Conservation of coastal wetlands of the Northern territory of Australia: The Mary River floodplain

Abstract The seasonally inundated coastal floodplains of the Northern Territory contain Australia's largest relatively unmodified wetlands. Past exploitation has been confined chiefly to extensive pastoralism, but that situation is changing rapidly. We report a floristic survey of the Mary River floodplain, and compare among river systems the vegetation pattern and factors influencing it. Management and conservation implications of changing land use on the Mary River and elsewhere are examined. Flora is cosmopolitan and plant communities relatively simple. Water depth and persistence of floodwaters determine vegetation patterning. Distribution and relative abundance of vegetation types within and between plains is influenced by an interaction between erratic rainfalls and inter-plain variation in topography and hydrology. Catchment differences in soils and hydrology add to inter-plain variation. But the conservation significance of the floodplains is founded neither in floristic uniqueness nor unusual plant communities. In combination the plains reliably provide large areas of favourable wildlife habitat. Diversity is maintained despite erratically variable rainfalls by temporal and spatial shifts of favoured habitats among and within river systems. Sustained diversity maintains larger populations than could persist under a uniform vegetation response to below average falls. Increasingly manipulative land management practices, introduced plants and saltwater intrusion threaten to reduce the total area of unmodified freshwater habitat, weakening the resilience of the ecosystem to natural environmental perturbation such as delayed or reduced rainfalls. The nature of the changes demand that ‘passive’ conservation measures such as creation of reserves be supplemented by active management practices. An ecosystem-wide approach to management is required to achieve conservation goals, especially maintenance of habitat diversity.

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