Approximately 50 years ago, Nile tilapia were accidentally introduced to Brazil, and the decline of pearl cichlid populations,
which has been intensified by habitat degradation, in some locations has been associated with the presence of Nile tilapia.
There is, however, little strong empirical evidence for the negative interaction of non-native fish populations with native fish
populations; such evidence would indicate a potential behavioural mechanism that could cause the population of the
native fish to decline. In this study, we show that in fights staged between pairs of Nile tilapia and pearl cichlids of differing
body size, the Nile tilapia were more aggressive than the pearl cichlid. Because this effect prevailed over body-size effects,
the pearl cichlids were at a disadvantage. The niche overlap between the Nile tilapia and the pearl cichlid in nature, and the
competitive advantage shown by the Nile tilapia in this study potentially represent one of several possible results of the
negative interactions imposed by an invasive species. These negative effects may reduce population viability of the native
species and cause competitive exclusion.