MaNGROVE THREATS
Human threats to mangrove ecosystems may be loosely divided into two categories:
consumptive, in which whole trees, or parts of them, are cut for
products such as timber, fuel, or pulp; and
non-consumptive, in which a forest is threatened by human activity
other than direct exploitation of the trees themselves (see below). Adverse effects on mangrove ecosystems as a result of both consumptive and non-consumptive activity are increasing in the tropics as a result of increasing human populations, and attempts to improve or modernize the living standards of coastal peoples.
In areas where trees are cut to supply timber, fuel or pulp, it is important that managers and coastal planners be aware of the need for careful planning to ensure that the forest is not overexploited, or threatened from other sources. On Zanzibar, mangrove trees are exploited to provide building materials and for making charcoal, and cutting is managed with the goal of long-term sustainability in mind. In some areas, such as Malaysia, India, and Bangladesh, mangrove forests are managed silviculturally on a sustainable-yield basis, just as are terrestrial forests. Even where mangroves are used extensively, many countries retain a protective fringe of mangrove trees along shorelines and waterways to reduce erosion and to provide a seed source.
Non-consumptive threats to mangrove ecosystems may be further divided into two categories: pollution and land-use conflicts. One of the more serious forms of pollution is oil, for this substance can foul gas exchange surfaces of the exposed roots, and lead to death as a result of oxygen starvation. Toxic substances in oil can also poison both the roots and important soil micro-organisms. Cintron et al. (1981) documented the effects of an oil spill in a mangal Puerto Rico and noted that in the areas receiving the heaviest impact, 50% of the forest canopy was lost after 43 days, and 95% after 85 days
1. Mangrove ecosystems are important for a number of ecological and economic reasons, and therefore their conservation (wise use) should be considered in coastal development and land-management planning.
2. Mangroves are sensitive to changes in water flow and drainage patterns. Therefore, any coastal developments which might alter water flow and drainage should be considered carefully by planners, and ways should be sought to minimize the damage of such developments to mangroves.
3. Mangroves are sensitive to certain pollutants, especially excess sediment, oil, and certain industrial effluents. Coastal development should be planned so as to minimize potential damage to mangroves from excess suspended sediment, oil, and industrial effluents.
4. When mangroves must be removed for development because there is no alternative, development plans should be designed to minimize the effect and size of the area cleared. The effects of disrupting fisheries that depend directly or indirectly on mangroves should be carefully considered before granting permission to clear-cut mangroves for development.
5. Any proposal to alter mangrove areas for aquaculture or other purposes should be fully evaluated for potential impact on village-level fisheries. It may be that the socioeconomic value of mangrove resources, such as mud crabs and mud lobsters, may be greater than the socioeconomic value of aquaculture development. Aquaculture schemes which favour the retention of mangroves should be favoured wherever possible.
6. Mangrove areas should be set aside and given reserve status to ensure the long-term conservation of some mangroves in the natural state, and to provide a baseline against which to estimate human inpacts outside of reserved areas. The zoning of mangroves for different purposes in areas of intense use should also be conducted.
Most of the threats to mangrove ecosystems can be placed under the broad heading of land use conflict. Changes in land use patterns may critically alter a number of features of the mangrove habitat, which may lead to the deterioration of a local mangal. Alternatively, mangroves are often clear cut and drained to permit certain types of land use to proceed.
Human activities which disrupt water flow patterns through a mangrove can have disastrous consequences. The diversion of fresh water away from river deltas by upstream dams can have serious impacts on riverine mangroves. River water brings in nutrients and particulate material which are necessary for the maintenance of mangroves in estuarine areas. In dry areas, soil salinity can increase rapidly as a result of high evaporation rates if the inflow of fresh water is disrupted, and this can cause the rapid mortality of mangrove trees.
The impoundment of water within mangrove areas can also lead to the degeneration of mangrove forests. Water flow is needed for nutrient provision, but more importantly the air breathing lenticels of the roots need periods of emergence to function effectively. If the roots remain continuously covered in water, the trees die as a result of oxygen starvation in the roots. In the 1970's the Natal Parks Board in South Africa built a bridge across a small stream at Sodwana Bay National Park. The bridge did not permit sufficient drainage, and water was impounded upstream from it. Most of the mangroves in the system were killed as a result of oxygen deprivation, and although the bridge was replaced with one that allowed better drainage, even in 1994 the area has only marginally recovered. A similar case has been documented in Puerto Rico by Cintron and Novelli (1983).
