Threats & Pressures
Top - Mangroves respond to deteriorating conditions with dieback and loss of habitat.
Top - Avicennia marina (lighter trees) has been specifically affected by agricultural runoff in the Mackay region, Queensland.
The arrival of Europeans in the eighteenth century saw the alteration of estuaries in southern and eastern Australia. As population growth increases along Australia’s coastline there has been a corresponding increase in usage and development. Coastal natural resources including mangroves have come under increased risk, particularly near large urban centres. Degradation of mangrove habitat by the direct loss or alteration of trees reduces its capacity to function effectively as a viable ecosystem. This in turn endangers the species that depend upon the healthy mangrove ecosystems. While it is currently estimated that none of Australia’s mangrove species are at risk, the coastal zone they occupy has the greatest number of threatened species. One example is the rare and endangered Rusty Monitor, Varanus semiremex - a small crab-eating goanna restricted to hollow trees in mangroves of north eastern Queensland. One of the few mangroves to have suitable hollows for this monitor, is Avicennia marina, the species threatened most by chemicals in agricultural runoff. The chief threats to mangrove habitat come from: conversion and landuse change and the indirect effects of sediments and chemicals in runoff from catchments degraded by clearing of upland vegetation and intensive agriculture.
Conversion and Landuse Change
Around 17% of Australia’s mangroves have been destroyed since European settlement. Mangroves near developing centres have been systematically destroyed and damaged. Moreton Bay, for example, is situated near the city of Brisbane where an estimated 20% of the pre-European mangrove area has been subject to reclamation landfill.
Compared with other countries, however, these impacts are relatively low. This is largely because most of Australia’s mangroves are located in the more sparsely populated northern regions of the country, like north Queensland, the Northern Territory and northern Western Australia. Mangrove forests in these regions have largely remained pristine, or in near pristine condition. But, where development has taken place, then the effects on mangroves is usually severe. For example, the development associated with the expansion of the Northern Territory’s major population centre of Darwin has resulted in the clearing of 2% of the pre-European area of mangroves in the harbour area.
The Rusty Monitor, Varanus semiremex, is rare and threatened because it depends on mangroves. Photo: Kieren Aland
Indirect Impacts of Sediments in Runoff
Increased sediment loads in catchment runoff are affecting mangrove distributions within estuaries. In recent decades, there have been unprecedented gains in mangrove areas at the mouths of at least four Queensland river estuaries, including Trinity Inlet, Pioneer River, Johnstone River, and Fitzroy River. It is expected that the pattern is similar in other states. The increase in such mud banks is indicative of increased clearing of catchment vegetation, and the construction of barrages and dams.
Indirect Impacts of Chemicals in Runoff
Agricultural chemicals in runoff appear to affect mangrove health. In five adjacent estuaries in the Mackay region of Central Queensland, more than 30 km2 of mangroves have been affected by the severe dieback of Avicennia marina. Twenty other species appeared unaffected, but A. marina occupied about 50% of the total mangrove area. Correlative evidence implicated herbicides used in sugar cane production as the most likely cause of this dieback. Key indicators of mangrove plant health were correlated with diuron concentrations in sediments, and planthouse trials demonstrated that salt-excreting mangroves like A. marina were more affected by herbicides than the more salt-excluding species.
- Landfill of mangrove areas continues in developing industrial areas like Port Curtis in Queensland.
- Depositional banks like those in the Fitzroy River, Queensland, are rapidly colonised by mangroves.