A 26-year study at the Arid Recovery Reserve reveals how removing invasive predators like cats and foxes triggers a dramatic reshaping of desert small mammal communities.
The removal of invasive feral animals like cats and foxes from a fenced reserve in South Australia has led to an astonishing ecological transformation, a study led by UNSW conservation scientists has found.
The study, published today in Proceedings of the Royal Society B , tracked the abundance and demography of 10 small mammal species across a 26-year period inside and outside the Arid Recovery Reserve - a 12,300 hectare reserve featuring six fenced enclosures that is Australia's largest feral-proof reserve.
Following the removal of rabbits, cats, and foxes almost three decades ago, the researchers observed a clear pattern of succession - a process where species gradually replace one another over time as ecological conditions change. Smaller rodents such as Bolam's mouse and the house mouse were the first to make the most of the new feral-free conditions, while larger species like the spinifex hopping mouse and plains mouse surged after five to nine years.
Professor Katherine Moseby , who was lead author on the paper, said nine of the 10 small mammals were natives, while one - the house mouse - had arrived along with European invasion.
"These are 10 small mammals occurring naturally in the ecosystem, so not reintroduced as part of any translocation project," she said." These include native rodents like the plains mouse, and small carnivorous marsupials like the charismatic stripe-faced dunnart."
"Over the time that we monitored, some of the small mammals inside the reserve - like the spinifex hopping mouse - increased dramatically with populations up to 33 times higher inside than outside the feral-free areas."
The largest differences between inside and outside the reserve occurred after big rainfall events suggesting that feral cats and foxes suppress small mammal population booms which has implications for recovery of threatened species.
"During droughts small mammal populations decline and shrink back to refuge areas. Breeding and population growth after rain allows them to repopulate those areas but predation from cats and foxes is suppressing this and leading to local extinction."
Surprise finding
Prof. Moseby said small mammal succession is often seen after fires or other events that remove the vegetation. Then as the vegetation starts growing back, it favours different small mammals at different times. For example, the grass returning might favour one species, then when the trees rebound, it favours others that outcompete the smaller ones.
"But we found a succession response just from removing cats and foxes, without having any sort of major vegetation change," she said.
"It occurred due to changes in competition and differences in how quickly species returned, with larger rodents taking longer to re-establish but then outcompeting smaller rodents. It was fascinating to see how dynamic this ecosystem is, and it continues to change 26 years later."
We should be thinking more broadly about the habitats we conserve to protect native small mammals once we've controlled predators - to understand they've much got a much larger niche than where we observe them now.
Snapshot of an earlier time
It wasn't just the flourishing of small mammal populations that impressed the conservation scientists. They noticed that when some of the larger rodents eventually outcompeted the smaller ones, they moved into habitats they weren't typically seen before - a shift that opens up new possibilities for conservation strategies.
"We saw this massive increase in abundance once we got rid of the cats and foxes," Prof. Moseby said.
"But it wasn't just about the numbers. We also had animals expanding into different habitats you don't normally find them in, giving us a bit of an understanding of how these ecosystems might have looked before the European invasion. It appears many of these animals had much larger niches than they do now."
For example, the spinifex hopping mouse has a preferred habitat of sand dunes. But within the feral-free enclosure, they expanded into swales, or clay-based lowlands. Likewise, the plains mouse moved in the opposite direction: known to prefer a swale habitat, they moved into sand dune areas of the reserve.
"Cats and foxes have not only caused significant declines in many mammal species but also changed the habitats they use. We should be thinking more broadly about the habitats we conserve to protect native small mammals once we've controlled predators - to understand they've much got a much larger niche than where we observe them now," said Prof. Moseby.
Feral-free future
The researchers will continue to monitor the population numbers of the small mammals inside and outside the fenced-off reserves. Prof. Moseby is hoping the data from this research will inform future conservation efforts of other threatened species as well as draw attention to the fact that feral predators affect not only the abundance of native animals but also the way they use their habitat and how they interact with other species.
"Our desert ecosystems have changed so much since Europeans arrived and brought cats and foxes with them. We studied the 10 small mammals that are still present at our desert study site but unfortunately 60% of the mammal species formerly present here disappeared after European arrival. We need to work hard to restore these desert ecosystems and reduce the impact of feral species."
