Linking to the recently-published article, Natasha Harrison discusses whether native predators inside ‘predator-free’ havens could be good for the conservation of threatened native prey species.
Predator-free havens
Since their introduction into the Australian landscape following European settlement, feral cats and red foxes have caused prolific population declines among our native fauna. The extinction of Australian mammals makes up more than two thirds of contemporary global mammal extinctions, and predation by foxes and cats is implicated in the majority of these. To protect native mammal populations from relentless predation by these invasive species, conservation managers are turning to predator-free havens – offshore islands or fenced reserves free from invasive predators.
These havens are incredibly effective at insuring populations from threatening processes such as predation, but we are finding that in isolation from predators, havened populations are losing their anti-predator responses (the ability to detect and respond to predators, to avoid being eaten). The loss of these traits is likely to mean that havened populations are unsuitable for reintroduction back into the wild where predators still persist, permanently confining them to the haven.
Anti-predator traits
The loss of these critical traits could be evolutionary – traits that are advantageous in avoiding predators, like vigilance, may disadvantage individuals when competing for limited resources. For example, animals constantly looking over their shoulder for non-existent predators will be last to secure food, mates, and shelter. Over time, vigilance and other anti-predator traits can be selected against because those who allocate more time to these behaviours will have lower survival and reproduction compared to more bold, risk-taking individuals.
The loss of anti-predator traits from havened populations could also be from the loss of learned behaviours as without predators, individuals lack the experience necessary to develop and maintain such responses. In either case, theory predicts that imposing predation pressure inside havens should provide both favourable selective pressures and the appropriate experience necessary to maintain anti-predator traits in our havened populations.
The study
Given that cats and foxes pose such a threat to our native mammals, we wondered whether we could apply a less intense predation pressure from native predators instead. Native predators, such as the chuditch, a small native marsupial carnivore, are smaller (requiring less prey) and only predate animals for food compared to foxes and cats that are known to hunt for fun. We measured anti-predators responses in a havened population of woylies (a small macropod species) that had coexisted with chuditch for 8 years, and compared these to a wild population of woylies.
We explored a variety of traits, including agitation behaviour during handling by humans (assuming those who are more agitated are more motivated to escape from a ‘predator’) and the length of their hind-feet (individuals with bigger feet can take longer leaps and run faster when escaping predators). Where other havened populations isolated from all predators have shown stark differences in these anti-predator traits compared with wild populations, here we detected no differences in anti-predator behaviour and morphology between our wild population, and the havened population with chuditch.
These findings suggest that the presence of chuditch in this haven has maintained anti-predator traits in woylies. While this result is promising, it doesn’t necessarily mean that these woylies will survive in the wild – we are making the assumption here that exposure to chuditch has kept them savvy enough to avoid foxes and cats, even though they have never encountered these predators before.
To test this important assumption, we took woylies from the chuditch-exposed haven, and wild woylies, and released them into an area of suitable bushland where foxes, cats, and chuditch persist. A number of woylies were fitted with VHF radio-collars which allowed us to track them and follow their survival. After 10 months in their new home, we found no differences in the survival rate of the havened and non-havened woylies.
This suggests that the presence of chuditch in the haven has maintained important anti-predator traits in our woylie population, keeping these individuals suitable for release into the wild. This exciting finding provides important evidence to inform the management of havened populations, ensuring that we may reintroduce woylies and other threatened mammals back into areas of their former range, to improve the cultural and ecological health of our landscapes.
Read the full article “Dancing with the devil: Could native predators inside ‘predator-free’ havens be good for the conservation of threatened native prey species?” in Journal of Applied Ecology.
