However, the Nile tilapia is a generalist species, and its ability to adjust readily to stream environments probably facilitated its rapid spread throughout the Brazilian river systems
http://plosone.org/article/info:doi/10.1371/journal.pone.0029746#article1.body1.sec1.p2

The cited reference concluded that Nile tilapia were associated with degraded river habitats. Whatever changes occurred in the rivers studied may have constituted degradation of habitat for some species but (by definition) not for Nile tilapia, which evidently benefited from those changes. Nothing in either article clearly supports applying the label "generalist" to Nile tilapia. They may be equally well (or better) be called specialists, specially adapted to survival under the newly prevailing conditions. The article incorporates advocacy by concluding that changes favoring one species over another constitute degradation. It could just as easily have suggested that Nile tilapia are "more fit", and that the "natives" are effectively no longer native. Unless there is a practical prospect of imposing and maintaining conditions conducive to the predominance of former natives, it seems problematic to complain that fishes persisting under prevailing conditions are the wrong fishes.