‘When the River Runs Dry’ is a familiar song in Australia. Some rivers in the arid center of the continent flow only after a stiff monsoon season, and smaller tributaries all over the country commonly shrink to puddled potholes and dry river beds during the dry season. But rivers also run dry in more temperate climes. Much of the upper reaches and feeder streams of the great rivers of North America, and even the mighty Amazon, dry out seasonally.
Cities in both, the US and Europe harbour more plant species than rural areas. However, plant species of urban areas are closer related to each other and often share similar functions. Consequently, urban ecosystems should be more sensitive towards environmental impacts than rural ecosystems.