Feral cat cull: why the 2 million target is on scientifically shaky ground


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The government’s target to kill 2 million feral cats sounds impressive, but lacks scientific rigour.
Author provided

Tim Doherty, Deakin University; Dale Nimmo, Charles Sturt University; Don Driscoll, Deakin University; Euan Ritchie, Deakin University, and Ricky Spencer, Western Sydney University

The Australian government’s target of killing 2 million feral cats by 2020 attracted significant public interest and media attention when it was unveiled in 2015.

But in our new research, published today in Conservation Letters, we explain why it has a shaky scientific foundation.

The target was developed for the Threatened Species Strategy. At the time of its launch in 2015, there was no reliable estimate of the size of Australia’s feral cat population. Figures of between 5 million and 18 million were quoted, but their origin is murky: it’s possible they came from a single estimate of feral cat density in Victoria, extrapolated across the continent.

A recent review estimated a much smaller population size — probably varying from 2 million to 6 million, depending on environmental conditions. Using this estimate, the proportion of Australia’s cat population to be killed under the government’s target is now likely in the range 32-95%, rather than 11-40% based on the original population estimate.




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Targets for the removal of pest animals should consider how they will affect an animal’s current and future population size. But because a scientific justification for the 2 million target was never provided, it is unclear whether or how the revised estimate would alter the target.

Feral cats culled in Queensland.
http://www.couriermail.com.au/news/queensland/news-story/746d8ad0366fe9f64ea9c26d36a41a37

Hitting the target, missing the point

For cat control to have a lasting effect on feral populations, it needs to be intense, sustained, and carried out over large areas. This is because cats can rapidly reproduce and re-invade areas. To benefit threatened species, cat control also needs to be undertaken in areas that contain — or could potentially contain — native species that are threatened by cats.

Research commissioned by the government conservatively estimated that around 211,500 feral cats were killed in 12 months in 2015–16 (ranging between around 135,500 and 287,600). This estimate was used to report that the first-year target to kill 150,000 cats was met with room to spare.

The benefit to threatened species of achieving this target is unclear, because we don’t know if the control efforts had a measurable effect on cat populations; whether they took place in areas that would benefit threatened species; or how (or if) the target and related activities contributed to the estimated 211,500 cat deaths.

Around 75% of the killed cats were attributed to shooting by farmers and hunters. It is questionable whether such approaches could keep pace with high rates of population growth and re-invasion from surrounding areas.

These and other issues were known before the target was set, leading experts to recommend that an overall cat culling target should not be set.

Cat image captured using camera traps in the Hawkesbury region, NSW.
Western Sydney University

Shifting focus

The focus on killing cats risks distracting attention from other threats to native wildlife. These threats include habitat loss, which has been largely overlooked in the Threatened Species Strategy.

Habitat loss is politically sensitive because its main driver is the clearing of land to make way for economic activities such as agriculture, urban development, and mining. The strategy mentions feral cats more than 70 times, but habitat loss is mentioned just twice and land clearing not at all. Australia has one of the world’s worst rates of land clearing, which has recently increased in some regions. For instance, clearing of native vegetation in New South Wales rose by 800% between 2013 and 2016.




Read more:
Let’s get this straight, habitat loss is the number-one threat to Australia’s species


A focus on feral cats is warranted, but not at the expense of tackling other conservation threats too. A comprehensive, integrated approach towards threatened species conservation is essential.

Any upside?

Despite its questionable scientific basis, it is possible that the ambitious nature of the 2 million target has raised the public profile of feral cats as a conservation issue. However, to our knowledge, there has been no attempt to measure the effectiveness of the target in raising awareness or changing attitudes, and so this remains a hypothetical proposition.

Raising awareness about the negative impacts of cats on native wildlife is important.

The Threatened Species Strategy has other targets that are more closely linked to conservation outcomes, such as the eradication of cats from five particular islands and the establishment of ten new fenced cat-free exclosures. Achieving these targets will make a small contribution to the culling target, but have a comparatively large benefit for some threatened species.




Read more:
For whom the bell tolls: cats kill more than a million Australian birds every day


Australia’s target to kill 2 million feral cats is a highly visible symbol of a broader campaign, but the success of policies aimed at reducing the impacts of feral cats should focus squarely on the recovery of native species.The Conversation

Tim Doherty, Research Fellow, Deakin University; Dale Nimmo, Associate professor/ARC DECRA fellow, Charles Sturt University; Don Driscoll, Professor in Terrestrial Ecology, Deakin University; Euan Ritchie, Associate Professor in Wildlife Ecology and Conservation, Centre for Integrative Ecology, School of Life & Environmental Sciences, Deakin University, and Ricky Spencer, Associate Professor of Ecology, Western Sydney University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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A numbers game: killing rabbits to conserve native mammals



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Controlling rabbit populations has a key role in conserving Australia’s native plants and animals
William Booth

Euan Ritchie, Deakin University; Damien Fordham, University of Adelaide, and Miguel Lurgi, Centre national de la recherche scientifique (CNRS)

Invasive species have a devastating effect on biodiversity. In Australia, introduced red foxes and feral cats have been implicated in the majority of the extinctions of the native mammal fauna, which has been decimated since European arrival.

