They are areas where water is the primary factor controlling the environment and associated plant and animal life.
Wetlands occur where the water table is at or near the surface of the land, or where the land is permanently or temporarily covered by water. Once thought of as mosquito-filled swamps or bogs, wetlands perform many valuable functions. They cleanse the water that flows into them, and can reduce flooding and maintain water tables.
In the past, those soggy areas of land were often drained and 'put to better use' but now we know they are essential, extremely productive places. In New Zealand they support the greatest concentration of wildlife out of any other habitat.
Wetlands act like the kidneys of the earth, cleaning the water that flows into them. They trap sediment and soils, filter out nutrients and remove contaminants; they also return nitrogen to the atmosphere.