The role of introduced species in the decline of a highly endemic fish fauna in Central Mexico

Central Mexico, home to a unique and imperilled freshwater fish fauna, high in endemism, is also a very densely populated region with considerable human activity. Aquatic systems in the region have been heavily disturbed by human activity in the form of species introductions, pollution, and habitat modification, which has resulted in the extirpation of native fishes and the alteration of the native fish fauna. An analysis of the roles of these factors in the decline of the fish fauna and an overview of its conservation status are presented. Both current and historic fish assemblages were characterized and water physicochemical parameters recorded at 62 localities within the region. Water quality, hydrological modification, and the presence of introduced species were used as variables in a linear model to explore their respective influence on the integrity of local fish assemblages. Analyses suggest that both water quality and the presence of introduced species have had adverse impacts of similar magnitude on the conservation status of native species, whereas hydrological modification has had no statistically significant effect. This survey revealed the profound alteration of the fish fauna. Only 40% of the expected native populations were found, and only 30% of the sampled assemblages were free of introduced species. As little as 17% of the sampled assemblages were taxonomically intact. The mean species richness per locality was only half of that indicated by historical records. The conservation value of springs, where many threatened microendemics can still be found, is highlighted, and we argue that their small size and isolation make them key targets for conservation efforts.