By Matt Walker
Editor, Earth News
Mangrove forest seen from under the water's surface
There is 13% less mangrove forest around the world than thought.
The unexpected decline in this unique coastland habitat is revealed by the first global satellite survey of mangroves, which grow in intertidal regions of the tropics and subtropics.
Mangrove cover has previously been calculated by collating reports from individual countries, say scientists.
But inaccuracies in this system have led to estimates of surviving mangrove forest being inflated.
Details of the new estimate of mangrove forests are published in the journal Global Ecology and Biogeography.
"I was expecting more mangroves than we found," says Dr Chandra Giri, the principal scientist at the International Land Cover and Biodiversity project run by the US Geological Survey based at Sioux Falls, South Dakota.
Mangrove forests are a unique habitat, growing in coastal areas of high salinity, temperature, sedimentation and muddy soils that contain little oxygen.
As well as providing breeding and nursing grounds to many marine creatures, they play an important role in stabilising shorelines, helping to prevent damage from natural disasters such as tsunamis and hurricanes.
The previous best estimates of mangrove forest cover were made by either using low resolution satellite images (taken at a resolution of 1km), or by compiling disparate and incompatible datasets collected by individual countries.
The new survey uses photographs taken by the Landsat satellite as part of the Global Land Survey, and archive images taken by the same satellite. The survey is prepared by the US Geological Survey and NASA.
Crucially, the Landsat photographs are accurate, being taken at a resolution of just 30m, and free from clouds, offering a clear view of the forest below. Mangrove forest is also distinct in satellite photographs, meaning it is relatively easy to measure and classify.
That allowed Dr Giri and a team of colleagues based in the US, Australia and Kenya to compile the first high resolution global map of mangrove forests.
According to their survey, the total amount of mangrove forest in the world in 2000, the most recent available date for which data is available, was 137,760 km² in 188 countries and territories.
The largest tract of remaining mangroves in the world: the Sundarban mangroves on the border between Bangladesh and India (in Red)
That is 12.9% smaller than the most recent previous survey conducted by the UN Food and Agriculture Organization (FAO).
The total mangrove area accounts for 0.7% of the total tropical forest in the world.
"We need to preserve the remaining mangrove forests with urgency otherwise they might disappear," says Dr Giri.
"13% is significant, especially as it is disappearing faster than inland tropical forests."
"Globally, between 20% and 35% of mangrove area has been lost since approximately 1980, and mangrove areas are disappearing at the rate of approximately 1% per year, with other estimates as high as 2-8% per year," Dr Giri warns.
Most mangroves (42%) are found in Asia, followed by Africa (20%), North and Central America (15%), Oceania (12%) and South America (15%).
Approximately 75% of all mangrove forests are found in just 15 countries, and under 7% are currently protected.
The new survey confirms that most mangroves are confined to the tropics and sub tropics and the largest percentage of mangroves is found between 5º N and 5º S.