FCM,
There are many possible reasons for adding Tilapia and the fact that they DON'T live year round in temperate water is what makes them so special. If they didn't die annually, they would no doubt devestate a pond with stunted fish. The following may not hold in every case or pond situation and is all "generally speaking". The only potential problem I can find with tilapia is the potential to cause turbidity if overstocked.
I have read hundreds of studies on the tilapia and especially the ones that state they are a potential invasive in temperate waters. The invasive fear claims are simply not based on fact in temprate waters but rather they base these clams on tropical water conditions that could some how magically occur.
Tilapia species in the USA that are stocked into ponds are all mouth brooders and will produce huge numbers of offspring every 3-6 weeks. These fry grow very fast, but the YOY of other fish will eliminate almost all of them before they grow much over an inch. Most of those few survivors will get eaten when they grow to an optimum size for larger predators, but several still make it to maturity and will reproduce a second or even a third generation in a pond in a single season.
When water temps start to cool, the tilapia become slow swimmers and any that are small enough to be eaten by another fish in the pond will become forage. All carnivores and omnivores in your pond will gorge on the dying tilapia. After all tilapia have died, you will only see the original stockers, if any are not eaten by turtles.
Although tilapia are omnivores, just like other fish, they waste as little energy as possible feeding. They will eat the eggs of an abandoned nest, but they won't fight off a fish protecting the nest unless easy food is unavailable. Tilapia can extract nutrition from almost anything due to a very low stomach PH so they will eat plants like algae, duckweed and others that no other fish is competing with them for. Tilapia also eat detritus, not because they eat dead leaves and other decaying organics, they are going after the bacteria on the detritus and consume the dead organics incidentally.
Tilapia do filter feed in a way also but are NOT filter feeders. A tilapia's gill rakers produse a sticky mucus that catches tiny particles of algae and microscopice animals. They will essentially hock up a loogey when enough nutients have been caught and swallow it. Tilapia do not filter feed enough to effect the growth of any other fish that rely on filter feeding to survive.
I have raised bluegill fry and mature tilapia in the same aquariums and unless a fry swims into an easy range for the tilapia OR if I don't feed enough, the bluegill will be safe.
The bottom line is, if properly stocked in the right numbers, the tilapia will actually INCREASE your ponds total carrying capacity and increase the health and numbers of every fish in the pond. By heavily reducing the oxygen depleting, toxic gas producing detritus along with carbon dioxide producing algaes and other over-abundant plants, and vigorous swimming mixing the water, tilapia produce results similar to adding an undersized aeration system.
By adding the proper numbers in a pond with sufficiant vegetation, tilapia will produce growth rates in other predators similar to using supplemental feeding. In a Bass-heavy pond, bass will have better growth with adding tilapia than with using pellets to feed the forage species.
Tilapia will eat the most abundant foods not competed for first and move on down the line to the next most abundant. Eventually competition will occur but it is usually only minor in nature. In a pond overgrown by plants, tilapia can eliminate the need for chemicals entirely.
The only article I have found that shows a fish potentially negatively impacted by adding tilapia is the Channel Catfish. Here is an ecerpt with a link to the full article below.
Tilapia can be cultured with channel catfish (Ictalurus punctatus) with only a minor reduction in catfish yields. Male tilapia stocked at a rate of 800/acre yield nearly 770 pounds/acre when channel catfish are stocked at 3,000/acre. At this stocking rate, net production of catfish declines by roughly
SRAC-280-3
170 pounds/ acre, but for every reduction of 1 pound in catfish
production, 4.5 pounds of tilapia are produced. Catfish production does not decline when cultured in combination with tilapia, silver carp (Hypophthalmichthys molitrix), and grass carp (Ctenopharyngodon idellus) at densities of 800; 1,000; and 20/acre, respectively. With no additional feed, total net production can reach nearly 4,120 pounds/acre compared to 2,370 pounds/acre for catfish cultured alone. The incidence of off flavor catfish may be less in catfish/tilapia polyculture than catfish monoculture.
OSU pdf 
