species can pose a major threat to endemic species in New Zealand.

    The National Cat Management Strategy Discussion paper was first released in 2017 and an updated report was released in 2020. This involved a range of national organisations interested in cat management, including the Gareth Morgan Foundation, SPCA and local and national government representatives. This paper aimed to recognise the needs of cats, cat owners and cat carers and the potentially negative impacts of cats on communities, other species and ecosystems.

    Keeping cats indoors

    There is a growing movement for cat owners to have environmentally friendly cats. Some conservationists might see this as a contradiction, but others who are keen to both own a cat and also protect native species take steps to try and minimise their pet’s impact, for example, by:

    • keeping the cat indoors – if not all the time, then at least at night
    • desexing
    • adding a bell to the cat’s collar
    • ensuring it is regularly and well fed
    • adding possum guards around trees to prevent cats reaching nests.

    Related content

    In Reading to learn about New Zealand birds and their conservation, find out how one primary teacher used activities on conserving our native birds with her year 7 students and how she integrated reading activities with the science lessons.

    Cats and their natural predatory instincts pose an ethical dilemma for many pet owners but can be a good subject for a socio-scientific investigation by students. The Ethics thinking toolkit provides a structured framework for scaffolding student thinking about an ethical issue.

    The Connected article Keep your cat inside is a great example of the integration of literacy and science. Use it to explore text characteristics and the nature of science.

    Find out more about the Predator Free 2050 initiative and see the comprehensive teaching resource (produced by ZEALANDIA, with support from WWF New Zealand) that supports New Zealand schools to explore the pest-free vision with students.

    The resource Food of wild cats illustrates how an Assessment Resource Banks (ARB) activity can be used to provide opportunities for students to strengthen their capability to use evidence to support ideas in the context of science.

    In Scientists using fake smells to stop predators killing endangered birds discover an alternative innovative pest control method.

    See the range of interesting resources in our Dogs, cats and science Pinterest board.

    Useful links

    This Radio NZ news story, Cats’ secret lives go public covers the story of 10 Wellington cats who had tiny cameras attached to their collars to try to find out what impact they had on our wild life.

    Locals in New Plymouth discovered their cats' secret lives under the CatMap Curious Minds project that was supported by the Taranaki Participatory Science Platform. Students at Welbourn School partnered with digital mapping experts at MAIN (Mapping, Analysis and Information Network) Trust New Zealand and added GPS trackers to their cats. Discover more in this Radio NZ article and interview from January 2020, including a slide-show of great images from the project.

    Visit the Department of Conversation website to learn more about the havoc feral cats create for our native ecosystems, then find out how to Make your cat conservation friendly.

    Read about DOC’s work with Predator Free 2050.

    This Scoop news article, Cats' impact on native wildlife - Experts respond, has more on Dr Yolanda van Heezik and Dr John Innes responses to the launch of the Cats-to-go campaign.

    Research, featured in this The Conversation May 2020 article, looks at the the huge problem cats play in the extinction of Australia's native species.

    After the discovery in Otago of 28 dead native lizards that had been regurgitated by a cat, the issue keeping cats indoors is looked at in this Spinoff article from October 2021.

    The book Cat Wars: The Devastating Consequences of a Cuddly Killer by Peter P Marra and Chris Santella, tells the story of how in 1894, a lighthouse keeper arrived on Stephens Island off New Zealand with his cat. In just over a year, the Stephens Island Wren, a rare bird endemic to the island, was extinct.

      Published 12 March 2013, Updated 2 October 2021 Referencing Hub articles
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