Cats and their control
Wherever Europeans settled in Australia they brought cats with them. (Abbott, 2002) They are now widespread along the coast and in the pastoral areas inland.
The newest research claims that cats roam over 99.8% of Australia’s land area and close to 80% of the island areas. (Doherty et al, 2017)
(also see: http://www.nespthreatenedspecies.edu.au/news/media-release-feral-cats-cover-over-99-8-of-australia)
In south-west Western Australia cats were introduced as early as the 1820s but it took about 40 years before the first feral cats were reported. (Abbott, 2002, Abbott, 2008a)
The also deliberately introduced rabbit provided a key prey species for the increasing cat populations but still spread so rapidly that the WA Government in 1898 supported the release of domestic cats financially in order to curtail the spread of rabbits from South Australia to the south coast of WA. (Abbott et al, 2014)
In WA a severe epizootic disease between 1880 and 1920 had already reduced a wide range of mammals in the critical weight range including ringtail and brushtail possums considerably (Abbott, 2006) and left the remaining low density populations particularly vulnerable to predation.
Cats like foxes are generalist feeders – but the fox more so. Cats rarely scavenge unless under food stress. (Read et al, 2015) They are almost exclusively carnivorous and live prey is their favourite food source. (Lazenby et al, 2014)
Research in Western Australia confirmed that feral cats consumed more live prey - birds, rodents and reptiles - than foxes that also ate carrion and invertebrates (Risbey et al, 1999) and that small mammals were consumed more than any other prey category. (Dickman, 2014)
While cats prefer smaller prey, an adult feral cat can still take an animal the size of an adult rabbit. Prey larger than approximately 1.5 kg are not at such a high risk of cat predation, while very small mammals usually have a high rate of reproduction which can compensate for losses through predation. (Abbott et al, 2014)
Newer evidence suggests that if a cat was able to successfully kill a difficult to hunt species, it can specialise on this selection and become a skilled hunter and killer of this very species. If the species is rare or endangered a single cat can threaten the persistence of the population and cause catastrophic predation particularly as many threatened species populations are fairly small. (Read et al, 2015)
Large feral cats can weigh about 6 to 7 kg which would be the equivalent to a female fox. (Short and Turner, 2005) The largest feral cat ever captured in WA (in 1995) weighed 10.4 kg. (Abbott et al, 2014)
Cats and foxes are both mainly active during the night but also frequently hunt during the day. (Abbott et al, 2014)
However, cats can climb high into trees and hunt arboreal mammals whereas foxes can only ascend trees with sloping trunks. (Abbott, 2011)
Cats are now regarded as one of the world’s most invasive species that contributed to bird, reptile and mammal extinctions globally. In Australia predation by cats is considered the single most significant threat to mammals of the critical weight range (Read et al, 2015) and they might be responsible for 16 mammal extinctions. (Lazenby et al, 2014)
They are recognised as a key threatening process under the EPBC Act.
In addition to direct predation, cats carry parasitic and disease organisms such as Toxoplasma gondii and the tapeworm Spirometra erinacei.(Johnson and Hemsley, 2008)
In Australia we now have more feral cats than pet or stray cats and their effect on native wildlife is disastrous. However, in urban areas our pets also cause considerable damage to ringtails in our backyards. Most cats are natural born killers no matter what their owners claim and no matter how well they are fed. They hunt and kill out of instinct. (Read et al, 2015)
The impact of cats needs to be reduced in all categories – pets, strays and ferals - if we are serious about fauna conservation. (Doherty and Calver, 2014)
For domestic cats collar-mounted alarms that emit a signal when the cat pounces and bibs which stop the cat from climbing trees or pouncing effectively have been tested, however their efficacy is not unequivocal. Bibs are manufactured from brightly coloured fabric that also warns potential prey.
Rubbish tips and landfill sites close to towns and bushland usually harbour dense populations of stray cats. (Dickman, 2014)
The difference between a stray and a feral cat might just be the time they have spent independently.
Feral cats are usually shy, elusive and highly suspicious of people and their devices and accordingly hard to control. They also rarely consume bait when live prey is available. (Read et al, 2015) This is not just a preference but based on their general hunting behaviour. They use sight and sound to locate their prey and a lifeless sausage would not attract their attention.
This makes it impossible to control cats using similar broad-scale baiting programs as for fox control.
Unlike foxes cats do not have a major impact on agriculture which most likely lowered the urgency of controlling them. Nevertheless researchers have investigated ways of controlling cats cost-effectively on a landscape scale for more than 20 years, but there still seems no break-through in sight. (Moseby and Hill, 2011) Our contemporary control techniques are clearly inadequate, ineffective and unpopular with the public. (Read et al, 2015)
Research is concentrating on poisoning as the method of control and the same types of poison – 1080 and PAPP – that kill foxes are effective in killing cats. There is no specific toxin for cats available. As cat bait would have to be placed on the surface, the risk for non-target species is particularly high. (Dickman, 2014)
A trial to encapsulate toxin in a pellet (Curiosity) instead of injecting liquid 1080 into meat bait ended in failure as it was wrongly assumed that native animals would reject the pellet. Bandicoots, chuditch and lizards all consumed the bait including pellet - which thankfully did not contain any toxin in the trial but a staining substance. (De Tores et al, 2011)
Varanid lizards have considerable dietary overlap with cats and are highly at risk of consuming what was meant for a cat. No poison will ever be safe but the risk associated with Curiosity was deemed unacceptable.
