The mangroves of Casamance, Senegal

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This image shows the mangroves in the estuary of the Casamance river in Senegal. The resolution is 20 m and the image was acquired by the HRV sensor aboard the SPOT 4 satellite.

What you see are not the true colours as it consists of the values recorded in the mid infrared, the near infrared and the red spectral bands. The urban areas are visible in pink, water in blue, the vegetation on land in light green and the mangroves in dark green; areas that are subject to regular flooding appear in purple.

Over-harvesting and decreasing rainfall threaten mangrove biodiversity.

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Mangroves are plant species that grow in shallow and protected coastal areas in tropical regions, in the tidal zones, such as estuaries, deltas, lagoons and coastal lakes. They grow in conditions where few other plants can survive: high salinity, soft substrate and lack of oxygen.

The mangrove tree is the dominant species. It survives in these hostile environments with the help of physiological adaptations, such as aerial roots that give it some unique features.

Mangroves are fragile ecosystems that must be protected because they play a crucial ecological and socio-economic role.

Mangroves protect and stabilize coastal areas

Through their root system, mangroves prevent coastal erosion and provide an effective barrier against the winds, waves and ocean currents. They protect the coast and its inhabitants against cyclones, storms and tsunamis. By stabilizing the sediment, they also prevent silting of coral reefs, sea grass beds and shipping lanes.

Mangroves are reservoirs of food and raw materials

Mangroves provide conditions and unique habitats which are inhabited by a large number of animal species: mammals, reptiles, amphibians, birds, and especially fish, crabs, shrimps and molluscs, for which they provide favoured breeding sites. Mangroves, although amounting to just 0.4% of the world's forests are indeed essential to the life cycle of most commercial fish species worldwide.

In their capacity as an extraordinary reservoir of wildlife, mangroves provide a major source of protein to many coastal populations, particularly in developing countries. It supplies these people with firewood and wood for building, even for consumption and medicinal purposes plants are found that are essential to their survival.

Mangroves are threatened

The steady increase of the population in coastal areas puts pressure on these fragile ecosystems. Many of mangrove forest zones are destroyed for the purpose of the production of rice or salt, aquaculture or property development.

The decrease in precipitation during the last 20 years is an additional threat to mangroves, which need relatively large amounts of rainfall to prosper.

As a result, over 3 million hectares of mangroves have disappeared in the past 30 years. In 2010, the total surface covered by mangroves was reduced to 15.6 million hectares.

The protection of mangroves requires a thorough knowledge of these ecosystems and their spatial distribution. For 15 years, remote sensing has played a crucial role in the study of mangroves and has established an almost complete overview of the status of these ecosystems worldwide.