Alternative ways to measure impacts of dam closure to the structure of fish communities of a neotropical river

Neotropical freshwaters host some of the most biodiverse ecosystems in the world and are among the most threatened by habitat alterations. The high number of species and lack of basic ecological knowledge provide a major obstacle to understanding the effects of environmental change. We assessed the impact of dam closure on the fish communities of a neotropical river by applying simple descriptions of community organizations: species abundance distribution and abundance biomass comparison curves. Fish data were collected using gillnets during three distinct time periods (1 year before, 1 year after and 5 years after dam closure), at eight sites located downstream of the dam, in the reservoir, transition zone and upstream. A total of 69 343 individuals representing 260 species were recorded. Dam closure was associated with changes in the organization of fish communities at all sites. Species richness tended to increase immediately after dam closure while evenness decreased. Changes in taxonomic structure (richness and evenness) were accompanied by a change in the distribution of biomass with the proportionate contribution by smaller individuals significantly increased relative to larger individuals. Five years on, richness had fallen to below pre-closure levels at all sites, while the comparative stability of assemblages in the transformed habitats was reflected by biomass-abundance distribution patterns that approximated pre-disturbance ratios. This simplistic analysis provided a sensitive and informative assessment of ecological conditions, which highlights the impact to ecosystem process and ecological networks and has particular value in regions where a lack of detailed ecological knowledge precludes the application of traditional bioassessment methods. Copyright (C) 2015 John Wiley & Sons, Ltd.