“Invasive species are organisms (usually transported by humans) which successfully establish themselves in, and then overcome, otherwise intact, pre-existing native ecosystems. Biologists are still trying to characterise this capability to invade in the hope that incipient invasions can be predicted and stopped. Factors may include: an organism has been relieved of the pressures of predators or parasites of its native country; being biologically "hardy", for example, has short generations and a generalist diet; arriving in an ecosystem already disturbed by humans or some other factor. But whatever the causes, the consequences of such invasions - including alteration of habitat and disruption of natural ecosystem processes - are often catastrophic for native species.”

IUCN/SSC Invasive Species Specialist Group (http://www.issg.org)

Many species of tilapia native to Africa and the Levant has been introduced by man to other parts of the world. In some cases, the introductions have been accidental; tilapia fish have managed to escape from fish farms or ornamental fish breeders. In other cases, the introduction has been deliberate, e.g. to combat mosquito or aquatic weeds. Tilapia has also been introduced as food fish to lakes and other bodies of water. If you are an aquarist keeping tilapia as a pet, it is every important never to release it into the wild. If you fail to find a new home for your pet, it should be euthanized.

Many species of tilapia are amazingly adaptable and this makes them especially prone to becoming problematic invasive species. Quite a few tilapias can adapt to brackish conditions and some are even capable of living where the salinity is 35 ppt, the average salinity of sea water. Tilapias are also famous for their opportunistic feeding style and many species can make use of a wide range of food sources, from aquatic invertebrates and small fish to decomposing organic matter (detritus), plants and plankton. In addition to this, many species and variants breed rapidly and grow fast. It should however be noted that most species are sensitive to cold since they hail from tropical environments.   

Tilapia can cause problems for native flora and fauna in several ways. They will compete with other species for food and can disrupt the ecological balance. Tilapia can also cause turbidity in clear waters since they are fond of digging. Turbidity will reduce the amount of available light in the water, which affects all organisms relying on photosynthesis.                                                           

Examples of tilapia as an invasive species

• In the United States, established tilapia populations of tilapia occur in the many different parts of the country. There is for instance a thriving population of Oreochromis mossambicus in Salton Sea, an inland saline lake in Southern California.
  
  
• Further south in the Americas, Tilapia has been introduced to the sensitive Galapagos Islands. The Galapagos Islands are located far off the coast of Ecuador in South America and their remoteness has resulted in a very high degree of endemic species. These endemic species are now threatened by the introduction of foreign species, such as tilapia fish.
  
  
• In Singapore, the problem with tilapia as an invasive species may be on its way of solving itself. During World War II, Oreochromis mossambicus was introduced to Singapore by the Japanese. Soon, it became a common fish in both fresh and brackish water, as well as in the sea off the northern coast. Since the late 1980s, these feral population have however been declining dramatically and this is believed to be caused by the introduction of other tilapias, probably the hybrid Oreochromis mossambicus x Oreochromis niloticus and possibly also Oreochromis urolepis hornorum and Oreochromis aureus. When Oreochromis mossambicus mates with certain species and hybrids, the resulting batches tend to have a much skewed sex ratio where almost all fishes are male. It is also common for hybrids to produce a lower number of fry per spawning. In addition to this, it is not unreasonable to assume that that offspring containing genetic material from Oreochromis niloticus risk losing the high salinity tolerance of Oreochromis mossambicus and instead get the low salinity tolerance of Oreochromis niloticus.