References

http://en.wikipedia.org/wiki/Introduced_species

http://en.wikipedia.org/wiki/Invasive_species

http://www.esa.org/tiee/vol/v1/figure_sets/species/species_overview.html

http://wwf.panda.org/about_our_earth/species/problems/invasive_species/

http://habitatnews.nus.edu.sg/pub/naturewatch/text/a063b.htm

http://www.issg.org/database/species/search.asp?st=100ss&fr=1&str=&lang=EN

http://www.nparks.gov.sg/cms/index.php?option=com_content&view=article&id=64&Itemid=159#

http://www.nss.org.sg/documents/Pages%2093-97.%20Tan,%20B.,%202013.%20Alien%20Invasive%20Species.pdf

http://iyb2010singapore.blogspot.sg/2010/03/bad-biodiversity-invasive-alien-species.html#.VFuMx_mUeFk