It is a common practice in some areas to convert mangrove forests into fishponds for the aquaculture of prawns and fish. In many areas, mangrove trees are clearcut, the cleared area is surrounded by dykes, and fertilizer applied. It has long been recognized that extensive conversion of mangrove areas to shrimp or fish ponds has decreased yields of other mangrove resources. There are usually coastal villagers who depend on mangrove resources for subsistence or commercial harvests. In many areas the access to village resources is protected by customary law. However, village leaders, managers and planners need to carefully assess the socioeconomic value of natural mangrove stands, before permitting their conversion to aquaculture ponds. Widespread destruction of mangroves for aquaculture can decrease the natural availability of the juvenile stages which form the seed-stock for many aquaculture operations. Wherever possible, aquaculture schemes which favour the retention of mangroves should be favoured over schemes which call for their removal.
Perhaps one of the biggest threats facing mangroves in certain areas is the clearcutting and drainage of mangals for development purposes. Clearing may be done for urbanization, including housing, industrial areas, and harbour facilities. For example, in Suva, Fiji, much of the low-lying area now occupied by housing, industry and port facilities were once fringed by mangroves that were removed during land reclamation and development.
Although mangroves are dependent on sediment, they are also sensitive to excess sedimentaton, particularly the sudden deposition of large quantities. For example, Cintron and Pool (1976) reported a case in Puerto Rico where sand was extracted from a coastal dune for construction of an airport. Subsequently, storm waves overtopped what was left of the original dune, and carried large amounts of sand into the adjacent mangrove forest. All mangrove trees were killed wherever the sand was deposited to a depth greater than 30 cm. Road construction upstream from riverine mangroves is well known as a source of mortality to mangroves, and can cause the mortality of trees over large tracts. Dredging for building materials, or to increase the depth of shipping channels can cause excess sedimentation, particularly when dredge spoils are dumped near mangrove forests.
Since mangroves often have mosquitos, crocodiles, and unpleasant smells, they are not popular with tourists or resort owners. Therefore, when tourist resorts are built, mangrove forests are usually cleared and attempts are sometimes even made to convert the coastline to a sandy beach. When asked to grant permission for a resort developer to clear an area of mangroves, it would be wise for coastal zone managers to consider requiring the developer to incorporate the mangrove fringe into his/her landscape plan, rather than planning for its removal.
consumptive, in which whole trees, or parts of them, are cut for
products such as timber, fuel, or pulp; and
non-consumptive, in which a forest is threatened by human activity
other than direct exploitation of the trees themselves (see below). Adverse effects on mangrove ecosystems as a result of both consumptive and non-consumptive activity are increasing in the tropics as a result of increasing human populations, and attempts to improve or modernize the living standards of coastal peoples.
In areas where trees are cut to supply timber, fuel or pulp, it is important that managers and coastal planners be aware of the need for careful planning to ensure that the forest is not overexploited, or threatened from other sources. On Zanzibar, mangrove trees are exploited to provide building materials and for making charcoal, and cutting is managed with the goal of long-term sustainability in mind. In some areas, such as Malaysia, India, and Bangladesh, mangrove forests are managed silviculturally on a sustainable-yield basis, just as are terrestrial forests. Even where mangroves are used extensively, many countries retain a protective fringe of mangrove trees along shorelines and waterways to reduce erosion and to provide a seed source.
Non-consumptive threats to mangrove ecosystems may be further divided into two categories: pollution and land-use conflicts. One of the more serious forms of pollution is oil, for this substance can foul gas exchange surfaces of the exposed roots, and lead to death as a result of oxygen starvation. Toxic substances in oil can also poison both the roots and important soil micro-organisms. Cintron et al. (1981) documented the effects of an oil spill in a mangal Puerto Rico and noted that in the areas receiving the heaviest impact, 50% of the forest canopy was lost after 43 days, and 95% after 85 days
1. Mangrove ecosystems are important for a number of ecological and economic reasons, and therefore their conservation (wise use) should be considered in coastal development and land-management planning.
