Quick Dips

It's True -- Killing Possums is a Good Thing

Landcare Research has produced some of the first documented evidence that reducing possum numbers can generate spectacular benefits for native forests -- evidence that adds to our understanding of how much possum control is needed.

It is well known that the estimated 70 million possums in New Zealand defoliate native forests, and diminish native bird numbers. But although there is ample anecdotal evidence that reducing possum numbers helps habitats bounce back, there is little published data.

Landcare Research scientists are studying forest areas at Motatau, near Whangarei in Northland, and Waihaha, west of Lake Taupo. At Motatau, the broadleaved native tree, kohekohe, is a major food source for possums. A vertebrate pest ecologist, Graham Nugent says in 1997, half of the 50 trees surveyed were almost totally defoliated. But by 1999, after possum number had been knocked back by more than 90%, only one tree had died, and most of the rest had foliage cover in the normal range. This is regarded as a spectacular recovery.

Nugent says dramatic recovery of native mistletoe was also seen after highly successful possum control at Waihaha.

"Mistletoe foliage was almost completely absent before possum control, and we were surprised to see it re-appear and flourish," he says.

However, there was a downside.

"The recovery only persisted while possum numbers remained very low. In the five years after possum control, numbers increased to the level at which about ten percent of all traps laid caught possums. Once trapping success rose above about five percent, the mistletoe recovery reversed".

This knowledge helps us understand the level of possum control needed.

"It's easy to say we need control, but not how much control we need," says Nugent. "This varies between different tree species, and different habitats. But the evidence backs up our hunch that the rarer and more heavily impacted a species is, the more intense the level of control required will usually be".

"In plant species other than mistletoe or kohekohe, the level of recovery has not been as dramatic. But these species tend to be slower-growing species, and not nearly as heavily impacted".

Earlier work by Landcare Research shows the impact possums have on bird life. Research on kokako in the Bay of Plenty and Waikato in the late 1980s and 1990s showed that the birds could only achieve 50% nesting success or better when ship rat and possum indices were below 5%.

In more recent research at Motatau, native pigeons (kukupa/kereru) showed a dramatic increase in nesting success after possum control. In 1996 and 97, before possum control in the area, 13 pigeon nests were found, but none produced chicks. However, in 1998 and 99, after possum control, seven out of the seven nests found fledged chicks.

Nugent says the information gathered in this research is proving highly useful to the Department of Conservation, and is helping regional councils, landowners and managers to decide how much to spend on possum control.