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Does Culling Work?

Fox carcasses are often strung up along fences, reminding landholders in the area of the continuing threat they pose to livestock. Credit: mattinbgn

Fox carcasses are often strung up along fences, reminding landholders in the area of the continuing threat they pose to livestock. Credit: mattinbgn

By Thomas Newsome, Lily van Eeden, Billie Lazenby & Christopher Dickman

Culling of pests such as foxes, feral cats and dingoes can have unexpected and completely undesirable effects.

Wildlife managers face the twin tasks of monitoring animal populations and intervening to correct their numbers if they are perceived to be heading in the wrong direction. On the one hand, declining populations may be targeted for recovery, especially if they represent threatened species or have other values that we wish to maintain. On the other hand, rapidly increasing or already-overabundant populations will often be subject to control.

“Traditional” pest species such as rabbits and foxes fall into this latter category, but so too do native species such as koalas and large kangaroos at certain times or locations. Culling is often advocated to reduce population numbers.

Culling is a management option that aims to remove some animals from a population and thereby reduce to acceptable levels the damage that they appear to be causing. However, what is considered “acceptable damage” varies greatly. Thus, modest culling of fox populations may be sufficient in sheep-grazing areas, especially at times when no lambs are present and at risk of being killed, but more aggressive culling might be needed if small or threatened populations of wildlife need to be protected.

In Sydney, for example, the small and endangered population of little penguins at Manly is sometimes exposed to marauding foxes. Here, the loss of any penguins is likely to compromise the survival of the population, and the only level of culling that will ensure its persistence is the complete, or almost complete, removal of the intruding predators.

Although culling is often considered essential, it remains a controversial option for managers to employ. If a species is introduced, widely accepted to be damaging to native wildlife or agricultural interests, or occurs in remote areas, culling will often be carried out with public approval.

For example, recent operations that culled hundreds of thousands of feral camels in central Australia raised nary an eyebrow, but initiatives to cull much smaller numbers of feral horses, or brumbies, from alpine and sub-alpine areas regularly raise howls of protest even though the damage caused by horses to high country environments has been well documented. Of course, people generally have more affinity and closer associations with horses than they do with camels, so emotional responses are not surprising.

Culling of native wildlife is also contentious, especially if it involves large, cute, charismatic or iconic species such as flying foxes, dingoes, kangaroos or overabundant populations of koalas. In all these examples, debates about culling are often loud and emotive.

In the case of dingoes, the merits of lethal control are continually being debated, yet shooting, trapping and poison baiting are carried out across much of the continent. Dingo or “wild dog” management is conducted primarily to protect livestock, particularly in sheep-grazing country south of the dingo fence.

The dingo is legally protected to some degree in most states, including Victoria, where it is listed under the Flora and Fauna Guarantee Act 2008. However, in a controversial move, the Victorian government recently announced that it will re­introduce a bounty to promote lethal control of dingoes and other canids. This change comes approximately a year after the previous bounty program was axed.

For all the resources and emotional energies that are spent on culling, you would be forgiven for thinking that the science of culling operations is well-known and that success is inevitable when operations are properly planned and executed. Well, yes and no. Culling should work if the size of a pest population is known, if removal methods are available and will reduce the population size and impact by a desired amount, and if the rate of recovery is known.

However, satisfying all of these “ifs” can be a tall order, and two recent studies show that small-scale culling operations can have not just unexpected effects but also completely undesirable ones.

In the first study we assessed whether ground shooting is effective at reducing fox density in the southern highlands of New South Wales where sheep, primarily, are grazed. To do so we estimated the density of foxes from spotlight counts before and after a fox cull. Prior to culling we estimated that the density of foxes was 4.18 individuals per square kilometre; a density similar to estimates in other Australian agricultural regions.

We thought we would greatly reduce the fox population by removing 47 individuals in 12 nights of ground shooting, but our results suggest that this did not happen. In fact, we made hardly a dent in the population, with the density estimate after control being 3.26 foxes per square kilometre – a reduction of only 0.92 individuals per square kilometre.

