Rivers as ecological systems

Abstract Streams and rivers as ecological systems are distinct from all others and have played a pivotal part in the development of human societies. They form a network of habitats connected by flow, coalescing into bigger rivers downstream. They are a dynamic part of the hydrological cycle, draining the landscapes with which they are closely linked and carrying precipitation and organic and inorganic materials downstream. They are architects of the landscape and are extremely ‘open’ systems. Running waters vary profoundly over a range of spatial and temporal scales and harbour a diversity of living things out of proportion to their relatively limited spatial extent. Most lotic species live on or in the stream bed, though some inhabit the water column. Many species occupy both aquatic and terrestrial habitats during their lives. River organisms are mostly (though not exclusively) small and short-lived and often have obvious adaptations to life in the flow.