But there’s a herbivore that also causes eco-catastrophe. Rabbits both compete with native animals for food and shelter and act as easy prey for abundant populations of cats and foxes. By over-grazing vegetation and reducing habitat complexity, they make hunting easier for introduced predators.




Read more:
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Food webs are complex. Because of this, once an invasive species is embedded in a food web, simply eradicating them without considering the potential knock-on effects to other species they interact with, could cause unintended and undesirable consequences. We modelled different rates of rabbit population reduction to assess what level of control might be best for aiding the conservation of native mammals and not causing negative outcomes.

Rabbit numbers boom and crash

Rabbits, famously, reproduce rapidly and can cope with a relatively high predation rate. This can cause “hyper-predation”, where rabbit-inflated cat and fox populations indirectly increase the predation pressure on native mammals. This is especially so when rabbit populations intermittently crash due to, for example, extreme environmental events (like severe and prolonged droughts) or disease. This causes predators to switch their diet and eat more native mammals.

Threatened species such as the greater bilby are likely to benefit from rabbit control.
Jasmine Vink

This logically suggests that reducing rabbit numbers might thus help reduce cat and fox populations, by removing their abundant prey. Collectively this should benefit native plants and animals, including many threatened mammal species. However, ecosystem and pest management is a complex game.

When controlling rabbits we need to look beyond one or two species. We should consider the potential consequences for the entire ecological community, which ultimately depend on how changes in one species percolate through the network of ecological interactions between them.

Our new research, recently published in the Journal of Applied Ecology, set out to examine these questions in more detail. We consider other key players in Australia’s arid regions, such as kangaroos and dingoes, when looking at the effects of rabbit control on small native mammals. Our aim was to provide a better understanding of how changes in rabbit populations might affect other species via the food web.

We developed a multi-species ecological network model to describe and quantify how changing rabbit abundance can affect species on different feeding levels. In addition to rabbits, small native mammals, and mesopredators (cats and foxes), our model also considers apex predators (dingo) and large herbivores (kangaroo) as part of the Australian arid food web. This model allowed us to examine changes in predator-prey interactions (including potential prey switching and hyper-predation) and how these could affect the survival of native prey through time.

Our model of an Australian arid ecosystem food web.
Author provided

We found that removing rabbits at rates between 30-40% appeared to benefit small mammals. This is approximately the rate at which rabbits are currently managed in Australia using biocontrol agents (introduced diseases).

Rabbit control in Australia typically involves a “press and pulse” approach. Rabbit populations are suppressed via biocontrol (press) and periods of warren destruction and poisoning (pulse). Finding that reducing rabbit populations by around 40% seems most beneficial to small mammals is important, as it informs how and when we combine these strategies.

The 40% rate corresponds well with the disease-induced (press) mortality rate in rabbit populations due to rabbit haemorrhagic disease and myxomatosis. These are the primary biocontrol agents used in arid Australia to control rabbit populations.

Our study supports rabbit-reduction strategies that involve sustained “press” control, that kill a moderate portion of a rabbit population, with less frequent removal at higher proportions of the population.

To effectively manage invasive species, it’s important to focus on entire communities. Targeting single species might not be enough – every animal exists within a complex web of interactions.




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Mourn our lost mammals, while helping the survivors battle back


There has been much focus by the current government on controlling feral cats, as a way to conserve many of Australia’s unique and threatened mammal species.

The ConversationHowever, more focus could be devoted to protecting habitat cover and complexity, by reducing the land clearing and over-grazing that makes hunting easier. We can also manage rabbits sensibly to reduce competition for resources, and indirectly control cats and foxes.

Euan Ritchie, Associate Professor in Wildlife Ecology and Conservation, Centre for Integrative Ecology, School of Life & Environmental Sciences, Deakin University; Damien Fordham, , University of Adelaide, and Miguel Lurgi, Postdoctoral research fellow, Centre national de la recherche scientifique (CNRS)

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

Without culling, Victoria’s feral horse plan looks set to fail


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Feral horses in the eastern Alps.
Griff en/Wikimedia Commons, CC BY-SA

Don Driscoll, Deakin University

Victoria’s new draft feral horse management plan, released on the last working day before Christmas, will be open for comment until February 2. But will it protect the Alpine National Park? The answers are yes on the Bogong High Plains, and no in the eastern Alps.

