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Public Radio's Environmental News Magazine (follow us on Google News)

Not-So-Awesome Possums

Air Date: Week of

The Brushtail Possum.

In celebration of Earth Month, Living on Earth dips into our archives and revisits some of our favorite stories. In 2001, Alan Coukell reported on the problem of possums in New Zealand. The non-native species ate tens of thousands of tons of forest each day and can spread tuberculosis to other animals. Host Steve Curwood talks with Philip Cowan from New Zealand’s Land Care Research to find out how the battle to control possums has progressed in the eight years since our story was produced.


CURWOOD: It’s Living on Earth, I’m Steve Curwood.

In the run-up to Earth Day, we’re revisiting some favorite stories from past years – and checking out what’s happened since.

Today – a report from 2001, from New Zealand. For most of its history, the New Zealand islands were isolated and remote with unique ecosystems. When human settlers arrived, some 800 years ago, they brought in new animals and plants, and often, native species had few defenses against them.

Among those new arrivals was the Australian possum, which thrived and spread nationwide, and became a threat to forests and livestock. As Allan Coukell reported, trapping and poisoning possums isn’t very effective so scientists have been seeking alternative control methods.


WAAYER: Around this time of night they'd be starting to come out of their burrows and hiding places. And they live off the nicest of foliage, which is why we're always after them.

COUKELL: It is dusk on this New Zealand farm near the town of Warkworth, about an hour north of the city of Auckland. Armed with a spotlight and 22-caliber rifle, Tom Waayer is preparing to defend his land against an alien invasion.

WAAYER: Here's our first one, see him in the tree there? See, here we go with this.


COUKELL: I think he fell.

WAAYER: He's fallen partway.


COUKELL: The enemy he's targeting is the Australian brush-tailed possum. Over the next three hours Tom Waayer will shoot nine more possums on his small farm. These few animals no longer pose a problem, but there are still at least 70 million more possums in New Zealand. And tonight across the country, like every night, they will eat about 20,000 tons of vegetation.

COWAN: We've been looking at the interaction between possums and vegetation since the mid-1960s.

COUKELL: Phil Cowan is an ecologist at the government-owned company Land Care Research. He says that the possums have been in New Zealand for more than a century. Their environmental impact has far from stabilized.

COWAN: What we’ve found over the last 30 years is that although possum numbers have basically remained constant, what we see is continuing degradation of the forest. Possums continuing to kill preferred tree species and changing the whole composition of the forest. And that, presumably, affects the whole way that the forest ecosystem operates.


COUKELL: Possums also snack on rare snails and insects, and prey on the eggs and young of critically endangered birds, such as kiwi and kokako. But just as important as the effect of possums on native forests is the risk to agriculture. At any given time, over 600 herds of cattle and farmed deer in New Zealand are infected with bovine tuberculosis, a disease passed on by the possums. In the course of a year about one-and-a-half percent of herds is infected, eight times the accepted international standard. New Zealand spends roughly $25 million a year on TB control in livestock. Cattle and deer are inspected, herds are quarantined, and infected animals killed. But fully half the money spent goes to trapping and poisoning possums. Morgan Williams is New Zealand's Parliamentary Commissioner for the Environment. He says getting rid of possums with these methods is nearly impossible.

WILLIAMS: Possums in many parts of New Zealand are in very rugged country, so it's not a matter of wandering around easily and putting out a little bit of bait or bit of trapping. And you've got to keep going back every few years because you never get the last possum. And with the amount of food that's in our forests, possum populations recover again quite rapidly.

COUKELL: Ecologist Phil Cowan agrees that the current methods of control are inadequate. He says what's needed is a cheaper and more effective way to eradicate the animals.

COWAN: We need to look for new technologies so we can do things much more cost-effectively and do possum control, not just in the highest priority areas, but over the whole of New Zealand, if at all possible. So what we're working towards is developing some kind of biological control that, together with conventional control, we can use to achieve that goal. What we're trying to develop in effect is a form of contraception for possums.


ECKERY: This is our possum breeding facility. We've made quite an effort in the first years of our research into possums and gaining a basic understanding of possum biology.

COUKELL: Doug Eckery is a reproductive physiologist at AgResearch, another government organization. He spends his time figuring out how to manipulate the reproductive activities of the marsupials. It's part of the overall goal of a vaccine that will cause the possums to lead happy, but childless, lives.


