SINGAPORE - In many areas across Singapore, one may find small, slender lizards - sometimes displaying a yellow or reddish orange flap under its throat - if they look intently enough.

These lizards, which can be found on the ground or trees, are called brown anoles (Anolis sagrei), and they have been spotted at an increasing frequency across the island. The catch is: they are not supposed to be here.

Brown anoles are found naturally in the Bahamas, Cuba and the Swan Islands.

It is one of two introduced species, the other being the greenhouse frog (Eleutherodactylus planirostris), that have been seen here more often, and in more places.

The greenhouse frog is native to the Bahamas, Cuba and the Cayman Islands.

For instance, two brown anoles were spotted in the balcony of a residential unit in Sembawang in March.

Introduced species refer to plants or animals that are not naturally found in Singapore.

They can be considered invasive when their presence causes harmful impacts on the local environment. This could happen if, for instance, they outcompete native species due to a lack of natural predators, high reproductive rates, and adaptability to different environmental conditions.

The National Parks Board (NParks) told The Straits Times that it is aware of the presence of the brown anole and greenhouse frog in Singapore and that it is monitoring the situation.

Dr Karenne Tun, group director for NParks’ National Biodiversity Centre, said the brown anole has outcompeted the native green anole in Florida, and in Taiwan, it has impacted local arthropod (animals including insects) communities in betelnut palm plantations. In Hawaii and other parts of the US, the greenhouse frog has also been documented as invasive, she added.

However, there is no evidence that these species have significantly impacted local ecosystems so far, she added.

Observers believe the two Caribbean species came into Singapore by “piggy-backing” on exotic plants that were brought in by the horticultural sector.

Dr Alex Figueroa, executive secretary of the International Commission on Zoological Nomenclature based at Lee Kong Chian Natural History Museum at NUS, said brown anoles continue to be in...