2. Mangroves are sensitive to changes in water flow and drainage patterns. Therefore, any coastal developments which might alter water flow and drainage should be considered carefully by planners, and ways should be sought to minimize the damage of such developments to mangroves.
3. Mangroves are sensitive to certain pollutants, especially excess sediment, oil, and certain industrial effluents. Coastal development should be planned so as to minimize potential damage to mangroves from excess suspended sediment, oil, and industrial effluents.
4. When mangroves must be removed for development because there is no alternative, development plans should be designed to minimize the effect and size of the area cleared. The effects of disrupting fisheries that depend directly or indirectly on mangroves should be carefully considered before granting permission to clear-cut mangroves for development.
5. Any proposal to alter mangrove areas for aquaculture or other purposes should be fully evaluated for potential impact on village-level fisheries. It may be that the socioeconomic value of mangrove resources, such as mud crabs and mud lobsters, may be greater than the socioeconomic value of aquaculture development. Aquaculture schemes which favour the retention of mangroves should be favoured wherever possible.
6. Mangrove areas should be set aside and given reserve status to ensure the long-term conservation of some mangroves in the natural state, and to provide a baseline against which to estimate human inpacts outside of reserved areas. The zoning of mangroves for different purposes in areas of intense use should also be conducted.
Most of the threats to mangrove ecosystems can be placed under the broad heading of land use conflict. Changes in land use patterns may critically alter a number of features of the mangrove habitat, which may lead to the deterioration of a local mangal. Alternatively, mangroves are often clear cut and drained to permit certain types of land use to proceed.
Human activities which disrupt water flow patterns through a mangrove can have disastrous consequences. The diversion of fresh water away from river deltas by upstream dams can have serious impacts on riverine mangroves. River water brings in nutrients and particulate material which are necessary for the maintenance of mangroves in estuarine areas. In dry areas, soil salinity can increase rapidly as a result of high evaporation rates if the inflow of fresh water is disrupted, and this can cause the rapid mortality of mangrove trees.
The impoundment of water within mangrove areas can also lead to the degeneration of mangrove forests. Water flow is needed for nutrient provision, but more importantly the air breathing lenticels of the roots need periods of emergence to function effectively. If the roots remain continuously covered in water, the trees die as a result of oxygen starvation in the roots. In the 1970's the Natal Parks Board in South Africa built a bridge across a small stream at Sodwana Bay National Park. The bridge did not permit sufficient drainage, and water was impounded upstream from it. Most of the mangroves in the system were killed as a result of oxygen deprivation, and although the bridge was replaced with one that allowed better drainage, even in 1994 the area has only marginally recovered. A similar case has been documented in Puerto Rico by Cintron and Novelli (1983).
It is a common practice in some areas to convert mangrove forests into fishponds for the aquaculture of prawns and fish. In many areas, mangrove trees are clearcut, the cleared area is surrounded by dykes, and fertilizer applied. It has long been recognized that extensive conversion of mangrove areas to shrimp or fish ponds has decreased yields of other mangrove resources. There are usually coastal villagers who depend on mangrove resources for subsistence or commercial harvests. In many areas the access to village resources is protected by customary law. However, village leaders, managers and planners need to carefully assess the socioeconomic value of natural mangrove stands, before permitting their conversion to aquaculture ponds. Widespread destruction of mangroves for aquaculture can decrease the natural availability of the juvenile stages which form the seed-stock for many aquaculture operations. Wherever possible, aquaculture schemes which favour the retention of mangroves should be favoured over schemes which call for their removal.
Perhaps one of the biggest threats facing mangroves in certain areas is the clearcutting and drainage of mangals for development purposes. Clearing may be done for urbanization, including housing, industrial areas, and harbour facilities. For example, in Suva, Fiji, much of the low-lying area now occupied by housing, industry and port facilities were once fringed by mangroves that were removed during land reclamation and development.
