"Compared to most of Hong Kong's native ants, they are better at discovering and defending food resources," says Dr Benoit Guénard, assistant professor and ant expert at the University of Hong Kong's School of Biological Sciences. "They thrive on open land where the vegetation has been cleared and native ants struggle to survive."

Each nest, a complex series of tunnels topped by a dome-shaped mound, is home to multiple queens.

"Most ants keep one queen in each nest - if she dies, the nest dies. With multiple queens, the death of one does not have a big impact." And the fire ants are unusually cooperative, even by ant standards. "With most ant species, each nest is an independent colony and the ants from different nests compete with each other. With fire ants, multiple nests form a single colony and all the individuals cooperate. This makes them much stronger - they can control a larger territory and exploit it more efficiently."

Red fire ants feed on plant seeds, insects and small animals. They also farm their own food by tending colonies of scale insects and aphids, which produce honeydew - a sweet syrup that the ants harvest.

"It's a bit like humans farming cows for milk," says Guénard.

Extremely resilient, they can cope with both drought and flooding. If water levels surge, the ants form a raft with queens on the inside and worker ants on the exterior. The raft floats until it hits a tree where the ants disembark and wait until the water subsides.

"We don't know how they got to Hong Kong," says Guénard, "but we think they arrived at least 10 years ago, possibly hidden in a potted plant. Many ants move around the world that way."

Once established, the ants expand their territory by breeding. After mating, a queen ant leaves her nest and flies or walks to a new patch of ground. She tears off her wings, digs herself underground and lays eggs that will hatch into tiny worker ants. The workers build the nest and bring food to the queen, which will spend the rest of her life in the nest laying eggs.

In February last year, Sai Kung district councillor Chan Kai-wai started receiving complaints that residents in his constituency were being stung by ants. He reported the problem to a number of government departments but, frustrated by the lack of action, set out to investigate the problem himself. He counted more than 1,000 fire ant nests covering an area of wasteland, traversed by paths and planted beds, that stretches from Ocean Shores, a Tseung Kwan O complex that is home to 27,000 people, including Chan and Lee, to Park Central, another massive residential development, a 10-minute walk away.

"Local residents use this area to relax, walk their dogs, jog and play with their children," says Chan.

He discovered that responsibility for getting rid of the ants lies with whichever government department manages the land. One six-metre-wide strip falls under the auspices of three departments.

"The planted flowerbed on the left is the responsibility of the Leisure and Cultural Services Department, the path in the middle is managed by the Food and Environmental Hygiene Department, and the wasteland on the right is the Lands Department," says Chan. Ants found by a road are dealt with by the Highways Department; those in drains by the Drainage Services Department; those on private land are the responsibility of the landowners.

The nests are treated with insecticide. The problem, says Chan, is that "each department treats their area at a different time". Only a fraction of the nests are destroyed.

"Last year, the various departments spent a combined total of HK$2 million," says Chan. "Not only did they fail, but the infested area has expanded."

In response to inquiries made for this article, a spokesman for the Agriculture, Fisheries and Conservation Department (AFCD), which is responsible for "enforcing regulations on plants, pesticides and animal controls", says that "departments concerned will also cooperate [to eradicate imported red fire ants] when the situation warrants".

"I first requested coordinated action last summer," says Chan. "Our situation is urgent - the ants' nests are now only 10 metres from the Ocean Shores buildings."

The dramatic consequences of a full-scale fire ant invasion are well documented in the United States. Fire ants arrived in Mobile, Alabama around 1940. Since then they have colonised 13 southern states. They damage agricultural crops and irrigation systems, and kill large numbers of lizards, rodents, birds, livestock and domestic animals.

"They are tiny compared to some of their prey," says Guénard, "but they attack in large numbers so they are able to overwhelm them."

The ants are attracted to electrical equipment and infest air-conditioning units, traffic signal boxes, telephone junctions, airport landing lights, computers and the electrical systems of cars. They chew on the insulation and carry in soil, causing short circuits.

"We have limited knowledge of their impact in Hong Kong," says Guénard, "but there is no reason to think they will behave differently here."

The Frenchman plans to launch a major, government-funded study of the red fire ant problem in Hong Kong.

"We think they are in the expansion phase at the moment. The climate suits them and, as far as we know, they have no natural predators here. As well as Tseung Kwan O, they have been spotted at the HKU campus, in the Lantau highlands, in the Deep Bay area, near Shenzhen and in southern mainland China. And the more we look, the more we will find."

Will Hong Kong ever be free of fire ants? Guénard is not optimistic.

"It's possible to control expansion, but it is impossible to eradicate them. They haven't managed to in the United States or Australia, despite having made huge efforts. We detect them only when the population is already quite large, and by then it's too late."

While fire ants colonise the land, a trio of foreign invaders dominates many of Hong Kong's rivers and streams.

The mosquitofish was brought to Hong Kong in the 1940s, to eat mosquito larvae.

"It's a nasty little customer," says David Dudgeon, chair professor of ecology and biodiversity at HKU. "The problem is that as well as mosquitoes, it eats everything else it can get its teeth into. It harasses the native fish, even much larger ones, nipping their fins and biting their scales. And it destroys frog and toad populations by eating their eggs and tadpoles. It's extremely aggressive and as tough as old boots."

Tilapia were introduced in the '50s to fish farms. Escapees invaded Hong Kong's rivers, and thrived alongside the mosquitofish.

