Large river ecosystems are often flanked by lowland riparian areas that are purported to harbor high rates of species diversity. Diversity and abundances, especially in transition zones, are often bolstered as a result of subsidies from adjacent ecosystems. However, many rivers worldwide have been subjected to increased regulation due to the presence of dams, weirs, and other water control structures that reduce the natural flow variability. As a result, many rivers have become channelised where the river is confined to the main channel and disconnected from the surrounding riparian areas. The purpose of this study was to determine the effects of channelisation on the community structure of macroinvertebrates in a semi-arid landscape. We used aquatic and riparian survey data from the middle Rio Grande in New Mexico at seven sample locations varying from semi-natural to channelised conditions. Sampling was conducted during the summer of 2008. Each sample location was divided into three habitats; the aquatic environment, the transition zone near the water’s edge, and the surrounding terrestrial environment referred to as the Bosque. Statistical analyses of the survey data was used to determine differences in the macroinvertebrate communities between channelised and semi-natural reaches and between habitats. Stable isotopes of carbon were used to distinguish between aquatic and terrestrially derived carbon sources.
Macroinvertebrate communities were similar in the Bosque at all sample locations. There were several differences between the Bosque and the transition zone. First, the abundance of arthropod predators was significantly higher in the transition zone (P<0.01). Second, the total abundances of macroinvertebrates were higher in the Bosque. Macroinvertebrate communities in the transition zone had reduced diversity and abundances between channelised and semi-natural reaches. Results from the aquatic survey data indicate that the density of aquatic organisms was 50% (P<0.01) lower in channelised reaches. Stable isotope analyses of carbon indicate that the predators in the transition zone receive the majority of their carbon from algae production indicating the presence of aquatic subsidies. Whereas, there was no indication that predators in the Bosque were receiving aquatic subsidies. These results demonstrate that channelisation can reduce macroinvertebrate densities and negatively impact diversity at the landscape scale by severing the linkages between the aquatic and riparian communities. These effects appear especially acute for macroinvertebrates in higher trophic levels, perhaps because preferred prey density is lowered.