MA XUEJING/CHINA DAILY

In many cases, the buyers have no idea of the ecological risks posed by their new "pets". If the owners fail to keep them securely locked in cages or tanks, the interlopers can break free, and a lack of predators means their populations grow more rapidly than those of native species, posing a severe threat to the ecosystem.

Not all invasive species arrive through the mail, though. In April, Fiona Sit, a well-known actress and singer from Hong Kong, provoked a storm of online criticism when she "freed", to use her own words, two Brazilian red-eared slider turtles she had bought at a food stall in a market and released them in the Bride's Pool, a stream with several waterfalls in the New Territories of Hong Kong.

When the 25-year-old posted photos of the turtles on her micro blog, she was inundated with comments from concerned netizens who pointed out the negative ecological impact the turtles would have on the local eco-system because of their lower age of maturity, higher rates of reproduction and larger body size, all of which give them competitive advantages compared with native species. The turtles are also banned in many countries because they are known to transmit a range of diseases.

"It's not uncommon for people to take Brazilian sliders to release them into lakes. They may see this as a philanthropic act, but what they don't realize is that these turtles are highly adaptable, rapidly reproducing predators," said Li Yiming, a researcher into animal ecology at the Institute of Zoology with the Chinese Academy of Sciences.

A customs officer displays ants found in test tubes imported from Germany in November. HUANG HAIPENG/CHINA DAILY

Red for danger

Southern China is also experiencing an infestation of red imported fire ants, a highly aggressive, toxic species that can destroy habitats. The insect's bite causes severe pain and skin blisters, and can even cause unconsciousness in people who are particularly susceptible to the venom.

The ants, which are native to South America, are predominantly found in southern China. They were first spotted in Taiwan about 15 years ago, and are thought to have arrived via cargo containers and soil used for ballast in ships.

They later arrived on the mainland via ships that docked at ports in the coastal province of Guangdong, and now the story is being played out again in the tourist haven of Hainan Island, China's southernmost province, where they are harming the local eco-system and posing a threat to public health.

The ants are highly aggressive and territorial. Wang Chunqiong, from Ding'an county in Hainan, has scars on her arms and legs after an encounter with the ants. "They usually hide in dry places in the crop fields, waiting for a chance to bite you when you are not looking. Almost everybody in the village has been bitten," the 65-five-year-old said.

The small, fast-moving insects sting their victims repeatedly, and are especially dangerous to the young and newborn babies.

In addition to the dangers they pose to humans and animals life, they are voracious, eating the buds and fruits of numerous crops, especially corn, soybeans, okra and citrus fruits. "They have eaten all our peanuts, but they are also destroying other crops and hampering agricultural production. We wish we could get rid of them," Wang said.

Her wish is easier said than done, though. When an alien species enter the natural environment, the creatures usually assume a place at the top of the food chain, and lacking predators, their numbers grow rapidly.

"The ecological hazards caused by invasive alien species include the eradication of certain local species, reducing biological diversity, polluting soil and water, and transmitting infectious diseases," said Li, from the CAS. "Once the population of an invasive species is established, it is very difficult to eliminate them completely. Even the most feasible methods－catching or killing them by hand－are too expensive."

Contact the writers at chengyingqi@chinadaily.com.cn and liuxiaoli@chinadaily.com.cn