Mai Po - the WWF-managed Hong Kong wetland where huge flocks of birds spend winter or stop to feed and rest en route from the Arctic to Australia - has been invaded.

Mosquitofish and tilapia dominate its ponds and apple snails chomp through its beautiful water lilies but, according to Michael Lau, WWF Hong Kong's assistant director of conservation, the biggest problem is with marauding flora.

"Plants are vitally important because they're at the base of the food web and shape the whole environment," he says.

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Mikania - which flourishes in many other areas of Hong Kong - is a subtropical American weed dubbed "mile-a-minute" because it grows so fast. With its heart-shaped leaves and tiny, fragrant white flowers, it does not look like a killer, but Mikania sprawls across plants and trees, including mangroves, using them as a climbing frame to gain access to sunlight, suffocating them in the process.

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"We uproot the younger Mikania and cut back the older, tougher plants, which cannot be pulled out," says Lau. A similar battle is being waged against Sonneratia, an invasive mangrove tree.

"It is taking over tidal mudflats along the seaward side of the mangrove belt all the way round Deep Bay, where wading birds such as plovers and sandpipers feed on worms and crabs," says Lau. "The [Agriculture, Fisheries and Conservation Department] hires contractors to remove it every year, but it always comes back."