Possum pests are dropping themselves in it

Researchers have found a new way to count possums ravaging New Zealand's native bush - using the DNA "fingerprints" of individual pellets of possum droppings.

"Possums produce more than 100 small droppings a day, so you have much more chance of finding possum poo than finding possums - and possum poo doesn't try to hide from you," project manager Graham Nugent said today.

Possums slaughter native birds and destroy their habitat, as well as threaten meat and dairy exports by spreading bovine tuberculosis.

The Animal Health Board, which carries out huge culls of possums to control the spread of TB, has funded Landcare Research scientists to improve its estimates of possum numbers surviving poisoning campaigns - a key factor in subsequent population recovery.

Current estimates of survivors are based on the number of possums caught in leg-hold traps, an expensive and time-consuming method which can be distorted if possums have become trap-shy.

The Landcare Research scientists have found if they can identify individual possums through the gut DNA in possum droppings calculations base on the number of times individual possums are "caught" in two or more surveys gives an objective way to count the survivors.

Mr Nugent said this technique for counting elusive wild animals was likely to prove cheapest when the animals were scarce.

"Only a few DNA samples are needed to 'capture' some of the possums at least twice when there are only a few animals," he said. "It is the proportion of recaptures that is used to estimate possum numbers."

The work showed DNA could be extracted from droppings up to 27 days old, depending on recent rainfall levels.

Landcare Research geneticist Dr Dianne Gleeson said that though examining possum droppings was not glamorous, the results had been very exciting.

"We did not expect to have any success," she said. "But by peeling the 'skin' off the droppings with a scalpel, we could find small numbers of DNA cells that have been sloughed off the walls of the possum's gut.

"We gain enough information to obtain individual genetic profiles of possums that remain at large."

The cost of the technique compared well to field counts using traps, and would drop further as procedures were further streamlined.

Possum plague

* The possum population is estimated at between 40 million and 70 million.

* It is estimated they chew through 20,000 tonnes of native bush every three nights.

* $60 million to $80 million a year is spent on killing possums.

* Possums are the main means by which livestock are infected with bovine TB.

- NZPA