Invasion of the aliens

A deadly alien invasion is quietly creeping into Singapore, and scientists plan to use DNA analysis to combat it.

Frogs have been found plagued with fungal diseases never before seen here, giant African land snails have shown up in the Botanic Gardens, and at least one hungry arowana has been found in Nee Soon's delicate freshwater swamp forests, where the fearsome predator threatens to chomp up rare native fish and crustaceans.

Singapore has a sizeable immigrant population of non-native animal, fish and plant species.

It's not clear exactly how many aliens there are but a National Parks Board list updated in 2009 threw up 67 species of such freshwater fish, birds, reptiles, amphibians and mammals. Plants, insects and marine life, however, were not counted.

Last month, about 100 researchers gathered at a symposium on climate change, aquatic invasive species and coastal restoration to discuss what has been found here, what the potential impacts are, and new ways of tracking down invaders.

National University of Singapore herpetologist or amphibian and reptile specialist David Bickford pointed to the religious practice of animal release as one way alien species enter the environment.

Last year, Assistant Professor Bickford and his colleagues found two wild frogs that tested positive for the chytrid fungus, now decimating frogs in the Americas, Australia, Africa and Europe.

The fungus can cause devastating skin infections in frogs, and the infected amphibians were found at sites where devotees perform the traditional practice of releasing animals.

Foodfish aquaculture, deliberate release and the pet trade are some other ways that non-native fish make it into Singapore's reservoirs and streams, said NUS ichthyologist or fish specialist Tan Heok Hui.

In January, researchers found a juvenile arowana, no longer than a pencil, in a forest stream at Lower Peirce. It was thought to have "swum into the swamp forest from the Upper Seletar Reservoir where it was probably born of feral parents", and was snacking on native fish.

The fish, which can be found in neighbouring countries, has a voracious appetite, lives at least 60 to 80 years, and can grow to 1.5m. Thankfully, researchers have found no arowana in the delicate Nee Soon swamp since then.

Alien species can also hitch rides on ships, plants and other imports. For instance, brown anole lizards - which are Caribbean natives - discovered in the Marina South area are thought to have arrived on plants bound for Gardens by the Bay.

Invasive species can be a major threat to native plants and animals because they often have no natural predators or competitors for food. Many are "generalists" which can survive in a wide range of climates, and are not fussy eaters.

No one has worked out the specific costs of invasive species to Singapore yet, in ecological and economic terms.

The good news is that not all non-native species necessarily do damage, and Singapore does not have a large agricultural sector that would be hard-hit by invasive plants or creatures.

Dr Lena Chan, director of the National Parks Board's National Biodiversity Centre, said some exotic plants were in fact brought in on purpose.

"For example, exotic plants such as the rain tree, brought in to green Singapore's landscape, continue to beautify our 'city in a garden', and they have not significantly affected our nature reserves."

They also provide habitats for native ferns, orchids and other species, she said.

However, Dr Chan noted that invasive species can compete with native ones for food and resources, and alter ecosystems.

What's more, there could be health and other impacts, pointed out Assistant Professor Luis Roman Carrasco of NUS' biology department, an expert on ecology and economics.

The largest impact, he said, would be human diseases spreading from animals, and the financial costs needed to cull invasive creatures such as house crows and rats.

He pointed out that technically, the dengue-carrying Aedes aegypti mosquito - which is endemic in the region now - is not native to Singapore either. Genetic analysis by US entomologists suggests it arrived from North Africa only in the 19th century.

Singapore should pay attention to invasive species entering South-east Asia through its port, he added.

"We, as a region, harbour one of the most important biodiversity hot spots of the world. Invasive species can be very detrimental to biodiversity conservation.

"Singapore is the gatekeeper of the main port of entry and thus the responsibility to carry out sound risk analysis and prevent the entry of harmful non-indigenous species is high."

So how are invasive species managed here?

There are no specific laws to safeguard against them, but the National Parks Board works with the Agri-Food and Veterinary Authority, which regulates imports of plants and animals; the Maritime and Port Authority, which makes sure ships adhere to rules about ballast water, in which foreign organisms can take a ride; and local research institutions to help keep such creatures in check.

Ms Cheryl Chia of NParks' conservation division said it has programmes to get rid of the fast-growing Zanzibar yam, the invasive shrub Koster's curse, and the giant African land snail.

But, she said, the process is not always straightforward.

"There's also an endangered species called the brown tree snail. We asked Botanic Gardens managers to help get rid of the African land snail, but they couldn't tell which was which."

So conservation managers have organised training sessions to help staff identify the unwanted gastropod.

Another conundrum is that unwanted aliens here can sometimes also be endangered elsewhere.

NUS' Dr Tan said of the Asian arowana: "It's a protected species under Cites (Convention on the International Trade in Endangered Species of Wild Flora and Fauna). But it has established feral breeding populations here. So do we get rid of it or sell it?"

It is possible to predict which species might get a foothold here and which will not adapt to Singapore's tropical environment, experts said.

If a species grows fast and can tolerate a wide range of conditions, it is more likely to thrive than a slow-growing, delicate one.

Now, local researchers are looking at a new technique to detect potential invasive species, said Dr Chan.

The technique, called environmental DNA, uses water samples to look for traces of animal DNA. That does away with the need to catch or see the animal in question, explained University of Notre Dame ecologist David Lodge, an invasive species expert, who pioneered its use in the US. He was in Singapore for the symposium.

Professor Lodge said: "Thirty years ago, we didn't have the tools and knowledge to forecast and prevent invasions. Now we do, but we're not using them. If you were to use recent science advances, most societies could save money that is lost due to invasions.

"We know how species get moved around, and can conduct risk assessments and predict which species are likely to be harmful or not. There's no need to use resources on trying to prevent or control things that aren't harmful."

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