Canals, ditches and drains are associated with agriculture and cities throughout the world. They take freshwater to and from urban and rural areas, particularly in temperate and subtropical regions. They can carry high nutrient and pollutant loads. Diversity of organisms is generally low, but may be high where there are earthen banks and fringing vegetation. Algae and macrophytes (where present) support microbes and waterbugs and other invertebrates often small fish, amphibians and crustaceans. They are also important pathways for dispersal of some aquatic species.