Twenty species of animals plentiful in the Bukit Timah Nature Reserve in the 1980s, including frogs, crabs and fish, are slowly being wiped out.
Preliminary findings by the National University of Singapore (NUS) are pointing to the acidity of a stream in the 80ha nature reserve, which is rich in plant and animal life. A four-year study led by Associate Professor David Higgitt of the university's geography department has noted that the stream, which covers 5ha of land, is more acidic after rainstorms.
The acidity or alkalinity of fluids is expressed as a pH value, with pH 7 being neutral. Values lower than seven indicate acidity, and above seven, alkalinity.
Researchers have found the water in the stream on the nature reserve to have a pH value of 4.4 to 4.7. Prof Higgitt believes it is more acidic now than 20 years ago.
Earlier studies have found that although animal species have evolved and adapted to the increasingly acidic environment, they are likely to be under stress. The animal population has come down and some crabs, for example, have developed harder shells.
Professor Peter Ng, director of the Raffles Museum for Biodiversity Research at the NUS, said a change of one unit in the water's pH represents a tenfold change in its acidity. This may be beyond the ability of the animals' bodies to cope with.
The acidity of the water comes from sulphur dioxide and nitrogen oxides in the atmosphere - from industrial pollutants and lightning, for example - that dissolve in rain water, which then falls into streams and other bodies of water.
The National Parks Board (NParks) is working with the NUS to find out how badly the water quality is affecting the diversity of the plant and animal life in Singapore's last remaining primary forest. Its assistant director of centre nature reserves Sharon Chan agreed that the changes in the pH of some streams make a closer study necessary, so freshwater habitats in the nature reserves can be better managed.
The pH level of rain water in some parts of the United States and western Europe was as low as two in the 1980s. But since then, with regulations curbing pollution and the use of cleaner fuels, their acidity levels have fallen.
Dr Erik Velasco, a post-doctoral fellow in the NUS' geography department, said the presence of acid rain here is to be expected, given the level of industrialisation and the presence of aerosols. Aerosols are tiny air particles that occur naturally and are caused by the burning of fossil fuels.
The National Environment Agency, however, said the acidity of rain water here - at pH 5 - is no different from that of urban cities around the world; it also said rain water is no more acidic now than in the 1990s.
Prof Higgitt suggested that one way to protect the biodiversity in the stream would be to add limestone - a naturally occurring alkali - to slow down acidification. But he cautioned that more studies are needed as this could affect the environment in other ways.