It was only then he discovered the rabbits had been keeping down kikuyu grass which grew over penguin's burrows, and tangled up the little birds with long runners, preventing them from reaching their burrows to feed their chicks.

"Ever since we have had to do kikuyu control," says Dr Braysher, a retired natural resources manager and adjunct professor at the University of Canberra.

The rabbits, feral pigs eating discarded bananas under planation trees and Australia's export of feral goats, which has created the biggest market of its kind in the world, remind him of the value of ferals and the futility of trying to eradicate them.

"I find it ironical a lot of the animals which are pests are also very valuable resources, like feral pigs, feral horses, camels, all brought in because we wanted to use them," Dr Braysher said.

"Then some escaped, so the ones that escaped are not wanted, people reckon they are feral. But they want to keep the pigs and goats and deer and horses which are in captivity."

A member of the Jerrabomberra Wetlands Nature Reserve management committee Dr Braysher believes Australia and the world could save much money abandoning expensive, fruitless attempts at killing feral animals.

"What we have done of course is modified the environment for those animals we bought in, for sheep, cattle and goats, so they will do well, and we get surprised when they escape and do well in the modified environment.

"One of the reasons pests do well is because they are highly adaptive, most breed very well and are quite happy to live in human-modified environments. The opposite is true for our natives, except for things like kangaroos, maybe galahs who like grain crops," he said.

Feral cats, Indian mynas, wild horses and carp are in environmentalists sites, but unless doing a risk assessment first and targeting a specific outcome, they are wasting money that could be spent on protecting the natural environment, says Dr Braysher.

"We only have about 2 per cent of our native grasslands, and 4 percent of yellow box redgum woodland left and there is no wonder those native animals have disappeared," he said.

"It is not due to cats and foxes and pigs and goats, it is due to the fact we have got rid of the habitat. People don't like to talk about that because it is very expensive to deal with those issues," he said.

At the Jerrabomberra wetlands rangers are managing feral foxes, including the one photographed, rather than trying to shoot and eradicate all of them.

Writing a textbook on best practice pest management in conjunction with the CSIRO, Dr Brayshaw cites native fish declining well before carp infested the Murray River catchment, because of weirs which stop big cod migrating.

Salinity has increased in the lower Murray River to a point that the water cannot be used for irrigation.

"Native fish can't survive but carp can survive in 50 per cent sea water," he said. In other sections of the river where snags had been left, the Murray cod, a predator who ambushes prey from the cover of snags, had flourished.