Predation discussion
Predation seems the main reason for translocation failure, for extinction particularly of critical weight range species (Burbidge and Mckenzie, 1989) and for decline in our native wildlife in general.
Predation is clearly a major threat and eradication of the invasive pests we have brought into the country ourselves would be wonderful – if it was possible at all. Even a target of killing 2 million cats in 3 years sounds like utopia to me.
We have two toxins that kill cats and foxes - 1080 and PAPP. We can inject the toxin into meat or eggs or encapsulate it. The broader the area of application, the better are the chances of success but also the greater the risk for non-target species.
1080 is relatively freely available (license system) and I wonder whether this will also be the case for PAPP as it is clearly more humane.
I fail to see any signs yet that the experimental use will lead to broad-scale implementation any time soon – no matter whether it is Curiosity (pellets) or Eradicat or a ‘hybrid Eradicat’ as mentioned in some articles .
The pellet is praised as safe for non-target species in some publications, criticised as useless in others. (see: 'Cats and their control')
The risks for non-target species is clearly the biggest obstacle and could most likely only be overcome through the development of species-specific bait. (Paull et al, 2011)
Attempts on cat eradication not always go according to plan. A team of researchers trapped cats in a specified area and camera monitored the outcomes. Numbers of trapped cats were high at the beginning, then decreased steadily suggesting that most cats had been removed. However, the cameras said something completely different. The abundance of cats had gone up not down! Dominant, resident cats were caught and young cats that would normally have died recolonised the area quickly as there was ‘unclaimed’ space and food available. They also had learned to avoid traps.
The only way of reducing cat abundance long- term would be a constant removal above the rate of survival, recruitment and immigration. (Johnson, 2015)
In peri-urban and urban areas cats may also be subsidised by human food sources which can promote increases not only in cat numbers but also in numbers of rats and ravens. Raven populations, a major threat to young ringtails in urban backyards, often explode when provided with direct or indirect nutritional subsidies. (Soule, 2010)
Even the mere presence of predators can influence behaviour and resource use of prey as they need to satisfy their vital requirements (food, water, shelter, mate) while avoiding to be killed by a predator. The fitness and reproduction of prey species can be suppressed through this ecology of fear. (Berger, 2008)
However, Australian fauna are naïve to the novel hunting behaviour of introduced species and ringtail possums would first have to learn fear of those predators they have not evolved with.
According to new research naïveté will be reduced through adaptation in the course of long coexistence. Bandicoots recognised dogs as predators in areas with dingos but not on dingo-free Tasmania.
While 4000 years of coexistence between bandicoots and dingos meant enough exposure, 200 years with dogs were clearly not. (Frank et al, 2016) Even though this is good news in principle I am doubtful that our already critically endangered western ringtail possum will have adequate time to adapt.
Predation is a habitat feature and manipulating the habitat to make it safer for wildlife and more uninviting for predators might be the most sensible way to go. However, we destroy more, fragment more, simplify more – and all this directly benefits predators and leads to an increase in predation, which we then try to fight with quite unsatisfying results.
As we cannot effectively control cats and foxes, we could however make hunting more difficult for them by increasing the quantity and quality of shelter and refugia. Complex habitat with dense vegetation, continuous canopy, protective understory including fallen trees and extensive burrow systems benefit wildlife and hinder predators, while walking trails and recently burnt areas with reduced vegetation cover provide particularly good hunting grounds.
Low-intensity fires leaving unburnt patches and intensive predator control directly after the burn could reduce the predation-related impacts of fire.
Our modern fire regimes have caused serious declines in fire-sensitive vegetation types such as those plants that are toxic to introduced species but to which co-evolved species have high tolerance.
Open homogenous habitat structures that are common along roads and creeks and at the edges of a habitat area increase the vulnerability of prey. (Lazenby et al, 2014)
By opening up areas through controlled burns and firebreaks or by putting in paths and trails (mountain bike trails are becoming extremely popular) we always cause loss of vegetation cover and make life easier for cats and foxes. Predation is reportedly always lower where vegetation density is higher. However, the type of vegetation seems clearly important too.
In areas of remnant vegetation, high fox control has a much better efficiency than in non-remnant vegetation, even if the non-remnant vegetation is within a 5 km radius of the habitat. (Wayne et al, 2006)
Predation impacts particularly on recruitment, young animals and those moving between habitat patches.
Any control programs should also be strategically timed for best results. Winter is the breeding season for foxes and a time when food is scarce and competition between foxes and cats is potentially high. Cats are supposedly easier to catch in late autumn and winter when young adults disperse and energy needs are high. (Molsher, 2001)
Autumn/winter is the time with the highest western ringtail mortality rates and predation is an important reason.
However, the influence of predators on any ecosystem is not only negative. Western ringtails could potentially destroy their own habitat through overgrazing if there was no predation threat – particularly in fenced sanctuaries.
In a limited but still adequate habitat supply, natural predators such as chuditch, pythons and raptors would however be enough to keep a balance without the purely destructive force by introduced predators.