Although mangroves are dependent on sediment, they are also sensitive to excess sedimentaton, particularly the sudden deposition of large quantities. For example, Cintron and Pool (1976) reported a case in Puerto Rico where sand was extracted from a coastal dune for construction of an airport. Subsequently, storm waves overtopped what was left of the original dune, and carried large amounts of sand into the adjacent mangrove forest. All mangrove trees were killed wherever the sand was deposited to a depth greater than 30 cm. Road construction upstream from riverine mangroves is well known as a source of mortality to mangroves, and can cause the mortality of trees over large tracts. Dredging for building materials, or to increase the depth of shipping channels can cause excess sedimentation, particularly when dredge spoils are dumped near mangrove forests.
Since mangroves often have mosquitos, crocodiles, and unpleasant smells, they are not popular with tourists or resort owners. Therefore, when tourist resorts are built, mangrove forests are usually cleared and attempts are sometimes even made to convert the coastline to a sandy beach. When asked to grant permission for a resort developer to clear an area of mangroves, it would be wise for coastal zone managers to consider requiring the developer to incorporate the mangrove fringe into his/her landscape plan, rather than planning for its removal.
MAIN USES AND THREATS
Rural populations in Asia have traditionally used mangroves as a source of wood and non-wood forest products for the production of charcoal, fuelwood, and timber and poles for houses, boats and fish-traps. In Indonesia, for example, the commercial exploitation of mangrove wood has a long history, and has been important particularly for the production of wood chips, timber (now banned) and charcoal, including for export. In addition, poles extracted from the Rhizophoraceae are used for building foundations. Malaysia has a long tradition of production of mangrove charcoal for national and international markets.
Besides these exceptions, mangroves have generally been used for local needs. Nypa fruticans is a common species in most Asian countries, and it has often been used as
thatching material for houses in Bangladesh, Brunei Darussalam, India, Indonesia,
Malaysia, the Philippines and Viet Nam.
Several fishing and rural communities depend on the fish and shellfish in mangroves as a source of income and food security; when mangrove forests are destroyed, a significant decrease in local fish catches may result. Indonesia, the Philippines and Thailand have major offshore fisheries, which represent a significant portion of national income and depend partly on mangroves.
The world’s mangroves 1980–2005
27
Many rural communities have used mangroves to produce honey, tannins and traditional medicines. Ecotourism activities are increasing in the region, providing further sources of sustainable income to local populations and creating awareness of the services and benefits provided by mangroves. Mangrove leaves are often used as fodder for camels, goats and sheep in the Near East, where mangroves are often the only coastal forest resource. However, overexploitation of the trees for this use may reduce mangrove growth and density, stunting trees and degrading stand quality. When the outer trees are overused, it is common practice to provide fresh, additional fodder for camels by opening the way to the inner mangroves. In some places this practice has affected the quality of the stand even if the mangrove area was not reduced significantly. The main causes of loss of area in Asia have been overexploitation and the development of shrimp farms. Because of its high economic return, shrimp farming has been promoted to boost national economies, as a potential source of income for local communities and as a means of poverty alleviation. However, this activity may cause loss of habitat and of ecosystem services provided by mangroves, and may damage surrounding areas through pollution caused by the chemical products used.
In some countries (e.g. India and Myanmar) government policies have promoted self-sufficiency in food production through the conversion of mangrove lands to rice paddies, while in other countries (e.g. China, Indonesia, the Philippines and Viet Nam) mangrove areas have been used for salt production. In addition, some mangrove areas have been clear-cut for urban or tourist development or converted to agriculture. The decrease in freshwater input or the interruption of flows caused by the construction of dams and diversion of water for irrigation may increase soil salinity, damaging the surrounding mangroves, as has occurred in Singapore and Pakistan.
During the Viet Nam war (1962–1972), mangrove forests often served as bases for military operations, and were consequently highly damaged by chemical warfare (herbicides and napalm). Further losses have been caused by the conversion of mangrove land to other uses such as shrimp ponds, agriculture, salt pans and human settlements.
