Wetland extent

  • Image, Wetland extent.

    Wetlands support high levels of biodiversity. They provide habitat for indigenous invertebrates, plants, fish, and bird species (eg fernbird, kōkopu, and eels), many of which live only in wetlands. Wetlands act as ‘kidneys’ and giant sponges – they clean the water of excess nutrients and sediment, control flood water and pollutants, and act as carbon sinks (removing carbon dioxide from the atmosphere). Wetlands have strong cultural and spiritual importance for Māori. They are a food source (eg eel, whitebait) and provide material for weaving (eg raupō, harakeke (flax)). Draining wetlands for agricultural and urban development over the past 150 years has led to significant wetland loss and deterioration. We report on the historic losses of wetlands from pre-human time to 2008, and the contemporary losses between 2001 and 2016.

    We classified historic wetland extent as a case study.
    We classified contemporary wetland extent as supporting information.

    Key findings

    Trend not assessed

     

    By 2008, wetlands had declined to 10.1 percent (about 250,000 ha) of the estimated pre-human extent of 2,471,080 ha – especially swamps. Analysis of change between 2001 and 2016 shows that decline continues and varies across regions.

    • Wetlands covered 0.9 percent of New Zealand's land cover, compared with an estimated 9.2 percent of land cover in pre-human times. Swamps have been reduced to 6.0 percent (89,920 ha) of their original extent (1,501,000 ha).
    • Of the wetlands we tracked between 2001 and 2016:
      • 1,247 hectares (0.5 percent) of total wetland area were completely lost over the period – 214 individual wetlands

      • 5.4 percent (746 individual wetlands) of total wetlands has experienced partial loss, but we cannot report how much loss in area this represents

      • 54.2 percent (132,954 ha) of our total 2016 wetland area did not change over the period – representing 80.6 percent of wetlands (11,063 individual wetlands).

    • We cannot report on 32.5 percent (79,646 ha) of wetland area – representing 12.4 percent of wetlands (or 1,699 individual wetlands) – because of improvements needed in spatial mapping.
    • The regions with the highest proportions of wetland loss are: Gisborne (3.1 percent), Marlborough (2.6 percent), West Coast (2.6 percent), and Canterbury (2.3 percent).
    • The regions with the greatest areas of wetland loss are: Waikato (13.4 percent; 329 ha), West Coast (12.5 percent; 308 ha), Southland (11.6 percent; 284 ha), and Canterbury (4.2 percent; 104 ha).

    Figure 1
    Interactive app

    Map showing wetland loss in New Zealand.

    View interactive app in full-screen mode.
    Figure 2

    Figure 3

    Note: The ‘unknown loss or change’ category includes wetland polygons identified as needing line work improvements in the Waters of National Importance (WONI) spatial data. We excluded wetland polygons flagged as errors in the WONI layer from analysis.

    Figure 4

    Note: The ‘unknown loss or change’ category includes wetland polygons identified as needing line work improvements in the Waters of National Importance (WONI) spatial data. We excluded wetland polygons flagged as errors in the WONI layer from analysis.

    Figure 5

    Note: The ‘unknown loss or change’ category includes wetland polygons identified as needing line work improvements in the Waters of National Importance (WONI) spatial data. We excluded wetland polygons flagged as errors in the WONI layer from analysis.

    The ‘partial loss’ category does not represent the total area that is actually lost (it could be a small amount or the entire amount of the individual wetland); it is the total area of wetlands flagged as being affected by partial loss from drainage or development (ie the wetland is getting smaller).

    Figure 6

    Note: The ‘unknown loss or change’ category includes wetland polygons identified as needing line work improvements in the Waters of National Importance (WONI) spatial data. We excluded wetland polygons flagged as errors in the WONI layer from analysis.

    The ‘partial loss’ category does not represent the total area that is actually lost (it could be a small amount or the entire amount of the individual wetland); it is the total area of wetlands flagged as being affected by partial loss from drainage or development (ie the wetland is getting smaller).

    Figure 7

    Map, Wetland extent, pre-human and 2008

    Definition and methodology

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

    The historic extent of wetlands (pre-human) was estimated from the national Fundamental Soil Layers (FSL) database, and refined using a 15m digital-elevation model derived from 20m digital contours. We used Geographic Information System (GIS) rules 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 have a high probability of having been wetland.

    Manaaki Whenua - Landcare Research mapped the 2008 wetland extent 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).

    Manaaki Whenua – Landcare Research identified recent declines (2001/02 to 2015/16) in wetland extent by visually comparing the Waters of National Importance (WONI), as mapped for 2001/02 from Landsat imagery, against Sentinel-2 satellite imagery in 2016. Where there were issues with the original WONI wetland polygon, it was assigned ‘improve linework’ or ‘error’. Where a wetland polygon was identified as completely lost and needing improvements in the polygon linework, it was assigned to the ‘complete loss’ category. We excluded wetland polygons flagged as errors from the analysis. The visual comparison only captures wetland loss, and does not capture new wetlands, increases, or changes in wetland condition.

    New Zealand is not currently able to report wetland extent with great accuracy, nor to quantify the rate of continuing loss or measure quality change. We intend to improve measurement of all freshwater wetland types – some are excluded from this measurement (eg geothermal wetland), and small or heavily wooded areas may be missing from the data. The methodology used shows the approximate magnitude and extent; however, this is likely to underestimate wetland loss and extent.

    Data quality

    Topic  Measure  Classification  Relevance  Accuracy 
     
    Freshwater ecosystems and habitats 
     
    Historic wetland extent Case study 

    Direct

    Medium

    Contemporary wetland extent Supporting information 

    Image, Partial relevance.

    Partial

    Low-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 (PDF, 6MB). Landcare Research Contract Report LC0708/158 for the Department of Conservation. Retrieved from www.researchgate.net.

    Belliss, S, Shepherd, J, Newsome, P, & Dymond, J (2017). An analysis of wetland loss between 2001/02 and 2015/16. Landcare Research Contract Report LC2798 for the Ministry for the Environment.

    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 2018).

    See Wetland extent (archived April 2017).

     

    Updated 19 April 2018

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