However, the Threatened Species Strategy again hails Curiosity as ‘the new humane feral cat bait’. If the pellet contains PAPP, death will be more humane than if it contains 1080 but the risk for non-target species is unchanged. I can only hope that the design of the pellet was significantly altered and native animals will now reject it.
While the former Threatened Species Commissioner (Gregory Andrews) marketed Curiosity (PAPP) as the ‘wonder weapon’, the former WA Environment Minister (Albert Jacob) praised Eradicat (1080) as the most promising course of action. In addition there is talk of a ‘new hybrid Eradicat’ (Moseby et al, 2015) but no more specific information is publicly available.
The sausage-style cat bait, Eradicat is now under experimental permit in Western Australia and captive trials demonstrated feral cats will consume the bait. However, there are still major concerns for non-target species and it is unclear whether Eradicat is effective under all environmental conditions. (Glen, 2014)
Non-target scavenging species will typically consume most meat baits and are at risk from either toxin – 1080 or PAPP. The delivery system is therefore the main issue.
A promising new approach attempts using the cats’ intrinsic predatory behaviour by providing them with live bait that has been rendered toxic - ‘Trojan prey’. (Read et al, 2015)
The best species to use would be one that is widespread, abundant, favoured by cats and tolerant to 1080. It would however be important to deliver a lethal dose as cats will develop an aversion to that prey if they suffered after ingesting a sub-lethal dose. Prey-switching to possibly a threatened species could be the highly undesirable consequence.
Translocation programs often fail due to cat predation and it would be an option to either incorporate a toxic pellet into the radio-tracking collar or implant it into a body part that is likely to be eaten by the cat. It would still be a loss if a translocated animal was killed by a predator, but at least the predator would be killed as well.
For ringtail possums the collar option seems problematic as it is very unlikely that the current tough collars would be consumed by cats. The implant could only be used for species that are highly tolerant to 1080, which applies to ringtails.
Even if the ringtail lived long enough for the toxin to degrade to sub-lethal levels, it would teach the cat to avoid ringtails as prey in the future.
As it is hardly possible to eradicate cats from ringtail habitat, this training of a predator could be helpful as a ringtail-avoiding ‘resident’ cat could also reduce influx from new cats.
This way of delivering toxin would probably reduce the number of non-target species at risk drastically.
This research also suggested rendering tissue of potential prey toxic to cats but this would make it necessary to keep the toxin levels up over time. Supplement feeding seed laced with 1080 would be possible, but they would of course not have any attraction for ringtail possums. I assume that it would also be extremely difficult to get the dosage right as the acceptance of supplementary feed would change with the availability of natural food.
Enhancing toxic plant species to lift the tolerance levels of our native wildlife would mean a major manipulation of habitat, which is more than unlikely to be a realistic option.
However, all suggestions in the above article would be extremely labour-intensive and costly and therefore unlikely for use in broad-scale control programs.
Broad-scale control programs rely on large quantities of bait, for cats 10 times more per square km than for foxes. (Moseby et al, 2015)
Camera monitoring revealed that even if cats find bait, they are not likely to take it unless they are very hungry indeed.
The so-called grooming trap is a way around the necessity to take bait but unfortunately also not suitable for broad-scale application.
If a cat walks past, a poisonous paste is sprayed automatically onto their fur. Being a compulsive groomer, the toxin will be cleaned off the fur and ingested.
I have trouble imagining how this could work in practice, but it is said that sensors will restrict triggering to target species; the trap can hold a high number of toxin portions and it can stay unattended for several months. (Moseby et al, 2015) I could not find any information on which toxin would be used and how it can be protected from degradation to sub-lethal levels.
The grooming trap is one of the innovations funded through the Threatened Species Strategy, even though it will hardly add much to the progress towards reaching the overly ambitious or likely impossible target of culling 2 million cats by 2020.
For small scale control or for the removal of a troublesome individual, traps are an option.
Wire cages and padded soft-jaw traps display similar trapping efficiency (Short et al, 2002).
Traps can be hidden in denser vegetation or covered with a hessian sack or card board.
There is a myriad of suggestions which lures have the highest success rate but chicken (fresh or fried) and fish (tinned or fresh) seem the most promising. Dried cat food with fish oil, lamb kidneys and mouse or day-old chicken carcasses seem also popular.
However in warm weather fresh food desiccates quickly and attracts ants.
Cat urine from feral cats sprinkled on the ground near the trap can also help to attract their curiosity.
Traps need to be checked frequently – particularly early in the morning. The usual non-target species that have a dietary overlap with cats might get caught in the trap as well.
(Stokeld et al, 2015, Read et al, 2015)
Some trials using trained trapping dogs claim that this provided an effective method for capturing feral cats with a much higher success rate than life trapping. (McGregor et al, 2016)
All of these control methods and also shooting are labour-intensive, expensive and difficult to maintain for long periods over large areas.
Eradication of feral cats is not possible unless small, prescribed areas are targeted and quick recolonization is prevented. (Lazenby et al, 2014)
Disease – feline immunodeficiency virus or feline leukaemia virus - is often hailed as the most effective control method with the least negative repercussions for pets as they could be inoculated.
These and other potential pathogens are already prevalent in Australian cat populations and it would be unlikely that they could have a major effect. There also is no effective vaccine available yet. (Dickman, 2014)
The use of a contraceptive vaccine would also be problematic and costly but it might still be the most feasible approach as it is more likely to be acceptable to the public.
Non-transmissible agents protect pets and other species (Moodie, 1995) but a sterilised cat would of course still feast and reduction in cat numbers would be postponed.
