Australia State of the Environment Report 2001 (Theme Report)
Australian State of the Environment Committee, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06751 5
Environmental indicators reported in this section:
|CO 1.1 | a | b |||Marine species, rare, endangered and threatened|
|CO 1.3 | a | b |||Seabird populations|
|CO 2.3||Coral reef area|
|CO 2.4||Dune vegetation|
|CO 2.7||Mangrove area|
|CO 2.8||Saltmarsh area|
|CO 2.9||Seagrass area|
|CO 3.6||Fish populations|
Australia's marine and estuarine habitats are diverse in nature, vary greatly in scale, and are interconnected in a variety of ways.
Coastal and estuarine habitats are closely linked with upstream catchments and the associated land uses, all of which have an effect on the condition of these habitats. The concept of a land water continuum is also well established in the Indigenous world view.
The following sections describe the current condition of significant coastal and marine habitats and the species that frequent them, where data and information permit. Where these habitats or species are under pressure, the pressures are described and any response to these pressures is reported.
Many of the indicators for habitats are for the areal extent of a particular habitat. However, the surface area of a viable habitat type is recognised as a very crude measure of ecological health or ecological value. Similarly, the protected area of a habitat does not give a definitive indication of the area that should be protected or be managed in different ways.
Comprising a diverse group of largely tropical trees and shrubs, mangroves live in the intertidal areas of sheltered marine shores, estuaries and tidal creeks.
Mangroves are our marine forests and are highly productive ecosystems. They are a vital habitat and nursery area for juvenile fish, crabs and prawns. Some Indigenous foods are obtained from mangrove environments, e.g. boring bivalves (Teredo), mud clams (Geloina) and mud crabs (Scylla). Mangroves also provide a natural barrier to storm surges; in more sheltered areas they stabilise large areas of mud and sediments, while in the tropics they also stabilise exposed low-slope coastlines.
Australia has the third largest area of mangroves in the world, totalling 11 500 km2 (Zann and Kailola 1995). The 43 species found in Australia represent 58% of the world's mangrove diversity, with one species occurring widely in southern Australia. The highest species richness occurs in the wet tropics.
For mapping purposes, mangrove populations are essentially linear features of the coastline. In these small maps it was necessary to enhance their display at the continental scale. Figure 4 represents total richness of mangrove species across a geographic segment of coastline. The actual distribution of mangroves around the coast is displayed in Figure 5. The two maps should be viewed in conjunction. A symbolic linear representation has been employed in Figure 5 to heighten visibility. Data for all patches of mangroves greater than 1 km2 in area, including those on islands, were mapped. Despite this enhancement the smallest areas mapped are not visible in a map of this size. These maps were compiled from all the data available, but mangrove mapping has not been completed for the whole of Australia.
Figures 4 and 5 indicate that mangrove species richness and area of mangroves are greatest in the northern part of the country and are closely related to both lower latitudes and greater rainfall. At present, no species are regarded as threatened so the area of mangroves (Figure 5) is a more critical indicator of ecosystem health.
Figure 4: Mangrove species richness for continental Australia.
Source: Environment Australia (2000)
Figure 5: Indicative mangrove distribution in continental Australia.
Source: Environment Australia (2000)
The area of mangroves appears to have increased naturally at a number of locations, usually colonising areas of sediment accretion, as in Trinity Inlet, Cairns (Wolanski and Duke 2001).
The major causes of the reduction in mangrove habitat are clearing for industrial development, agricultural and urban expansion, and changes to drainage. Significant habitat reduction has occurred near several major cities, and localised destruction has occurred near many smaller coastal cities and towns. Mangroves at the southern end of the range, for example in Port Phillip Bay and Barwon Heads, are at risk. Population growth in rapidly expanding areas of the coastal zone continues to be the major pressure upon mangroves.
Some of Australia's mangroves are protected through such means as marine protected areas or fish habitats. In Queensland, for example, there are currently 79 declared Fish Habitat Areas covering 603 000 hectares of tidal wetlands (Beumer et al. 1997).
The consequences of local destruction of mangroves are lower fish productivity and loss of water quality.