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Invasive alien species are one of the main components of global ecological change, the second known cause of animal extinctions, and very costly in terms of ecosystem services. Invasive alien species and damming are two of the most impacting alterations in freshwater ecosystems, and understanding the processes that govern biological invasions in these habitats is of enormous conceptual and practical importance. About 39 competing and overlapping hypotheses have been proposed in invasion biology that have been recently grouped in four (or five) concept clusters, namely the: propagule, resource availability, biotic interaction and Darwin’s clusters. We analyzed the relative importance of three of these concept clusters in Neotropical fish assemblages, using data from 29 reservoirs and variation partitioning analyses. We show that alien fish assemblages respond to variation in limnological characteristics in a way similar than native species, usually with positive effects of increased temperature, conductivity and chlorophyll-a concentration and decreasing turbidity. Overall, we found support for some hypotheses included in resource availability and Darwin’s clusters, such as increased resource availability and biotic acceptance, and no evidence of strong biotic resistance, marked effects of human disturbance, as measured by land-use changes, or propagule/colonization pressures. We discuss the potential reasons and management implications of these findings. Our study illustrates that analyzing the importance of classical hypotheses of invasion biology in tropical freshwaters and other ecosystems enhances ecological understanding and provides practical implications to prioritize management interventions and mitigate ecological impacts We express our appreciation to the PRONEX-MCT/CNPq for financial support and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brazil, process number 88882.344464/2019-01) and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil, process number 165319/2017-8) for the fellowships to graduate students. AAA received scientific productivity fellowships from CNPq. We are also grateful for the funding provided by CAPES through “Programa de Doutorado-sanduíche no Exterior” (PDSE, processes number 88881.361907/2019-01 and 88881.361938/2019-01) that allowed a research stay of CMM and MMG at the University of Girona. Additional financial support was provided by the Spanish Ministry of Science and Innovation (projects RED2018‐102571‐T and PID2019-103936GB-C21) and the Government of Catalonia (ref. 2017 SGR 548)

The Xingu River, located in northern Brazil, is already under the impacts of one of the most harmful projects in the history of the Amazon – the Belo Monte hydroelectric dam. A new megaproject called the ‘Volta Grande Project’, led by Canadian gold mining company Belo Sun will be situated adjacent to the same stretch of the Xingu that is dewatered by the Belo Monte complex. Various aspects of the project are controversial. There are reports that the mining company acquired public lands illegally, the affected area has high biodiversity and endemism of several taxonomic groups, and mining operations would be approximately 9.5 km from indigenous lands. Synergistic impacts of impoundment and mining operations are expected to dramatically alter the Xingu's biodiversity and ecosystem services that support indigenous people and riverine populations. Moreover, the mining activities will generate tons of toxic waste that will be stored in a high-risk tailings dam, which exposes the Xingu River to catastrophic consequences similar to those of the Doce River disaster.

Publisher Summary This chapter emphasizes the ecological responses of fishes to spatial and temporal variation in tropical stream habitats. It also discusses the steps to alleviate human impacts and restore degraded streams to protect the species of the fish. It reveals that freshwater fishes comprise 25% of the vertebrate species on earth. The South and Central American rivers and streams contain the greatest number of species on Earth, with recent estimates ranging as high as 8000 and 25% of global fish species richness. The highest fish species richness in the Neotropics is within the Amazon Basin. The ultimate environmental driver for variations in habitat quality and quantity, and hence breeding events, is probably stream hydrology. Tropical stream fishes occupy almost the entire spectrum of trophic niches that can occur in aquatic communities. Aquatic invertebrates are important food sources for stream fishes throughout the tropics. Terrestrial plants tissues, especially flowers, fruits, and seeds are very important resources for fishes in tropical streams. The primary threat to the ecological integrity of tropical streams and the long-term survival of their fish faunas is degradation of watersheds by a variety of human activities. The largest impact is from deforestation and conversion of land to agriculture besides over-fishing, dam construction, use of pesticides and herbicides, and introductions of exotic species, etc. Protection of vegetation cover, maintenance of the integrity of riparian zones, and reductions in point- and non-point-source pollution are essential components of conservation and management strategies for stream fishes.

To describe the Upper Parana River Floodplain (the last non-dammed stretch of the Parana River, Brazil) a food web model was quantified using ECOPATH. The modeled ecosystem showed maturity because of the total primary production/total respiration ratio (close to 2), Finn’s cycling index (7%) and overhead (65%). The first model elaborated had 40 compartments/groups, but its transfer efficiencies among trophic levels did not reduce in despite the trophic level increasing. To solve this, the effect of two grouping methods on system-level information and other ecosystem attributes was investigated. The first series tested, named “classic” (researcher intuitive way and by food preferences) also did not reduce transfer efficiencies. In the second series, named “by pathways”, the first species grouping were those with higher number of input pathways and longest mean length of pathways. Thereby, the news groups from aggregation decreased the number of components and system’s richness, but stability (measured by overhead) did not change, including the model with only eight compartments. The great number of the ten compartments that showed these characteristics was piscivores, increasing the redundancy within highest trophic level. The use of pathways (number and length) can be useful to lumping species since it reduces compartments and do compromise neither maturity nor stability, diminishing grouping subjectivity.