Published on March 28th, 2008 | by Beth Bader3
Mangroves Are a Critical and Threatened Marine Resource
March 28th, 2008 by Beth Bader
Clearing of mangroves in the name of land reclamation — land to be used for shoreline development — is a worldwide activity. According to University of Virginia and University of Georgia scientists William Odum and R.E. Johannes, more acres of mangrove may have been cleared worldwide than any other type of area except desert, an estimated “many hundreds of thousands of hectacres.” Clearing often results in high, long-term costs of battling increased erosion.
Mangroves are not areas to be “reclaimed.” They not only belong where they are, but they are actively reclaiming and preserving land on their own. The whole process is cyclical, beginning with the mangroves’ role as recyclers, and ending with protection of the outlying coral reefs.
The Role of Mangroves in Protecting Coral Reefs
The reclaiming process is as simple as it is vital. The mangrove roots act as natural filters, trapping sediment and run off from the land, thus preventing or slowing erosion.
Removal of mangroves causes a chain reaction. If the mangroves are cleared the sediment and pollutants run unchecked into the sea grass beds. The nutrient-rich effluent promotes the growth of phytoplankton and filamentous algae, which, combined with the sediment clouds the shallow water and blocks out light necessary for sea grass beds to flourish. The environment becomes anoxic, and the sea grass ecosystem dies.
The combined filtration of soil-runoff and pollutants by healthy mangroves and sea grass beds help keep the water clear of sediment. Sedimentation is a primary cause of coral reef destruction. The reason most mangroves are removed is for shoreline development and that development further increases the amount of sediment runoff from shore. Outlying coral reefs are now placed in double jeopardy.
Coral reefs, in turn, protect mangroves: their presence buffers the strength of waves and currents that could uproot the vulnerable marine plants inshore.
The Foundation for a Vital Marine Ecosystem
Mangroves not only preserve land, but they recycle themselves as well. Leaves from the trees fall into the water, sink, rot and become a rich source of both nutrients and that characteristic swamp smell. It is estimated that one acre of red mangroves sheds over 3 tons of leaves per year. Many organisms feed on the rotting leaf litter, also called detritus. Other species that use the mangroves as a nursery habitat will later populate the reefs and open waters, including sharks and other predators in reef ecosystems. All of these species feed on the detritus-eaters and small marine species associated with mangroves, and live as juveniles in the protected environment of dense mangrove roots.
In some areas, mangroves are protected, but trimming is permitted. Trimming, while somewhat better than total removal, cuts productivity of the mangrove ecosystem by reducing the amount of leaf litter, or detritus, produced. The impact of mangrove trimming was the subject of a study by Jim Beever of the Florida Department of Natural Resources. In the area of the study, trimming 2/3 of the mangrove height cut productivity by as much as 84% initially. Sightings of the visible animal life decreased by 79% after trimming.
In a natural state, mangroves are specialists at survival. The leaves of the plant have a waxy texture like the leaves of desert plants. This evolved leaf design allows mangrove trees to retain moisture and excrete salt, but at a high “energy cost” to the tree. Tannin in the mangrove bark makes the leaves inedible for most would-be predators — except humans.
Because of the degree to which mangroves are specialized, they are extremely vulnerable to change and cannot adapt well. Unable to cope, adapt, or re-colonize quickly under stresses, the delicate mangroves are lost. Gone also, is the base of the ecosystem and the protected nursery grounds for marine life.
A Rich and Fascinating Biodiversity
Perhaps a mangrove forest does look like wasteland to some. It’s true that it draws more mosquitoes than tourists. But life at the base of the trees is fascinating. Mangrove habitats possess a certain subtle beauty. To really appreciate them, mangroves should be viewed from the roots, or rather prop roots. Here, space, not food, is the limited resource. Barnacles, clams, snails and oysters live in a competition for living quarters.
Some of the most prevalent prop root inhabitants are species of tunicates (about 30 different species in the Caribbean alone). Some species look a bit like sponges, even more like lumps of two-month old pudding with openings. Some species are crust-like. All species look like stuff I wouldn’t want to step on if I saw it on the sidewalk.
Yet, underneath this slightly disgusting exterior beats a genuine heart next to intestines and a stomach. This lump is no spineless sac of animal matter, it is a full-fledged Chordate. It’s notochord, similar to our spinal cord, is a feature found only in the larvae. The chord disappears along with the larvae’s muscular tail as the adult forms.
Above the peaceful underwater habitat, it’s anything but quiet. Roosting in the mangrove trees are several species of marine birds, including egrets and the magnificent blue heron. Lizards and land crabs thrive. Mangroves are the home of the largest land crab of North American and Caribbean waters. A large male can wield one 12″ pincher.
At night, it’s a shark-eat-shark world as larger sharks come in to feed on the fish and the smaller juvenile sharks that inhabit the relatively protected mangroves. Other predators like stingrays and barracuda can be seen cruising the flats on the edge of the mangroves in search of midnight sashimi.
Mangroves are a place rich in life, a far cry from “wasteland” awaiting bulldozers and concrete. The ecosystem itself is a primary link in the marine environmental chain, a vulnerable link which needs protecting just as much as the coral reefs. The two are linked, and you cannot hope to protect one without also protecting the other.
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