CURWOOD: It's Living on Earth. I'm Steve Curwood. For most of its history, New Zealand's ecosystems evolved in isolation in the South Pacific, cut off from the rest of the world. Then, less than a thousand years ago, humans arrived, bringing creatures against which the native birds and forests had few defenses. Among them, the Australian possum. It was brought to New Zealand in the 1800s by settlers hoping to establish a fur trade. The market collapsed in the 1980s but the possum population has grown, and now the marsupials occupy almost all of the island nation. As Allan Coukell reports, trapping and poisoning have failed to control their numbers, so scientists are looking at alternatives.
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. (Shoots rifle) He's fallen partway. (Shoots)
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 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.
(Possum calls; a door shuts)
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. Even so, getting the public to accept the technology may not be easy. Parliamentary Commissioner Morgan Williams has been consulting with groups of ordinary New Zealanders, and asking them for their thoughts on possum control. He says they recognize the problem but they also have concerns.
WILLIAMS: New Zealanders, no matter where they are and whether they're deeply urban or deeply rural, have a pretty clear understanding about how big a threat possums are. When it came to focusing on delivery mechanisms, mechanisms that would only target the possum and prevent spread into the wider environment were the ones that were favored. So some form of bait delivery, using a genetically-modified carrot material, something like that, was the most favored option. Genetically modifying something like a virus was certainly not favored.
COUKELL: Farmer Tom Waayer shares the concerns, but he also recognizes that for effective possum control nationwide, there may be no other choice.
WEIR: There's a lot of worry with genetic engineering, whether or not it is safe. And sort of tests done every few years, whether or not that's adequate. I think in controlling the possums in New Zealand, that's about the only way to do it.
COUKELL: 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.
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CURWOOD: It's Living on Earth. I'm Steve Curwood. And this is NPR, National Public Radio. Just ahead: When 100,000 farm-raised salmon escaped into the Gulf of Maine, hungry seals were happy, but federal regulators were not. Stay tuned to Living on Earth.
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