The government deserves congratulations for planning to remove all horses from the most sensitive alpine areas around Falls Creek by 2020. These areas of the Bogong High Plains have fewer than 100 horses, but also rare snow-patch and bog communities that are extremely vulnerable.

But elsewhere, the goal of removing 400 horses a year from the eastern Alps doesn’t seem to go far enough. And by refusing to countenance the idea of culling, the state government is passing up the only realistic chance of getting feral horse numbers under control.


Read more: The grim story of the Snowy Mountains’ cannibal horses


The bulk of the plan provides grounds for cautious optimism. It acknowledges that feral horses threaten a range of native mammals, frogs and lizards, as well as displacing kangaroos and wallabies. Horses have enormous impacts on vegetation in alpine bogs and streams, and in many other ecosystems too.

The plan also makes clear that reducing horse numbers is a legal requirement. Victoria’s Flora and Fauna Guarantee Act 1988 lists “degradation and loss of habitats caused by feral horses” as a threatening process. The Victorian National Parks Act 1975 calls for “exotic species” such as horses to be exterminated or controlled within national parks.

The plan also sets a realistic time frame for review (annual reviews and major review after three years), and suggests that management plans will be altered if adequate environmental protection is not achieved. All of this is extremely promising, suggesting the state government is genuinely interested in delivering tangible environmental benefits.

Numbers game

But while the aspirations are good, the details present some problems. The draft plan promises to “explore all possible control options” to deliver a low horse population in the eastern Alps.

But the proposed reliance on trapping and removal, rather than culling, suggests the government is reluctant to enter what would be a tough debate against the often vocal pro-brumby lobby groups. This reluctance is to the detriment of our native species and apparently at odds with legislation.

The problem is that the New South Wales government has already tried trapping and removing horses in Kosciuszko National Park, and it hasn’t worked. Horses have continued to spread northward onto the main range, where environmentally sensitive alpine tarn and snow-patch communities occur.

It is unclear whether Victoria’s “aspirational goal” of removing 400 horses each year over three years will actually be enough to reduce horse numbers, or even to stabilise them. The report mentions modelling showing that the population can be stabilised by taking 200 horses per year, and that it would start to decline if 400 were taken per year.

But none of this modelling is published, so it can’t be evaluated in detail. And simple calculations suggest that these figures are incredibly optimistic.

The report says there were 2,350 horses in the eastern Victorian Alps in 2014. Horse populations can increase at up to 20% per year, so by now there could be more than 4,000 feral horses.

This means that even if the government does manage to remove the full quota of 400 horses each year, it would only take a 10% population growth rate for the numbers to keep rising. At a rate of 20%, there could be well over 5,000 horses by 2020, even with trapping and removal.

Culling option

Based on this rough calculation, the plan needs to eradicate many more horses. The draft plan claims that feral horses in the eastern Alps are “well established and are considered beyond eradication using currently available control tools”. Yet this claim ignores aerial culling, which is the cheapest, most effective, and most ethical way to reduce feral horse numbers.

Highly trained sharp-shooters and helicopter pilot teams can destroy more than 50 horses per day (based on previous culls in NSW, in which three teams of three people destroyed 606 horses over three days). Three teams could solve the feral horse problem in the Victorian alpine country in a month, and at lower cost.

It cost taxpayers more than A$1,000 for each horse trapped and removed from Kosciuszko National Park. Using the NSW cull as a guide to the resources required, and assuming A$300 per day per person, and A$10,000 per day per helicopter, it might have cost around A$150 per horse using aerial culling. That’s roughly 15% of the cost of trapping and removal.

Despite the risks to wildlife canvassed in the draft plan, and similar reports from NSW, there is no peer-reviewed research that defines the threats to native animals. A revised plan must include research to understand both the impacts of feral horses on native animal populations and their welfare.

The debate over culling horses typically ignores the unseen suffering that horses cause to native animals. Quantifying that suffering will be crucial for making informed decisions around feral horse management.


Read more: The ethical and cultural case for culling Australia’s mountain horses


It is great that we have a plan for managing horses in the Victorian Alpine National Park – albeit one that seems unlikely to work in the eastern Alps. But the Victorian government needs to show courage and leadership on the issue of culling feral horses. Our alpine natural heritage will continue to decline until horses are taken out of our national parks, and that will only happen when managers can include culling among their suite of management tools.

The ConversationIn NSW, the feral horses in Kosciuszko National Park are growing in number, and doing real damage to Australia’s highest mountains. Hopefully both states can take back the reins of feral horse management from single-issue lobby groups and exercise some real control over their feral horses.

