Invasive predators are eating the world’s animals to extinction

Invasive species are a threat to wildlife across the globe – and invasive, predatory mammals are particularly damaging.

Our research, recently published in Proceedings of the National Academy of Sciences, shows that these predators – cats, rats and foxes, but also house mice, possums and many others – have contributed to around 60% of bird, mammal and reptile extinctions. The worst offenders are feral cats, contributing to over 60 extinctions.

So how can we stop these mammals eating away at our threatened wildlife?

Counting the cost

Our study revealed that invasive predators are implicated in 87 bird, 45 mammal and 10 reptile extinctions — 58% of these groups’ contemporary extinctions worldwide.

Invasive predators also threaten 596 species classed as vulnerable, endangered or critically endangered on the International Union for the Conservation of Nature Red List. Combined, the affected species include 400 birds, 189 mammals and 149 reptiles.

Twenty-three of the critically endangered species are classed as “possibly extinct”, so the number of extinctions above is likely to be an underestimate.

Until now, these shocking statistics have been unknown, and the heavy toll of invasive predators on native biodiversity grossly underappreciated. Species extinctions attributed to invasive predators include the Hawaiian rail (Zapornia sandwichensis) and Australia’s lesser bilby (Macrotis leucura).

Australia’s lesser bilby, now extinct.

Who are the worst offenders?

We found that three canids (including the red fox and feral dogs), seven members of the weasel family or mustelids (such as stoats), five rodents, two primates, two mongooses, two marsupials and nine species from other families negatively impact threatened species. Some of these species, such as hedgehogs and brushtail possums, don’t immediately spring to mind as predators, yet they are known to prey on many threatened species.

Feral cats threaten the most species overall (430), including 63 that have become extinct. This equates to one-quarter of all bird, mammal and reptile extinctions – making the feral cat arguably the most damaging invasive species for animal biodiversity worldwide.

Five species of introduced rodent collectively threaten 420 species, including 75 extinctions. While we didn’t separate out the impacts of individual rodent species, previous work shows that black rats (Rattus rattus) threaten the greatest number of species, followed by brown rats (R. norvegicus) and Pacific rats (R. exulans).

The humble house mouse (Mus musculus) is another interesting case. Despite their small size, house mice have been recorded eating live chicks of albatrosses, petrels and shearwaters.

Other predators that threaten large numbers of species are the domestic dog (Canis familiaris), pig (Sus scrofa), small Indian mongoose (Herpestes auropunctatus), red fox (Vulpes vulpes) and stoat (Mustela erminea).

Invasive mammalian predators (clockwise from top left): feral dog, house mouse, stoat, feral pig, feral cat, brushtail possum, black rat, small Indian mongoose and red fox (centre).
Clockwise from top-left: Andrey flickr CC BY 2.0 https://flic.kr/p/4M2E7y; Richard Adams flickr CC BY 2.0 https://flic.kr/p/7U19v9; Mark Kilner flickr CC BY-NC-SA 2.0 https://flic.kr/p/4D6LPe; CSIRO CC BY 3.0 http://www.scienceimage.csiro.au/image/1515; T. Doherty; Toby Hudson CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:BrushtailPossum.jpg; CSIRO CC BY 3.0 http://www.scienceimage.csiro.au/image/10564; J.M.Garg CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Herpestes_edwardsii_at_Hyderaba.jpg; Harley Kingston CC BY 2.0 https://flic.kr/p/ceWFr7 (centre).

Island species most at risk

Species found only on islands (insular endemics) account for 81% of the threatened species at risk from predators.

The isolation of many islands and a lack of natural predators mean that insular species are often naive about new predators and lack appropriate defensive responses. This makes them highly vulnerable to being eaten and in turn suffering rapid population decline or, worse, extinction. The high extinction rates of ground-dwelling birds in Hawaii and New Zealand — both of which lack native mammalian predators — are well-known examples.

Accordingly, the regions where the predators threatened the greatest number of species were all dominated by islands – Central America and the Caribbean, islands of the Pacific, the Madagascar region, New Zealand and Hawaii.

Conversely, the continental regions of North and South America, Europe, Africa and Asia contain comparatively few species threatened by invasive predators. While Australia is a continent, it is also an island, where large numbers of native birds and mammals are threatened by cats and foxes.

Along with feral cats, red foxes have devastated native mammals in Australia.
Tom Rayner

Managing menacing mammals

Understanding and mitigating the impact of invasive mammal predators is essential for reducing the rate of global biodiversity loss.

Because most of the threatened species studied here live on islands, managing invasive predators on islands should be a global conservation priority. Invasive predators occur on hundreds of islands and predator control and eradication are costly exercises. Thus, it is important to prioritise island eradications based on feasibility, cost, likelihood of success and potential benefits.

On continents or large islands where eradications are difficult, other approaches are needed. This includes predator-proof fencing, top-predator restoration and conservation, lethal control, and maintenance of habitat structure.

