Biotic homogenization, the process of gradual replacement of native biota by non indigeous and locally expanding nonnative species, is rapidly diminishing the regional distinctiveness of global terrestrial and aquatic ecosystems. In the last years the introduction of peacock-bass species (Cichla spp.), a voracious predatory fish, in Neotropical reservoirs, has been considered a major problem for the local reservoir ichthyofauna. Although the empirical study of biotic homogenization is substantial and still growing, the mechanisms underlying its dynamics remain poorly understood. An ecosystem simulation model was used to investigate potential role of Cichla in ichthyofauna homogenization. The model was calibrated and validated based on field observations in four lateral lagoons and in the main channel of Rosana Reservoir (Parana River Basin), where Cichla kelberi was introduced in 2004. The samplings were conducted in February and September of 2004, January, March, May and August of 2005, February and June of 2006, February and July of 2007, February and October of 2008 and February of 2009. The large sized fish were caught using gill nets, while the small sized fish were sampled using throw trap. The same sampling effort was applied in each point and period.
Results/Conclusions
Field observations showed that fish diversity changed during the months following the introduction, especially regarding small sized fish. Despite that, this homogenization effect was minimized due to the non establishment of this species in the environment until the end of field observations. However, the model results supported that Cichla will get to establish at long time due to its exceptionally voracious predation. The models also showed that the introduction may seriously threaten native fishes, compromising the assemblage diversity or even causing regional extinction of some species. Hence, an interesting result from modeling was the linear loss of diversity over time. The decline of fish assemblages was cyclic and spread at each eight/ten years, with remarkable decreases in fish density and richness at summer periods. The reproduction of C. kelberi during warm months (November and December), with the outbreak of juveniles during the summer, can increase predation pressure mostly at the littoral zones, which are inhabited by these juveniles. This model reveals an important homogenizing force behind this predator and stresses the need to create policies to prevent new fish fauna interchange among South American basins. Thus, the model herein may help predict future patterns of homogenization by considering its underlying ecological processes and mechanisms.