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Wetlands (rivers, lakes, bogs, etc.), the biodiversity hotspots, provide habitats for biology on Earth and play an important role in biodiversity protection around the world. However, they are damaged by dam construction all over the world. Many studies have been conducted on the effects of dam construction on biodiversity, but a review about it is missing. The objectives of this review are: (1) to discuss the effects of dam construction on biodiversity in freshwater wetlands; and (2) to identify the future directions for research in this field. To achieve these objectives, 347 significant publications from 2000 to 2017 were selected (based on a rigorous screening processes and a systematic literature review approach) from the ISI Web of Knowledge database. Dam construction decreased the water fungal biomass and richness in reservoirs and downstream reaches, but increased the amount of soil microorganisms in downstream lake wetlands. The studies about effects of dam on benthos mainly focused on macroinvertebrate, periphyton and mussel. Most studies about the effects on macroinvertebrate claimed that dam construction caused increases in biomass and decreases in taxa richness in downstream reaches. The studies about effects of dam on plankton mainly focused on phytoplankton, zooplankton, planktonic microorganisms and ichthyoplankton. Effects of dam on fish (including aquatic mammals) included blocking migration route, habitat fragmentation, changing from lotic to lentic water in the impounded area, release of hypolimnetic cold water of reservoir, and changes of water flow in downstream reaches. Studies about effects of dam construction on botany mainly focused on the riparian plants, but there were few studies on floating plants and submerged macrophyte and effects of dam construction on botany in downstream lake wetlands. There were only few studies that examined these effects of dam on bird. We also pointed out the future directions for research in this field.