Keep out!

Climate change means alien invaders are more likely to wreak havoc on our wildlife, farms and economy. Those charged with repelling the threat say our isolation is no longer enough.

- SALLY BLUNDELL

On a clear day in autumn, ecologists Alex Reid and Mark Yungnickel peered into the shallow waters of Bob's Landing on Lake Karapiro. They were doing an assessment of the lakeshore and surrounding wetlands when they noticed, in about 10cm of water, the white underside of a dozen unusual-looking ribbed shellfish.

"It didn't look like anything we knew," says Reid. "Seeing them scattered over the substrate, we knew immediately we were looking at something strange."

Grabbing some waders, they fished a couple of the molluscs out and took photos. These were sent to the University of Canterbury, then to the Australian Museum in Sydney. On returning to the site, the two men found, camouflaged in the yellow-brown gravelly sand, a scattering of the shellfish, living and dead. A sample was sent to Biosecurity New Zealand, which sent it on to Te Papa, where staff identified it as the highly invasive gold clam Corbicula fluminea, also known as the Asian clam.

Should we be worried? The clams are prolific breeders, able to produce 400 juveniles a day and up to 70,000 a year. They have the potential to destroy native habitats and clog water supplies and hydro dams. In Europe and North and South America, says Yungnickel, they have resisted eradication efforts.

Surveys reveal the two-to three-year-old population of clams stretches one kilometre upstream and 45km downstream from where they were found - so far. Mercury Energy has reported about a dozen clams in one of its water-intake pipes at Karapiro Power Station - not enough to clog the intakefor now. How did they get here? No one knows, as yet.

So far. As yet. For now. For an island country reliant on primary produce exports and tourism, with nearly 80% of our plants endemic, these phrases ought to send a shiver down our collective spine.