Explain how the reduction in biodiversity can effect the ecological balance in the dam

Damming a river has a variety of effects on the freshwater ecosystem, more than just altering the flow from A to B.  Dams create calm bodies of water, changing overall temperature regimes and sediment transport, leading to conditions which tend to favour generalist species. Loss of specialist species, particularly endemics, changes the community structure and leads to biotic homogenization.  A dam will withhold sediment in the reservoir, not just decreasing the amount of substrate available to local freshwater species, but even impacting diadromous, estuarine and marine species much further downstream.  The competition between resident species for food and breeding sites will increase as damming isolates populations, and perhaps more importantly, damming completely restricts migratory fish species.  Isolation may lead to decreases in genetic diversity and therefore puts species at greater risk from disease.  All of these effects may be exacerbated by changes in the surrounding land use.  Overall, damming river flow will lead to both a loss of native species, but also an increase in exotic species which are more likely to become established in degraded habitats. For this reason, dams are one of the greatest global threats to freshwater biodiversity.