Ducksterman,

It will eventually have the ecosystem it can support, but once people start interfering it can greatly affect the outcome. For instance, take a river with prolific stone, caddis, and mayfly hatches, and a nice healthy population of cutthroat, sculpin, and lamprey (species chosen for fellow Oregonian). Add something like modsnails and hatches decrease and the effects cascade up and down the foodchain. Another thing that could happen is that the insect that we introduce is a temporary host for some parasite that will infect the fish (whirling disease works this way).

Probably more than you wanted to know...

Adam