Wetland extent

  • Image, Wetland extent.

    Wetlands support unique biodiversity and provide important services. They clean water of excess nutrients and sediment, help absorb floodwaters, and act as carbon sinks (remove carbon dioxide from the atmosphere). They also have cultural importance for Māori, and provide valuable food and materials (eg flax). Draining wetlands for agricultural and urban development over the past 150 years has significantly reduced their extent, leading to a loss of biodiversity and natural function.

    We classified Wetland extent as a case study.

    Key findings

    In 2008 wetlands occupied about 249,776ha of New Zealand’s land area. This is 10.1 percent of the estimated pre-human extent of 2,471,080ha.

    In 2008:

    • wetlands covered 0.9 percent of New Zealand’s land cover, compared to an estimated 9.2 percent of land cover in pre-human times
    • the proportion of remaining historical wetland extent was greater in the South Island (16.2 percent) than in the North Island (4.9 percent)
    • swamp (89,992ha) was the most common wetland type, accounting for 36.0 percent of New Zealand’s wetland area
    • pakihi/gumland (56,909ha) was the second most-common wetland type, accounting for 22.8 percent of wetland area
    • the West Coast region had the greatest area of wetlands (84,396 ha), followed by Southland (47,231ha) and Waikato (28,226ha).

    Figure 1

    Figure 2

    Figure 3

    Wetland extent, pre-human and 2008 – interactive map

    Definition and methodology

    New Zealand’s freshwater wetlands include areas that are permanently or intermittently wet, shallow water, or margins of land and water that support a natural community of plants and animals adapted to living in wet conditions (Resource Management Act, 1991). Wetlands occur in many locations, ranging from estuaries to mountaintops.

    The contemporary and estimated pre-human extent of wetlands were mapped at 1:50,000 to a minimum size of 0.5ha. Seven classes of wetlands were mapped according to their function. Wetlands were categorised as bog, fen, inland saline, marsh, pakihi/gumland, seepage, and swamp, based on Johnson and Gerbeaux (2004). Ephemeral wetlands, saltmarsh, and shallow water wetlands were not mapped.

    The historic extent was estimated from the national Fundamental Soil Layers (FSL) database, and refined using a 15m digital elevation model derived from 20m digital contours. Geographic Information System (GIS) rules were used to identify wetland soils, based on soil survey descriptions that included drainage properties and the presence of peat and wetland vegetation. Soil drainage is divided into five classes in the FSL, from poorly drained (class 1) to well-drained soils (class 5). Soils in classes 1 to 3 were considered to have a high probability of having been wetland.

    The contemporary extent was mapped using 26 Landsat Enhanced Thematic Mapper (ETM+) satellite imagery, and wetland point and polygon data. The data were collated from recent surveys, field work, or photo-interpretation held by local and central government. Point and polygon data were checked against the satellite imagery, and the wetland boundaries were corrected or delineated using the imagery (Ausseil et al, 2008).

    Data quality

    Topic Classification   Relevance Accuracy 
     Freshwater ecosystems and habitats  Case study

     

    Direct

     

    Medium

     See Data quality information for more detail.

    References

    Ausseil, A-GE, Gerbeaux, P, Chadderton, WL, Stephens, T, Brown, DJ, & Leathwick, J (2008). Wetland ecosystems of national importance for biodiversity: Criteria, methods and candidate list of nationally important inland wetlands. Landcare Research Contract Report LC0708/158 for the Department of Conservation.

    Johnson, P & Gerbeaux, P (2004). Wetland types in New Zealand. Available from www.doc.govt.nz.

    Resource Management Act (1991). Available from www.legislation.govt.nz.

    Archived pages

    See Wetland extent (archived April 2017).

    Updated 27 April 2017

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