What is worse is that we spent a considerable amount of time and money conducting the cull: 66 hours at a cost of around $1500. This included driving around for about 400 km in a 4WD or quad bike looking for foxes. We also drove 100 km to procure ammunition and equipment. At the end of it all, we wondered whether the effort was worth it.

While we didn’t estimate whether the fox cull helped to reduce fox predation on sheep, we do know that our culling program was ineffective at reducing fox density. This result was unsurprising in some respects; other studies have suggested that small-scale and isolated control programs do not effectively reduce fox density. Our study confirmed that assumption. Overwhelmingly, rapid immigration of foxes from neighbouring areas is cited as the key reason why uncoordinated control operations frequently fail.

But despite our study demonstrating that ground shooting is ineffective at reducing fox density, recreational hunters, land managers and property owners in Australia and elsewhere throughout the world continue to promote and use this management strategy.

Perhaps the results of our second study below will help to sway public opinion. In this study in southern Tasmania, we monitored the effectiveness of small-scale control of feral cats at three sites. Feral cats were monitored with remote cameras before, during and after a 13-month pulse of low-level culling at two of the sites.

Surprisingly we found that the number of individual feral cats identified by remote cameras actually increased by 75 and 211%, respectively, at the two culling sites by the end of the culling period before stabilising to pre-cull levels.

This result demonstrates that the rate of culling was not greater than the rate of replacement. Given that culling was conducted over a short period of time relative to the reproductive rate of feral cats, we suspect that the removal of dominant cats released their territories for other cats, and this resulted in a temporary influx of new individuals from surrounding areas – just like foxes in the first study.

So the question remains: does culling work? Under some circumstances we can confidently say yes, but under others the answer is more likely to be no. Here’s why.

On the one hand, culling can be an effective tool in some situations, particularly if the objective is to eradicate. For instance, feral cats were successfully extirpated from Macquarie Island as part of an ongoing and coordinated management action. Similarly, culling may be justified if the culling operation successfully removes problematic individuals or results in fewer losses of native wildlife and/or stock.

But our studies on foxes and cats demonstrate the need for cullers to better understand how the target animals will respond to the cull. If the culling effort cannot be sustained over a long enough period and is not intensive, then neighbouring or young dispersing individuals will simply fill the void that we create for them in the landscapes.

In some cases the cull could also disrupt the social structure of the target population and result in more breeding individuals or more nuisance animals. As an example, the efficacy of dingo culling varies considerably depending on the intensity of the operation, and in some instances population reductions can be short-lived. A study in Queensland also showed that culling can result in changes to the age structures and group sizes of dingo populations, and this sometimes leads to increased predation on cattle.

Similar conclusions have been reached from studies on coyotes in the USA. In fact, it’s often argued that recreational hunting can help to reduce the negative impacts of coyotes on human enterprises such as livestock ranching. However, even if the offending predators are removed, they are quickly replaced by other individuals. Just like foxes and feral cats in our studies, coyotes are far too resilient to be affected by most eradication programs, let alone from derbies or recreational hunting because high rates of human-caused mortality are required to reduce coyote abundance.

So where does this leave wildlife managers who have the responsibility of controlling rapidly increasing or over­abundant populations?

Despite considerable advances in our understanding of the driving forces behind the distribution and abundance of animal populations over the years, wildlife managers are often left making decisions based on incomplete information. Ecological research can play a pivotal role in addressing some of these knowledge gaps. So, too, can advances in technology, such as remote cameras that allow for the monitoring of cryptic species such as foxes and cats.

In the meantime, any management intervention that may involve culling should be preceded by a sound plan that includes ongoing monitoring of the success of the operation. We know that culling can work, but we need to improve our record of documenting successes as well as failures so that we can increase the odds of management success in future.

Thomas Newsome is a postdoctoral researcher at Deakin University and The University of Sydney. Lily van Eeden is a PhD candidate and Christopher Dickman is Professor in Ecology at The University of Sydney. Billie Lazenby is a wildlife ecologist in the Tasmanian Department of Primary Industries, Parks, Water and Environment.