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An intertidal forest. Mangrove Trees spend half of their lives with its roots submerged.
Photo: Genevieve Johnson

November 30, 2001
Intertidal Forests
  Real Audio

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey as it makes its way down the remote coastline of Western Australia toward Shark Bay. The other day, some of the Odyssey crew went out to explore one of the many tropical estuary systems found in Northern Australia. An estuary is the seaward end of a river where the fresh water mixes with salt water. The first thing we noticed was the thick forest of mangrove trees. These trees are unlike any other, growing between high and low tide in a region known as the intertidal zone. They grow from muddy soil, spending half their lives with their long roots submerged.

Very little is actually known about mangroves and their forest ecosystem. We know that they are an integral component of the marine environment. However, the mangrove ecosystem is different from adjacent systems, such as coral reefs and the open ocean, but is critically linked to and totally dependant upon them.

Mangroves are the coastal equivalent of tropical forests on land and are among the most productive ecosystems in the world. They are usually formed by between 3 and 8 species of woody trees and shrubs and are an excellent example of convergent evolution - meaning that many families have all adapted in a similar way. They are essentially the primary producers of the intertidal zone. They are the backbone of the living system, providing leaves and timber for herbivores (plant eating animals), which in turn are preyed upon by the carnivores (meat-eating animals).

Many different terrestrial as well as marine animals use the large root system of the mangroves as shelter, zones for mating and nursery areas. We know that many species of both commercial and traditionally important fish begin their life among the web of mangrove roots are later harvested as offshore adults.

Some species are residents that live there year round such as crocodiles and snakes, while others such as turtles and jellyfish are transients or short-term visitors depending on the season. This in turn invites larger predators into the mangroves to feed. Flying foxes eat fruits, while birds and reptiles eat insects, small fish and crustaceans that hide in the shallows at low tide. The high tide then brings in the large fish to feed on them. Top predators, such as saltwater crocodiles as well as sport fisherman, feed on the large fish. As we observed in this particular river, not even the apex predators are safe, with traps set to remove crocodiles that may pose a potential threat to humans from the area.

The above ground roots (called 'prop' roots) that make mangroves so distinctive, capture oxygen, an adaptation that allows the plant to survive in water. They also anchor the trees and prevent erosion of the soil, in fact, they are actually able to catch soil, effectively reclaiming the land from the sea. They can also help protect local communities against hurricanes and floods.

A crocodile trap set among an intertidal forest.
Photo: Genevieve Johnson

For many centuries, mangroves have provided a multitude of resources to the local population. It has been established that a single hectare of mangrove forest generates between 1,100 and 11,800 kilograms of fisheries catches per year, much higher than the 10 - 370 kilogram average for coral reefs. However, mangrove forests are facing an uncertain future and are currently being lost at an alarming rate around the globe. Presently, mangroves cover an area of 181,000 square kilometres throughout more than 100 countries in the world's tropical regions, but over the last 50 years, more than 50% have been lost.

They are removed to make way for housing developments and tourist resorts due to a rapidly expanding population. While chemical pollution and overfishing are all having a negative impact on the remaining intertidal forests and the oceanic ecosystems to which they are connected. By far the greatest force behind the appalling loss of mangroves over the last decade has been their conversion to breeding ponds for shrimp farming in order to generate a single product with a high market value. The products derived from the loss of these unique ecosystems can be found at dinner tables in Europe, Japan and the United States. Due to the support of governments and grants from bodies such as the World Bank, shrimp industries continue to emerge in developing countries throughout South East Asia and Central America. Unfortunately mangrove systems, together with the traditional livelihood of hundreds of coastal fishing communities, are being lost forever for the sake the shrimping industry. Ironically, we are now seeing these shrimp ponds drown in their own contamination and abandoned, leaving a destroyed ecosystem.

Mangrove forests, like all ecosystems, are impossibly complex. We are in danger off losing their unique web of flora and fauna forever at their current rate of removal. Some species can only survive in mangroves and nowhere else. If managed properly these intertidal forests could be valuable producers for humans and its ecosystem for many years to come.

Log by Genevieve Johnson

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