Non-native species, also known as alien species or exotic species, are species that have been introduced into an area from a different geographical region. These species are not endemic to the region. They can be beneficial, or they can be harmful, depending on how they interact with the ecosystem of the place they have been introduced to.
In some areas, non-native species thrive and grow well, maybe due to an ideal environment, or an unexplored niche that they can take over. Some non-native species that do well outside their geographical region may threaten the existence of the endemic species, especially so when they compete for the same resources. However, many non-native species are harmless, or even helpful in many ways.
A self-sustaining ecosystem
In Malaysia, the Putrajaya Lake and wetlands are man-made. The lake also has several man-made islands that are home to several non-native species. In the beginning, Putrajaya Lake and wetlands were just large, swampy areas, needing a lot of human interference to keep the balance in the ecosystem as well as to prevent the degradation of the area.
Things changed when the migratory birds stayed on instead of continuing on their migratory route to their final destination. Slowly, there were evidences that showed that the birds have settled permanently in the Lake and Wetlands, as there were nests on the man-made islands. The Putrajaya Lake and Wetlands now is slowly becoming a self-sustaining ecosystem due to the dynamic system initiated by the migratory birds which populated a new niche.
Symbiosis between the native and non-native species
Another example of how non-native species can revive an ecosystem can be seen in Puerto Rico. As farming practices grew on the island, the trees were chopped down causing massive soil erosions, and the island’s many native species were close to extinction.
When industrial development rose, and agriculture was not very lucrative anymore, poor farmers migrated to towns, leaving the farms abandoned. The soils of the farmlands were now a hostile habitat for native species. However, as nature took its course, repopulation of these farmlands were led by exotic species brought in by European colonists, such as mango trees, grapefruit, and the African tulip tree.
The island which was close to being barren was now recovering. There is evidence of symbiosis between the native and non-native species. The native coqui frog population has recovered and now make their homes in the African tulip trees. The non-native trees host a variety of organisms that are spreading the seeds of the native trees, adding to the ecosystem recovery of the island.
Reasons to worry?
While many scientists and environmentalists might disagree with the benefits that a non-native species can bring, it is important to bear in mind that most species in any particular ecosystem might have found their way there from another geographic area at any point of time.
It might be argued that the non-native species are hardier, as seen in Puerto Rico, thus they may be able to successfully overcome the native species and thrive. However, when a balance is achieved, as also seen in Puerto Rico, as well as Putrajaya, there is no need to worry. Many non-native species are harmless, and when they are useful, they contribute a lot to the local ecosystem.
 http://www.ukm.my/jsm/pdf_files/SM-PDF-42-10-2013/18 M. Zakaria.pdf