Don Driscoll, Professor in Terrestrial Ecology, Deakin University

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

A cull could help save koalas from chlamydia, if we allowed it


Desley Whisson, Deakin University

Whether it’s sharks, crocodiles or kangaroos, culling animals is always a contentious topic. But when the iconic koala is the species for which culling is being advocated, it sparks even more interest and debate.

Such was the case this week when researchers from Queensland and New South Wales published a study recommending that koalas be culled in the name of conservation.

Their proposal is for the selective culling of individual koalas suffering from chlamydia in an attempt to reverse the disease’s impact on vulnerable populations.

Koala chlamydia

Chlamydia is a sexually transmitted bacterial disease (a different strain to that which afflicts humans) that causes infertility and blindness in koalas, and is one of several factors thought to be behind the decline of koala populations in the eastern states. Koalas suffering the disease gradually become weak, stop eating, and die.

Although affected koalas sometimes may be found and taken into care, to date there have not been any systematic programs to combat the disease in wild populations. Given the negative effects of chlamydia on koala populations in some regions there is an urgent need to look at management options, including one that may seem quite radical – culling diseased individuals.

The current study considered a declining population on the “Koala Coast” of south-east Queensland. The researchers used computer simulations to model several disease management scenarios. The simulation that had the most positive effect on long-term population growth involved culling chlamydia-infected koalas that were already sterile and dying, and treating other infected koalas with antibiotics.

The study found that, to grow the Koala Coast population, around 10% (or 140 individuals) of koalas would need to be captured and culled or treated each year.

Killing for conservation

The idea of culling diseased individuals to manage disease and its impacts on wildlife populations is not new, and has met with both success (such as with Chronic Wasting Disease in deer in North America) and dismal failure (in the case Devil Facial Tumour Disease in Tasmanian Devils).

The effectiveness of these programs depends largely on the behaviour and ecology of the host species, and the distribution and nature of the disease. When enough is known about these aspects, computer modelling is useful for determining the potential effectiveness of a selective culling approach and for helping guide management actions.

But while modelling may inform us that culling is scientifically the best management approach, deciding whether and how to go ahead is complex, even more so when koalas are involved.

Koala management is closely scrutinised both nationally and internationally. The koala is the only native Australian species for which culling has been consistently dismissed as a management option (for overabundant populations in the southern states).

Although the current proposal for selectively culling diseased koalas isn’t “culling” as defined in the National Koala Conservation and Management Strategy, it still raises a question about killing koalas for conservation.

In 1997, culling was proposed as a component of an integrated strategy to manage high density populations of koalas on Kangaroo Island, South Australia. Despite having a sound scientific basis and the endorsement of many experts, it sparked much outrage and ultimately led to a decision at the Commonwealth level that culling will not be considered for management of koalas.

This decision has resulted in millions of dollars being spent on fertility control and translocation programs in Victoria and South Australia over the last two decades. These programs attempt to address situations where overpopulation of koalas is causing significant damage to local ecosystems.

Although some have brought positive outcomes after many years of intensive effort (for instance at Kangaroo Island and Mount Eccles in Victoria), these interventions are logistically challenging, extremely costly, and sometimes may have poor welfare outcomes for individual koalas.

Consequently, “do nothing” is the default management approach for many situations. But this can have drastic consequences for koalas, their habitats, and the other species that rely on those habitats.

Such was the case at Cape Otway in late 2013 when the Victorian government’s “do nothing” approach led to unsustainably high koala population densities, causing widespread defoliation of trees and the starvation of thousands of koalas. Around 700 koalas in irreversibly poor condition were killed when the government finally intervened on animal welfare grounds. Meanwhile, thousands of koalas likely suffered a slow death out-of-sight.

Although some trees recovered following the dramatic decline in koala numbers, high fertility has resulted in the population increasing again, and another imminent starvation event.

We do it for other animals, why not koalas?

Many wildlife researchers and managers would argue that a better approach for these situations would be to cull some koalas when it is clear that even more koalas will die if no action is taken.

This is not to suggest that culling be undertaken indiscriminately, nor in all situations. But it should be considered in circumstances where science indicates that it is the most effective approach to maintaining a healthy ecosystem and population of koalas.

It is the same approach that is used for numerous other native species in Australia and worldwide, so why shouldn’t it be considered for koalas, too?

Considering the outrage over killing Cape Otway’s starving koalas to reduce suffering, it seems that there may be little public support for culling koalas for any reason. It will be interesting to see how this new proposal to cull diseased koalas in Queensland and New South Wales will be received.

There likely will be opposition to culling and more support for a “treatment only” approach, despite its lower predicted effectiveness. However, one would hope that decision-makers place more weight on the scientific rigour of the research behind the proposal rather than the emotive argument that it is wrong to cull koalas.

The Conversation

Desley Whisson, Lecturer in Wildlife and Conservation Biology, School of Life and Environmental Sciences, Deakin University

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