"Both mosquitofish and tilapia use strategies that give them an advantage over local species," says Dudgeon. "Mosquitofish don't lay eggs. Instead, the eggs develop inside the female and she gives birth to live young, which are fairly well-developed and able to look after themselves. This gives them a big edge over egg-laying fish because eggs are very vulnerable to being eaten. Female tilapia keep their eggs safe by brooding them in their mouths. A single pregnant mosquitofish, or one female tilapia with a mouthful of eggs, can give rise to a whole new colony.

"Our native fish can't compete."

Dudgeon says the majority of Hong Kong's streams are now dominated by these two fish.

"They cope much better than native species in rivers that are polluted or have been channelised. If the natives are still hanging on when they arrive, the invasive fish will be the final nail in the coffin. However, the invasive species are less successful in healthy habitats - if you can find an unpolluted hill stream, you'll see more local resistance."

The third water-dwelling terror is the apple snail. Despite its cute name, it is a serious pest.

"Apple snails are like the superheroes of the snail world because they're indestructible," says Dudgeon. "They've got very thick shells and none of our native animals can eat them. They're equipped with gills and lungs, so they can breathe both in and out of water. And unlike other water snails, apple snails lay their eggs on plants above the water, so they are safe from hungry fish."

The snails have voracious appetites: "They eat plants, native snails and even the eggs and embryos of frogs and toads. They feed in water by day, but can come onto land to munch the surrounding plants at night. A wetland might start off with a rich mix of species, but soon after apple snails get busy, all that is left is an empty pond - with just some large apple snails in it. Plus the loss of plants allows algae to bloom, so the water turns green."

The only way to get rid of the snails is to collect them, and their bright pink eggs, by hand - a task that is impossible on a large scale.

Dudgeon has examined lowland streams and wetlands in the New Territories, finding some that are home to "mosquitofish, tilapia, apple snails, and nothing else . It's like a wave of barbarian invaders has arrived".

And there could be more on their way.

"Currently, anyone can import any animal into Hong Kong unless it's an endangered species," says Dudgeon. "There's no ban on invasive species, even though they are known to be high risk, and could cause severe damage to the environment."

He points to the case of the American red-eared slider turtle. It features on the IUCN's 100 worst invasive species list, yet thousands are imported into Hong Kong every year through the pet trade.

"You can see them for sale all along Tung Choi Street, in Mong Kok. They're usually sold as babies, when they're around the size of a HK$2 coin. But they can live for 20 years and grow to the size of a dinner plate. They become difficult to feed, they bite, they're prone to fungal infections, they can carry salmonella and they're very dirty. So people tire of them and chuck them in reservoirs and ponds. The problem is they're aggressive predators and will eat any animals they can get in their mouths."

Pet owners are not the only culprits - red-eared slider turtles are a popular choice for merit release, in which Buddhists buy animals and release them into the wild, in a bid to generate good karma.

"I'd like to see all releases of non-native species banned," says Dudgeon, "and a proper public education programme put in place." Countries including Australia and Singapore have adopted more stringent measures to keep invasive species out. "They issue a 'red list' of species that cannot be brought in, and a 'green list' of species which are known to be safe. Only those species can be brought in."

Guénard has identified two potential threats "knocking at Hong Kong's door". The Argentine ant - which can build supercolonies that stretch for thousands of kilometres - has reached southern Japan and "could arrive here at any time". The little fire ant, also known as the electric ant on account of its shocking sting, has invaded Indonesia and many islands around the Pacific Rim. It likes to sting victims on their eyeballs.

"The ants inject venom which causes cataracts and the animal eventually goes blind," says Guénard. Pet dogs and cats, which snuffle in the grass, are easy targets. "The little fire ant has an economic impact, too - it destroys orchards by taking over the trees, making it impossible to safely harvest the fruit."

The Hong Kong government signed up to the United Nations' Convention on Biological Diversity in 2011.

"To comply with the convention, you're supposed to control the spread of invasive species within your territory," says Dudgeon. "Clearly one of the best ways to do that would be to stop importing them."

The government is developing the Biodiversity Strategy and Action Plan to fulfil its responsibilities. Although it promises to "conduct biodiversity surveys", the plan's public consultation document, published in January, says "we do not intend to initiate major changes at this stage, such as changing existing policies and legislation".

"This does not augur well," says Dudgeon.

Introducing stricter import controls and quarantine measures costs money and interferes with free trade. The AFCD spokesman says, "… the government will handle the issue in a prudent manner. On one hand, we have to assess the impact on the local ecology and the effectiveness of the existing control measures. On the other hand, we have to consider the potential impact of introducing other legislative regulations on the society and the economy, as well as the relevant requirements of international trade."

Dudgeon and Guénard are convinced this is the wrong approach and that if the government does not invest in preventative measures, it risks being forced to spend more in the long run, to keep invasive species under control. They also argue that we have a duty to limit the risk of exporting invasive species to other parts of the world.

"It's not just a matter of money," says Dudgeon. "They say extinction is forever, but introduction is, too. If we want to see diverse wildlife around the world, with distinctive sets of plants and animals in different countries, then we have to prevent the competitively dominant species - the fire ants and apple snails - being introduced and wiping out the native species. Business has already become homogenised. Go to a shopping mall almost anywhere in the world and you'll see a McDonald's and a Starbucks. The same homogenisation is happening with wildlife. If invasive species take over, the natural environment becomes the same everywhere.

We need to protect the species that make Hong Kong the unique place it is, before they disappear."