The role of marginal vegetation in some waterfowl habitats

[First paragraphs...]Because of the demand for greater production, the farmers' attention is constantly being directed toward those areas of their land which are non-productive. Inevitably wetlands—swampy gullies, lagoons or lakes—come under close scrutiny. Soils underlying wetlands are usually very fertile and some of New Zealand's best dairy lands have been derived from them. Balham (1952) recorded the extent of this development in the Manawatu district at that time. In a submission to the Peatlands Committee in 1965, the Wildlife Service outlined the scope of wetland drainage in the Waikato and Hauraki Basins—one of the major waterfowl areas of New Zealand. According to this survey, 118.000 acres of Crown- and privately-owned swamp remained in this area; but 74,000 acres (63%) were set aside for immediate drainage or were being drained. Another 12,000 acres would be drained at a later stage when projects became effective. And although much of this massive swamp was considered good water-bird habitat, by the time present schemes are completed barely 8,000 acres will remain; and there is no certainty about the future survival of this area.
Waterfowl are not losing their habitat solely by drainage, however. Natural ecological succession is playing an important role. The encroachment of semi-aquatic plants, of which raupo (Typha muelleri) is by far the most aggressive, onto established water areas has converted many previously ideal waterfowl habitats into useless bogs. Raupo appears to be limited in its spread only by water depth, movement and salinity. Since World War II, concurrent with a major increase in the use of agricultural fertilizer, raupo has made a dramatic and widespread appearance on wetlands throughout New Zealand—to the extent that it has become an economic problem in some places through blocking waterways.