NT's mangroves under threat 


The mangroves of Darwin Harbour, considered among the richest in the Asia-Pacific region, are at risk from rising sea levels, increased cyclones, urban development and, surprisingly, hungry turtles.

Dr Kristin Metcalfe (pictured), who was awarded her PhD at Charles Darwin University’s October graduation ceremony, has just completed one of the most comprehensive studies of animal diversity of any mangrove forest in the world.

Her thesis documented the distribution, diversity, and abundance of fauna in the mangroves of Darwin Harbour. The research also assessed the impacts of urbanisation on mangrove biodiversity.

It also examined some of the factors delaying the recovery of mangrove forests damaged by urban expansion and cyclones.

Several field methods for forest rehabilitation were developed during the project which may fast-track recovery of these important ecosystems.

With some global warming specialists predicting an increase in extreme weather events, including cyclones, Dr Metcalfe’s research may be significant for rehabilitation of mangrove ecosystems across the world.

She said Cyclone Tracy’s destructive path through Darwin’s mangroves in 1974 was still evident.

'More than 30 years later, there is still a very clear gap where the tallest forests grew before the cyclone,' she said.

Dr Metcalfe said mangroves were critical for a healthy estuarine environment, as these highly productive forests buffer the coastline from strong winds, waves and erosion, while supporting an extraordinarily diverse, specialised and abundant animal life.

Northern Australia was fortunate to have such large areas of relatively pristine mangrove forests, she said.

Areas of South-East Asia including Indonesia and the Philippines have lost more than 70% of mangrove cover through human activity.

‘People don’t realise that these buffers between land and sea perform many vital ecological roles and functions as well as being an important feeding ground for many fish,’ she said.

During a two-year experiment, she tested the effects of shade, damage from floating debris and the feeding habits of animals in bulldozed and cyclone-damaged mangroves.

Dr Metcalfe found the recovery of the Darwin mangroves was hampered by sea turtles eating mangrove seedlings in damaged areas close to the seaward edge.

‘I could virtually see a grazing line where the turtles had been feeding on seedlings at high tide,’ she said. ‘In mangroves further up the shore, recovery was delayed by strong tidal currents, which washed away floating seeds.’

Predicted rises in sea levels will also mean the research on faunal diversity will be a benchmark for assessing future changes in Darwin Harbour.

Mangroves in Darwin Harbour generally occur in four distinct zones known as the hinterland margin, tidal flat, tidal creek and seaward zones.

More than 70 species of birds, 24 mammals and 305 invertebrates – including worms, crustaceans and molluscs – inhabit Darwin’s mangroves.

Each zone has its own distinct flora and fauna that could be destroyed by an increase in sea levels.

‘Vegetation and invertebrate species are specific to each zone and could be wiped out by a rapid rise in sea level,’ she said.