Australasian Science: Australia's authority on science since 1938

Are Two Fences Better Than One?

By Kate Helmstedt

Conservation fences are very effective in allowing threatened animals to breed, but when the population grows too much, managers must decide between extending the existing fence or building a new enclosure.

Fences are a key strategy in the conservation of threatened native species. Australia has more than 37 large conservation fences enclosing 27 species of bird, marsupial and reptile in more than 35,000 ha of predator-free habitat. On the Australian mainland, many of these species can no longer be found outside chain-link and electrified wire.

These species are perfectly adapted to the Australian environment but have been driven to the brink of extinction by cats and foxes. Once they’re protected behind a fence from these predators, their numbers can increase dramatically. That’s great, but this positive outcome creates its own problems. What happens once the population overshoots the carrying capacity of the fenced area?

Fence managers often decide to build upon success and secure funding for another fence. There are two basic ways this new funding can be spent. They can either expand the existing fence or build a second enclosure.

The Arid Recovery project at Roxby Downs in South Australia used the first strategy. The Mount Rothwell biodiversity reserve outside Geelong in Victoria is applying the second option.

Both options have benefits. For a given amount of money, a fence expansion can add much more land than a whole new fence. That means more protected habitat per dollar of budget, and more protected animals.

On the other hand, an entirely new fenced enclosure creates two independent protected areas. That ensures that conservation eggs are spread across two baskets. If something terrible happens to one fence, not everything is in jeopardy.

Catastrophes like fires and floods do damage conservation fences, and the effects can be devastating. Studies show that predators constantly prowl fence perimeters, looking for a way in.

So how can we choose between one large fence and two smaller fences – Single Large or Two Small (SLOTS)? After a meeting between university scientists and conservation managers from Western Australia’s Department of Environment and Conservation (DEC), researchers from the ARC Centre of Excellence for Environmental Decisions have come up with a method for measuring the benefits of each option in the SLOTS problem.

Given a set conservation budget, this approach quantitatively weighs up the costs, benefits and probabilities of future events for each alternative. Based on a population model for the conservation species, this method can tell managers which approach is going to give the best bang for their conservation budget.

It turns out that either strategy could be the best, but under different conditions. If travel between the two fences is quite difficult or expensive, a single fence is often a better option. However, with increasingly frequent environmental catastrophes it becomes more important to spread the risks between two fences.

The SLOTS method was applied to the conservation fence in Operation Rangelands Reconstruction, based at the Lorna Glen property in central Western Australia. The current fence at Lorna Glen is home to three locally-extinct marsupial species, and wildfires, floods or cat incursions could jeopardise these populations at any time. Given a new injection of conservation funds, managers could either expand their existing 543 ha fence or build a second enclosure elsewhere on the property.

The SLOTS method gives clear advice. If the managers at Lorna Glen can find a suitable location that’s closer than 60 km from the existing fence, they should build a second fence. It turns out that as long as the fences are separated by a fire buffer, there is some risk-spreading benefit – any spatial catastrophe that happens to the original fence (e.g. a wildfire) won’t necessarily damage the second fence as well.

Fences that are more than 60 km apart cost too much money to manage over the long run. If no suitable locations exist closer than 60 km, managers should enlarge their existing fence.

Of course, these results are specific to the situation at Lorna Glen – the fence they’re building, the species they’re protecting and the threats they’re trying to manage. However, the SLOTS approach can be applied to any conservation fencing project, and it turns out that making the right decision could save fencing projects a lot of money. The required investment per animal per year can be reduced by more than one-third if SLOTS is included in the decision-making process, resulting in savings of millions of dollars over the lifetime of a fence.

Kate Helmstedt is a researcher with the Environmental Decisions Group. She is based at the University of Queensland.