ECKERY: So, these are sort of the spoiled possums here. We do monitor the reproductive activity from them. That entails just taking a urine sample from them every day, but they're on a reward system, so when they give their urine sample they get a little piece of bread with jam on it. So most of them are more interested in getting their jam sandwich than worrying about what we're up to.


COUKELL: The aim of the research is to sterilize as many possums as possible. So scientists are enlisting the animal's own immune system using proteins from possum sperm and egg to create a vaccine that will prevent fertilization. Initially, the plan is to introduce the vaccine in the form of genetically modified carrots. But such a bait-delivered vaccine will still only reach a small percentage of the entire possum population. So Joanne Meers, a virologist at Massey University, is working to find a virus that could be used to spread sterility from one possum to another.

MEERS: Viruses have an advantage over other delivery systems, in that it gives two chances or two hits at being specific for possums. Not only have we got a virus that will only infect possums, but we also have a bit of - the protein is also specific for possums. So we have two prongs in the attack of being specific for possums.

COUKELL: Developing an infectious agent that spreads sterility requires a great deal of caution. The scientists will have to proceed carefully to ensure that the virus affects only possums and not humans or other animals.

Scientists predict that a one-third reduction in possum numbers will safeguard New Zealand agriculture. But they also know they will have to do much, much better than that if the forests are to recover and birds such as the kokako are to sing again. For Living on Earth I'm Alan Coukell in Auckland, New Zealand.


CURWOOD: And listening with me now to Allan Coukell’s report is ecologist Phil Cowan from New Zealand’s Land Care Research. And Phil, you’re still on the possum control beat – what kind of success have you been having?

COWAN: I suppose it’s mixed in some places and really good in others. In terms of the TB situation, the success has been really good. When we last discussed this I think New Zealand has about 600 infected herds while the latest figures are less than 140, so that’s really positive. In terms of impacts on native biodiversity and native forests, progress has been more mixed. But that’s partly because possums are just part of the problem. A number of endangered species are also threatened by some of the introduced predators, particularly rats and stoats and weasels. And so control has now switched from a single focus on possums to a focus on a suite of species that are all impacting on forests and on native birds.

CURWOOD: By the way, how is the kokako doing?

COWAN: Kokako is doing great. Kokako populations have been increasing steadily over the last five years, and that’s largely because of the continued pressure of pest control in keeping rats and possums down.

The Australian Brushtail Possum.

CURWOOD: You know one part of this report looked at efforts to try to figure out some possum birth control. How well did any of those methods pan out?

COWAN: We’ve continued with the research on fertility control, and we’ve been able to show that we can get infertility if we inject the possum egg proteins, for example. The issue has been how can we deliver those to the supposed seventeen million possums. And so we’ve been looking at trying to develop a method of delivering these orally in a bait. So that’s a real challenge. In terms of the virus work which was aimed at producing a method of biological control that would transmit naturally from possum to possum – the virus work got put on the back burner, but we have continued similar work, but this time looking at using a possum specific parasite, a worm that lives in the possum’s intestines, as a means of transmitting fertility control from possum to possum. While we’ve made significant progress in the lab, we’re still a long, long way from any idea of being able to release that into the field. And there would be a very long process involving a lot of public consultation before something like that could ever be released for practical use.

CURWOOD: By the way, how did possums wind up in New Zealand anyway?

COWAN: They were brought to New Zealand in the 1850s to set up a trade in the fur. At that time Australia used to export a very large number of pelts from possums to the Hudson Bay Company for auction on the world fur market. And so this was seen as another way in which the early colonists of New Zealand could increase their income.

CURWOOD: So what about bringing back the possum coat? Maybe that would get you some help from traders and entrepreneurs.

COWAN: Yeah, we – New Zealand stills harvests probably a million possum skins a year which get made into a variety of articles, but the big change in that industry has been that rather than using the skins so much these days, what people do is they pluck the fur from the possums once they’re dead and they blend that fur with merino wool and make very high quality garments like sweaters. Possum’s hair fibers are very light and silky and they have very good insulting properties and they make really high quality garments.

CURWOOD: Ah, so move over the cashmere goat, huh?

COWAN: That’s right.

CURWOOD: Phil Cowan is head of the Science and Pest Control Technologies team and New Zealand’s Land Care Research. Thanks so much.

COWAN: Okay. Thanks, Steve.



Introduced Birds and Mammals in New Zealand and Their Effect on the Environment, 1984

For more on the Australian possum, click here.


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