Despite the shocking statistics we have revealed, there remain many unknowns. For example, only around 40% of reptile species have been assessed for the Red List, compared to 99% for birds and mammals. Very little is known about the impact of invasive predators on invertebrate species.

We expect that the number of species affected by invasive predators will climb as more knowledge becomes available.


This article was co-authored by Al Glen from Landcare Research, New Zealand. Landcare Research is a government-funded research organisation that conducts research into a range of conservation issues, including pest management. It did not provide funding for this research.

The Conversation

Tim Doherty, Research Fellow, Deakin University; Chris Dickman, Professor in Terrestrial Ecology, University of Sydney; Dale Nimmo, Lecturer in Ecology, Charles Sturt University, and Euan Ritchie, Senior Lecturer in Ecology, Centre for Integrative Ecology, School of Life & Environmental Sciences, Deakin University

This article was originally published on The Conversation. Read the original article.

Killer cats: The devastating impact of feral cats on Australian biodiversity

I recently published a Hot Topic on feral cats for the Ecological Society of Australia.

Hot Topics are evidence-based syntheses of topical environmental issues and they include a database of studies relating to the topic. You can find the summary text below and access the full synthesis here.

JD_cat_v2
Photo by J Dunlop

 

The domestic cat (Felis catus) was introduced to Australia by Europeans and now inhabits all of mainland Australia, Tasmania and a number of offshore islands. Feral cats are opportunistic, generalist carnivores and their potential impacts include: predation, competition and disease transmission.

The recent Action Plan named feral cats as the number one threat to endangered mammals and Federal Environment Minister, Greg Hunt, has called for the ‘eradicat[ion] of all significant populations of feral cats’ within a decade.

Cats prey on at least 400 native and introduced vertebrates in Australia, including 123 birds, 157 reptiles, 58 marsupials, 27 rodents and 21 frogs. Cats have played a key role in the extinction of at least 20 mammals, and are a current threat to several threatened mammal and bird species. Most evidence for the population-level impacts of feral cats has been gleaned from historical patterns of decline and extinction, or from the failure of reintroduction attempts for threatened mammal species. The small amount of experimental work that has been done confirms that cats can suppress and exterminate small mammal populations.

The impacts of competition and disease transmission are less clear. Overlap in resource use between cats and native predators suggests competition may exist, although this has not been confirmed experimentally. Cats carry the parasite Toxoplasma gondii, which can cause death for many marsupials. High prevalence of Toxoplasma antibodies has been recorded in some marsupial populations, although it is not known how infection might alter their behaviour or susceptibility to predation.

Further research into the non-lethal impacts of cats is necessary, as well as a better understanding of the relationship between cats, other predators and their prey.

Media round up on feral cats

Over the past week or two there’s been a lot of media and opinion pieces about the impacts and management of feral cats in Australia. A couple of papers have been published too.

I thought I would compile all the various articles here because it’s hard to keep up with everything!

Background Briefing Feral cats rewrite the Australian story

Background Briefing The ethics of the dead cat shot

Andrew Cox for ABC Environment Feral cats: how we can solve this problem

Jim Radford for ABC Environment Feral cats will never be eradicated

John Woinarski for ABC Environment Cats are leading a new wave of extinction

Euan Ritchie for ABC Environment Top dogs: Australian predators can provide 24-7 feral cat control

Gregory Andrews for ABC Environment Calling for community support to beat feral cats

ABC Environment Greg Hunt a ‘hero’ for addressing feral cats: Walmsley

ABC Online Feral cats tear through last wild bilby population in Queensland’s Astrebla Downs National Park

ABC Online Scientists call for Tasmanian devils to be reintroduced as mainland predators to combat feral cats

Me for The Conversation To eradicate feral cats, we need to know how many are out there

ABC Online Sniffer dogs track feral cats in the Kimberley as part of wildlife project

The Guardian Environment Endangered species Australia pledges to halt loss of native mammal species by 2020

Chris Dickman and Tom Newsome in Applied Animal Behaviour Science Individual hunting behaviour and prey specialisation in the house cat Felis catus: Implications for conservation and management

Penny Fisher et al. in Applied Animal Behaviour Science How does cat behaviour influence the development and implementation of monitoring techniques and lethal control methods for feral cats?

Hugh McGregor et al. in PLoS One Landscape Management of Fire and Grazing Regimes Alters the Fine-Scale Habitat Utilisation by Feral Cats

Ferals, strays, pets: how to control the cats that are eating our wildlife

By Tim Doherty, Edith Cowan University and Mike Calver, Murdoch University

A feral cat in the bush. T Doherty
A feral cat in the bush. T Doherty

Feral domestic cats are a global threat to biodiversity and were recently named as the biggest threat to endangered Australian mammals.

