This point can also be considered to be an ecological effect. However, I decided to explain this individually as it does not directly affect the balance of nature. The biological effect may be less severe.

When invasive species are present in the ecosystem, they might hybridise with native species, causing negative genetic results. This can also cause native species to be threatened with extinction as hybrids can pollute the original species genetic pool. This might reach a stage where the original genes of the native animal no longer exist on it’s own. This process is unintentional and is extremely difficult to manage in the wild. There is not much documented study regarding this issue in Singapore, however one example of such genetic pollution is the red wolf and coyote breeding in eastern North Carolina.

Below is a picture of the comparison between the Red Wolf and Coyote, with the Red Wolf on the left:

Image from: http://en.wikipedia.org/wiki/Red_wolf