The distribution of Andean freshwater fishes is the result of the interaction of historical and contemporary factors such as basin geomorphology and the physicochemical characteristics of water bodies. Dramatic changes along river networks due to waterfalls or dams generate abrupt changes in longitudinal slopes that function as ecological barriers to dispersal and thus have an effect on the composition and richness of fish assemblages. We expect the amount of variation in beta diversity along the elevation gradient (between 700 and 3,500 m a.s.l.) of the eastern slope of the Cauca River basin to be explained by changes in the aquatic environment and connectivity among sites. We measured connectivity in terms of the distance along the stream channel between sampling sites considering changes in slope. We used a Generalized Dissimilarity Model (GDM) to evaluate the contribution of connectivity and other water mass characteristics (dissolved oxygen, pH, conductivity, temperature, and elevation) in predicting changes in beta diversity. The GDM models explained 33% of the total deviance in species turnover, suggesting that there are additional variables that have not been considered, such as available habitats along the rivers. Elevation was the variable with the largest relative importance in the model and connectivity explained only seven percent of the total variance when all sites were included. However, when only the sampling sites in the headwater streams were included (the most geographically isolated sites), the GDM models explained 51% of the total deviance and the contribution due to connectivity increased. Isolation of stream headwaters, in conjunction with extreme conditions present at high elevations may influence the fish assemblage turnover. Our results provide evidence that elevation has a strong influence on beta diversity of Andean fish assemblages. Species turnover upstream above 1,200 m a.s.l. is strongly influenced by channel connectivity and there are additional environmental variables that need to be included in the models to improve their prediction power.
La elevación en los Andes es un factor definitivo en la estructura de los ensamblajes de peces y los ecosistemas en que habitan. Los ensamblajes en el río Magdalena se estructuran mediante la interacción entre el tipo de ecosistema, la elevación y el ciclo de lluvias. En este capítulo se analiza la información ecológica disponible en diferentes fuentes sobre los peces del río Magdalena y se presentan algunas características de los ensamblajes de peces. La mayor riqueza de especies se encuentra en las zonas bajas de la cuenca del río Magdalena y, en particular, en los ecosistemas de corrientes (ríos y quebradas); mientras que la mayor diferencia en la composición de especies entre cuerpos de agua se observa en las zonas arriba de los 1.600 m s.n.m. Esta característica les confiere particularidades a los ensamblajes en términos de la riqueza de especies en cada uno de los gremios tróficos, de las estrategias de vida y de los nichos funcionales en los que se estructuran los patrones de la ictiofauna en los Andes nor-occidentales. En la cuenca se reconocen cuatro grandes gremios tróficos: carnívoros, detritívoros, omnívoros y planctófagos. El gremio de los carnívoros es el más rico en especies. La estrategia de vida más diversa en esta cuenca andina es la oportunista. Estas características en la riqueza de especies por ecosistema, por gremio trófico y por estrategia de vida, causa que el ensamblaje del río Magdalena finalmente se estructure en 25 nichos funcionales y, de nuevo, el mayor número de nichos se encuentra en las zonas debajo de los 1.200 m s.n.m. El conocimiento de la ecología de los peces en el río Magdalena aún es incipiente, pero es posible aportar a partir de la información disponible para la toma de decisiones por parte de los diferentes actores de la cuenca.
We describe the change in the fish community of the Porce River in Magdalena River Basin, Colombia, following the construction of the Porce III hydropower reservoir based on 13 years of monitoring data. The results show a clear reduction of the number of native species, which have been supplanted by colonizing non-native species, especially in the reservoir. Four native species detected prior to dam construction have apparently disappeared, but 12 new species were registered post-construction. We analyzed spatial changes in beta diversity in the aquatic environments surrounding the dam. The new environment generated by the reservoir presents a unique species composition and contributes significantly to the total beta diversity of the system. Altogether three distinct new fish assemblages emerged following reservoir formation and there are now six assemblages where there had previously been three. This dramatic change, already visible within a decade of construction, highlights just how strong of an impact dam construction has on habitats and how rapidly fish communities react in this hotspot for endemic fish diversity. Our findings demonstrate the importance of monitoring fish communities for revealing the impact of damming on river ecosystems and informs potential complementary fish diversity inventories elsewhere in the Magdalena River basin.
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