Victoria Vaughan Straits Times 6 Nov 10;
ALIENS are among us, curling their tendrils around the edges of our forests, peering out of ponds and skittering up trees.
But they are not from outer space. They are plants and animals from across the world that have become permanent residents in Singapore.
Alien species refer to plants and animals which are non-native or exotic to a country.
And their numbers are growing, according to a recent study, Introduced Species In Singapore: An Overview, by Assistant Professor Darren Yeo of the National University of Singapore and Ms Cheryl Chia of National Parks Board (NParks).
The study recorded 142 alien species here - an 84 per cent jump from a previous study in 2003. However, this seemingly huge increase is likely to reflect the fact that little attention has been paid to these visitors until now.
Prof Yeo, 39, said: 'Concern about alien species has been growing worldwide and we are catching up now.'
More than 60 per cent of these 'aliens' are from Asia and 19 per cent are from Central and South America. More than 70 per cent have made Singapore home, the study found.
But not all alien species are considered invasive.
Invasive species - a major cause of the loss of biodiversity - are defined by experts as those which become self-sustaining in an ecosystem, changing it and threatening native plant and animal life.
Singapore has no species which are considered invasive.
Still, scientists say alien species could have an adverse impact on native species, as they compete for food and space or make prey out of local species.
Among the 'aliens' here, 117 are vertebrate species and 25 are invertebrate, according to the study. Freshwater fish and reptiles make up the bulk of the non-native species known here so far.
The pet and live food trade is one route of their introduction; the release of unwanted pets or mercy releases especially around Vesak Day, a Buddhist festival, is another.
Some alien species hitch a ride in the ballast tanks of container ships which take on water at foreign ports to balance the weight of the ship and then dump it in other ports.
The majority of the 'aliens' here are found in man-made habitats such as parks, gardens, wasteland, canals, reclaimed land and ponds.
Many of the non-native fish find their way into reservoirs and drains as they prefer the water there which is less acidic than natural forest streams.
According to the study, there are also 34 potentially invasive plant species that were brought here as ornamental plants.
While some of the alien species are not yet established in Singapore, they have a history of being invasive and posing a threat to the local species and ecosystems elsewhere.
There are eight such species here and they should be monitored closely because there can be a delay before an alien species becomes established and/or turns invasive, says Prof Yeo.
These species include the snakewood plant found at the edge of forests, as well as the red claw tropical freshwater crayfish from Australia and New Guinea which has established itself in reservoirs near Nee Soon Swamp Forest - an important refuge for native biodiversity.
Prof Yeo suggests a broadening of the definition of invasive species to include nuisance or pest species such as the African giant land snail, the Javan mynah, the house crow and the American cockroach.
His paper also recommends studies on the alien species here and risk assessments to manage potentially invasive species.
Mr Alan Ow Yong, chairman of the bird group Nature Society of Singapore, pointed out that non-native birds, such as the Javan mynah and white-crested laughing thrush from Java, lineated barbet ranging from Nepal to northern peninsular Malaysia, and the house crow found across Asia, have caused problems for the local species.
The white-crested laughing thrush, which was brought in as a pet in 1925 and has since bred in the wild, has been competing with native species such as babblers, which also live and feed in the lower branches and on the ground.
'It obviously competes with native birds that are near the ground,' said Mr Ow Yong. 'But these birds are not as aggressive as the Javan and Indian mynahs which are not afraid of humans and push other birds away.'
NParks is also keeping a close watch on alien plant species.
Mr Wong Tuan Wah, director of conservation at NParks, said the authorities are mindful of the potential adverse impact of these species.
'Our urban environment comprises many introduced species. These species include trees and ornamental plants that make up our Garden City, such as the rain tree, originally from the tropical American region,' he said.
'For example, the Dioscorea sansibarensis (Zanzibar yam), a fast-growing climber, can potentially smother the forest canopy if allowed to invade and proliferate in our rainforests.'
NParks manages the situation by actively clearing certain introduced plant species from the nature reserves in Singapore. It has also introduced community outreach programmes to encourage the public not to release non-native animals here or take native flora and fauna out of the country.
The Agri-Food and Veterinary Authority of Singapore also has regulations on the import, export and transshipment of plants and animals.