wetlands are dominated by vascular plants that have adapted to saturated soil. there are four main types of wetlands: swamp, marsh, fen and bog (both fens and bogs are types of mire). wetlands are the most productive natural ecosystems in the world because of the proximity of water and soil. hence they support large numbers of plant and animal species. due to their productivity, wetlands are often converted into dry land with dykes and drains and used for agricultural purposes. the construction of dykes, and dams, has negative consequences for individual wetlands and entire watersheds. their closeness to lakes and rivers means that they are often developed for human settlement. once settlements are constructed and protected by dykes, the settlements then become vulnerable to land subsidence and ever increasing risk of flooding. the louisiana coast around new orleans is a well-known example; the danube delta in europe is another.