Mangroves—Seafaring Land Builders
WITH their maze of tangled roots gripping the mud and sand of tidal flats, mangroves do a remarkable work of land building. They also provide an important link in the food chain of land and marine creatures that find a haven in their protective covering.
Mangroves are found practically everywhere in the tropics—in Africa, on the shores of Central and South America, and on coastlines of countries in the Indian and western Pacific Oceans. One of the most outstanding mangrove forests in the world is found in southern Florida (U.S.A.), where they blanket more than 700 square miles (1,800 km2) of coast and thousands of islands.
From the air, mangrove forests look like an evergreen wilderness of islands and estuaries—all interwoven with shallow bays, lagoons and creeks.
From the surface, however, the picture is entirely different. They present an almost impenetrable wall of twisted and intermeshing trunks and roots. In mature or older mangrove swamps, the root systems are jumbles of arches. Rising from the watery surface, the roots cascade in so many directions that it is quite difficult to determine where one root begins and another one ends.
Red Mangroves
The forest is dominated at sea edge by the red mangrove. It is the pioneer builder of this unique landscape. Because of the adaptability of these mangroves to different coastal environments, they offer a contrast in shape and size. Some are tall and have straight trunks reaching heights of 80 to 100 feet (24 to 30 m). Others are domed shrubs held aloft by their network of roots. Another variety may grow horizontally, rerooting as it moves parallel to the ground.
Behind the red mangrove line and growing on higher ground are several unrelated trees. But these trees are also called mangroves, loosely so.
Ecologically Important
Viewing mangroves up close may cause one to wonder how they can be anything but useless. Generally, they are infested by hordes of mosquitoes and are responsible for a strong swamp odor. However, early explorers of the Americas, India, and the East Indies noted that people used the mangrove for food, the bark as a cure for diabetes and burns, and the roots as a sedative. Man has sought the tannin in the bark for dyeing and the mangrove wood for charcoal production.
During the past decade environmentally minded scientists discovered that mangrove forests represent a complex ecosystem. This has helped to dispel the view existing in some industrialized nations that mangroves are a nuisance to man and a hindrance to land development. Now scientists know that mangroves help to preserve and expand the land. At the same time, the forests create an environment important to the welfare of a host of land animals, birds and marine creatures.
The seedling of the mangrove is the vanguard of the land-building activity. When released by the parent tree, the seedling is a rudimentary tree. It may take root in the shallow waters below, or drift into the sea current for a voyage that could take it thousands of miles away from the parent tree before getting lodged on an oyster bar or a mud flat. If necessary, the seedling can survive afloat about one year.
Once it gains a foothold, it sends out arching fingers that quickly become a mass of contorted prop roots. These roots support the trunk of the tree above the seawater, bring up sap, and supply oxygen not found in the mud. They also act as a strainer for sea debris—shell and coral fragments, driftwood, seaweed and soils. When the debris consolidates, an island is born, one perhaps only a few square feet in width. As more seedlings are produced by the solitary mangrove, they may take root in the mounting debris below. These add more debris-catching power to the spreading prop roots.
After a few decades, an island may attain considerable size, rising several feet above sea level. Over the years, the sea-voyaging mangroves may even link island to island or an island to the mainland. In the meantime, a variety of wildlife seeks shelter under them.
Wildlife Boardinghouse
The arching roots from the tree trunk and limbs become a boardinghouse. On the roots are myriads of white-shelled coon oysters and swarms of crabs. Within the forest, raccoons, frogs, turtles, alligators, snakes, wildcats, deer and other creatures forage. In the branches, terns, pelicans, eagles, storks, cormorants, egrets, ibises, herons and other tropical birds find sanctuary.
One mangrove island in the famed Rookery Bay Sanctuary on the southwest coast of Florida has become a birdwatcher’s favorite because of the huge flocks of different birds that roost there at night. The island becomes so loaded with white plumage by evening that from a distance it appears to be covered with snow.
Food Factories
Besides being a haven for wildlife, mangrove forests are veritable food factories. Biologists in Florida have observed that creatures feed on the leaves while they are still attached to the trees, and especially after the leaves fall and start to decompose.
To speed the breakdown, hungry crabs and scavenging amphipods no bigger than a grain of rice shred the leaves. The leaf particles become covered with bacteria and fungi that are rich in vitamins and protein.
As these leaf particles are chopped smaller and smaller, diminutive sea creatures feed on the bacteria and fungi. The indigestible cellulose bit of the leaf is excreted and subsequently becomes a vehicle for a whole new colony of bacteria and fungi.
This food web supplies at least 80 to 90 percent of the nutrition for several varieties of crabs, worms, insect larvae, shrimp and fish. In turn, these creatures are themselves food for many other species of fish, including important commercial varieties.
Though man’s knowledge of many of these things in relatively recent, the value of mangroves to the earth’s cycle of life reflects a creative wisdom that existed long before humankind.