Mexican mojarra can dominate non-native convict cichlids even when outnumbered

Abstract

The non-native, aggressive, convict cichlid Amatitlania nigrofasciata is often regarded as a cause for the decline of native Mexican mojarra Cichlasoma istlanum in Mexican rivers. Convict cichlids are assumed dominant in areas they invade. Cooperative territorial defense has been observed in convict cichlids, suggesting that they can dominate Mexican mojarras by coalition. Behavioral observations were recorded and analyzed to experimentally test hierarchical dominance establishment (ES) and maintenance (MN) between uneven numbers of both species. Counts of aggressive interactions (bites, chases, and confrontations) between species were used to measure dominance in ES and MN interactions. More chases and bites were made by Mexican mojarra than convict cichlids in both ES and MN tests. Mexican mojarra attained and maintained dominance against two convict cichlids when competing for food resources via increased antagonistic interactions. Intraspecific confrontations were more common than interspecific confrontations for convict cichlids. Our results offer insight into competition mechanisms potentially occurring in natural settings, suggesting that while convict cichlids may be exploitative and interference competitors to Mexican mojarra, behavioral interactions between these species may not be the sole cause of observed Mexican mojarra population declines in freshwater ecosystems throughout western and southern Mexico. Successfully established non-native freshwater fishes are often regarded as better competitors than native fishes. Populations of the native fish Mexican mojarra, Cichlasoma istlanum, have been hypothesized to be negatively affected by the introduction of the non-native convict cichlid, Amatitlania nigrofasciata. Behavioral observations were analyzed to demonstrate that a native fish can outcompete and establish and maintain hierarchical dominance over non-native fish, even when at numerical disadvantage. We expected to find a cooperative territorial defense strategy as a mechanism to obtain dominance and hypothesized that hierarchies among these species would be determined by the number of individuals of each species present in contests. Mexican mojarra consistently attained and maintained dominance over convict cichlids via increased antagonistic interactions despite being outnumbered. Our results suggest that behavioral interactions between these species may not be the sole cause of observed Mexican mojarra population declines.