But what about your pet cat, or the local stray? While any kind of domestic cat can kill wildlife, there’s no “one size fits all” way to manage their impact.

Before we figure out how best to manage cats, we first need to distinguish between the different categories.

What’s in a name?

There are likely more than 10 million feral cats in Australia, and each year tens of thousands of stray cats are put down in Australian cities.

Feral cats in Australia (Invasive Animals CRC)

But there are several ways to classify cats. They include: pet cat, house cat, domestic cat, stray, feral, semi-feral, unowned, colony cat and free-roaming, amongst others.

Despite the plethora of systems, we can make a couple of generalisations. Common characteristics that are used in defining different groups of cats include their degree of socialisation, ancestry, ownership, fear of humans and reliance on humans for their care.

The broadest two groups that can be identified are “owned” cats, which live in a household where they are fed and cared for by humans, and “unowned” cats that do not live in a household and may or may not be fed or cared for. The term “free-roaming”, on the other hand, can include both owned and unowned cats, since it generally refers to a lack of confinement.

Both owned and unowned cats can kill wildlife, so the term free-roaming is useful for management in cities where both groups co-exist.

Stray or semi-feral cats are those that were once owned and part of a household but have since become lost or dumped by their owners. Stray cats are partly or wholly dependent on humans for the provision of food and shelter and colony cats are stray cats that live in groups fed by members of the public, but are not officially owned or cared for.

Feral cats on the other hand, have no degree of socialisation or dependence on humans.

A feral cat in the bush in Western Australia. T Doherty.
A feral cat in the bush in Western Australia. T Doherty.

Claws for concern

In Australia, feral cats outnumber stray and pet cats in terms of both absolute numbers and their relative impacts on native fauna. Feral cats prey on hundreds of species of birds, mammals and reptiles and have contributed to the extinction of more than 20 mammal species. It’s estimated that tens of millions of native animals are eaten by feral cats each night.

Make no mistake though, some stray and pet cats are natural born killers too. The majority of wildlife taken by cats in urban areas are likely to be rats, mice and common bird species, but cats are also a potential threat to endangered species that live in urban areas, like the western ringtail possum in Busselton, Western Australia and the southern brown bandicoot in south-eastern Melbourne.

A matter of life and death

Whether a cat is named as stray or feral could mean the difference between life and death for the animal.

Under New Zealand’s Animal Welfare Act, feral cats can be killed. Killing is acceptable for stray cats too, except where there is evidence of ownership, in which case the cat must be sent to a shelter for at least seven days.

The regulations in Australia are more complex, because each State and Territory has its own set of legislation. Nevertheless, feral cats are recognised as a pest and hence are subject to lethal control across Australia.

Under the guidelines of the International Society of Feline Medicine, stray cats qualify for rehoming, whereas feral cats do not. However, both groups of cats qualify for trap-neuter-return (TNR; a vexed issue in itself), and then “occasionally euthanasia” as a last resort.

Not everyone agrees on the best way to manage cats. One study from the US, perhaps unsurprisingly, found that the opinions of cat colony caretakers and bird conservation professionals were highly polarized, and a study from the UK found that cat rescue workers considered that most cats could be tamed and rehomed, whereas veterinarians generally considered that this was inappropriate for feral cats.

In Illinois, rural residents were more likely to support lethal control of feral cats compared to urban residents , and in Western Australia, cat owners were less concerned about wildlife impacts and were less likely to favour controls than non-cat owners.

How to help

Managing the impacts of different groups of cats on wildlife requires different approaches. Keeping your pet cat indoors or confined to your property can prevent it from killing wildlife.

Collar-mounted predation deterrents like bells or pounce protectors can reduce predation rates. Also, having your male cat desexed will prevent it from breeding with other free-roaming cats and hence contributing new kittens to the stray population. Desexing your female cat avoids the problem of rehoming kittens.

Trap-neuter-return of stray and colony cats does nothing to prevent individual cats from killing wildlife. Feeding is ineffective too because cats may hunt even if they’re not hungry. Removal of stray cats from urban areas is the only way to stop them from eating wildlife.

Landscape scale control of feral cats in Western Australia can be achieved through annual aerial baiting and this is currently being tested in other parts of Australia too. Unfortunately though, the cats always come back, especially in good rainfall years. Predator-proof fencing and predator-free islands are important sanctuaries for threatened fauna that can’t survive even low levels of cat predation.

The future is looking pretty grim for Australian wildlife and reducing the impacts of cats in all categories is an essential component of fauna conservation plans, especially for our endangered birds and mammals.

The Conversation

Tim Doherty receives funding from Earthwatch Institute Australia and the Holsworth Wildlife Research Endowment.

Mike Calver receives funding from the Australian Pet Welfare Foundation. He is affiliated with the Cat Welfare Society of Western Australia. This article represents the views of the authors and not of the society.

This article was originally published on The Conversation.
Read the original article.