Cats, rats and foxes have wrought havoc on Australian wildlife and ecosystems. Known as “invasive mammalian predators”, these are species that have established populations outside their native range.
Responsible for numerous extinctions across the globe, this group of species also includes American mink in Europe, stoats and ferrets in New Zealand, and mongooses on many islands.
One common solution is to kill these predators. However, research published this week in the journal Biological Conservation shows it’s much more complicated than that. Killing invasive predators often doesn’t work and is sometimes actually worse for native wildlife.
Killing for conservation
Management of the threats to biodiversity posed by invasive predators has focused on reducing their populations using lethal control. This includes poison baiting, trapping and shooting.
Such management programs often occur with little regard for how they might interact with other threats that are impacting ecosystems. This has led to unpredictable outcomes of invasive predator control. Sometimes it doesn’t work or, worse, it results in a negative outcome for wildlife.
We identified six disturbances with strong potential to increase the impacts of invasive predators: fire, grazing by large herbivores, land clearing, altered prey populations, the decline of top predators and resource subsidies from humans (such as increased food or shelter availability).
These disturbances interact with invasive predators in three main ways.
First, disturbances such as fire, grazing and land clearing result in a loss of vegetation cover, which makes prey more vulnerable to predation.
For example, small mammals in the Kimberley region of northern Western Australia experienced more predation by feral cats in an intensely burnt area, compared with patchily burnt and unburnt areas. Grazing by livestock similarly removes protective cover. Research shows that feral cats prefer to hunt in these areas because of the improved hunting success.
Second, increases in food or declines of competing top predators can allow populations of invasive predators to increase, thereby increasing their impact on native species.
For example, introduced prey species, such as rabbits in Australia, can support larger predator populations. This can lead to increased predation pressure on native species – a process termed “hyperpredation”.
The extinction of the Macquarie Island parakeet was attributed to this process. The parakeet co-existed with feral cats for more than 60 years, but declined rapidly to extinction following the introduction of rabbits to the island in 1879. Resource subsidies, such as garbage or hunters’ carcass dumps, can also support larger predator populations, leading to greater predation pressure.
Third, many of these disturbances also have a direct impact on native species, which is exacerbated by invasive predators. For example, habitat fragmentation reduces population sizes of many native species due to habitat loss. Increased predation by invasive predators can therefore make a bad situation much worse.
Getting it right
Our synthesis shows that management of invasive predators is likely to benefit from employing more integrated approaches.
Maintaining habitat complexity and refuges for prey species is one way that invasive predator impacts can be reduced. This includes improved management of fire and grazing. Lower-intensity fires that retain patchiness could reduce the predation-related impacts of fire on native species. Such approaches may be the best option where no effective predator control method exists, such as for cats in northern Australia.
Native top predators such as wolves in Europe and North America or dingoes in Australia can have suppressive effects on invasive predators. “Rewilding” is an option in some places where these species have declined. Where native predators conflict with livestock producers, guardian animals can often protect livestock from predation instead of lethal control.
Reducing resource subsidies is a simple way of reducing food resources for invasive predator populations.
If lethal control is used, it should be applied with caution. Selectively removing individual pest species from ecosystems can do more harm than good. Multi-species approaches are the best way to avoid such surprises and the order in which species are removed is an important consideration.
Rather than focusing on single processes, conservation managers should consider the multiple disturbances operating in stressed ecosystems and use management actions that address these threats in unison. Such integrated approaches are essential if further extinctions are to be avoided.
The paper is free to download until July 30 2015.
Tim Doherty is PhD Candidate at Edith Cowan University.
Chris Dickman is Professor in Terrestrial Ecology at University of Sydney.
Dale Nimmo is Lecturer in Ecology at Charles Sturt University.
Euan Ritchie is Senior Lecturer in Ecology, Centre for Integrative Ecology, School of Life & Environmental Sciences at Deakin University.