Among the current threats to mangrove ecosystems, the ever-increasing human pressure on coastal areas is one of the most serious. In addition to human-induced threats, natural hazards such as cyclones, storms and floods frequently occur in this region, threatening several coastal ecosystems, including mangroves. Trees in the front lines are often uprooted and damaged during these events. However, the loss of this ‘sacrificial belt’ (Takle, 2007) may contribute to the overall protective function of dense forests, as occurred during the cyclone in Orissa, India, in 1999. It is also reported that many people in Bangladesh were protected by mangroves during the tidal wave that hit the Chakaria Sundarbans in 1960 (which at that time was still covered by dense forests), and in Viet Nam, when typhoon Wukong hit three northern provinces in 2000, but left no damage to the dykes behind the regenerated mangrove.
Rural populations in Asia have traditionally used mangroves as a source of wood and non-wood forest products for the production of charcoal, fuelwood, and timber and poles for houses, boats and fish-traps. In Indonesia, for example, the commercial exploitation of mangrove wood has a long history, and has been important particularly for the production of wood chips, timber (now banned) and charcoal, including for export. In addition, poles extracted from the Rhizophoraceae are used for building foundations. Malaysia has a long tradition of production of mangrove charcoal for national and international markets.
Besides these exceptions, mangroves have generally been used for local needs. Nypa fruticans is a common species in most Asian countries, and it has often been used as
thatching material for houses in Bangladesh, Brunei Darussalam, India, Indonesia,
Malaysia, the Philippines and Viet Nam.
Several fishing and rural communities depend on the fish and shellfish in mangroves as a source of income and food security; when mangrove forests are destroyed, a significant decrease in local fish catches may result. Indonesia, the Philippines and Thailand have major offshore fisheries, which represent a significant portion of national income and depend partly on mangroves.
The world’s mangroves 1980–2005
27
Many rural communities have used mangroves to produce honey, tannins and traditional medicines. Ecotourism activities are increasing in the region, providing further sources of sustainable income to local populations and creating awareness of the services and benefits provided by mangroves. Mangrove leaves are often used as fodder for camels, goats and sheep in the Near East, where mangroves are often the only coastal forest resource. However, overexploitation of the trees for this use may reduce mangrove growth and density, stunting trees and degrading stand quality. When the outer trees are overused, it is common practice to provide fresh, additional fodder for camels by opening the way to the inner mangroves. In some places this practice has affected the quality of the stand even if the mangrove area was not reduced significantly. The main causes of loss of area in Asia have been overexploitation and the development of shrimp farms. Because of its high economic return, shrimp farming has been promoted to boost national economies, as a potential source of income for local communities and as a means of poverty alleviation. However, this activity may cause loss of habitat and of ecosystem services provided by mangroves, and may damage surrounding areas through pollution caused by the chemical products used.
In some countries (e.g. India and Myanmar) government policies have promoted self-sufficiency in food production through the conversion of mangrove lands to rice paddies, while in other countries (e.g. China, Indonesia, the Philippines and Viet Nam) mangrove areas have been used for salt production. In addition, some mangrove areas have been clear-cut for urban or tourist development or converted to agriculture. The decrease in freshwater input or the interruption of flows caused by the construction of dams and diversion of water for irrigation may increase soil salinity, damaging the surrounding mangroves, as has occurred in Singapore and Pakistan.
During the Viet Nam war (1962–1972), mangrove forests often served as bases for military operations, and were consequently highly damaged by chemical warfare (herbicides and napalm). Further losses have been caused by the conversion of mangrove land to other uses such as shrimp ponds, agriculture, salt pans and human settlements.
Among the current threats to mangrove ecosystems, the ever-increasing human pressure on coastal areas is one of the most serious. In addition to human-induced threats, natural hazards such as cyclones, storms and floods frequently occur in this region, threatening several coastal ecosystems, including mangroves. Trees in the front lines are often uprooted and damaged during these events. However, the loss of this ‘sacrificial belt’ (Takle, 2007) may contribute to the overall protective function of dense forests, as occurred during the cyclone in Orissa, India, in 1999. It is also reported that many people in Bangladesh were protected by mangroves during the tidal wave that hit the Chakaria Sundarbans in 1960 (which at that time was still covered by dense forests), and in Viet Nam, when typhoon Wukong hit three northern provinces in 2000, but left no damage to the dykes behind the regenerated mangrove.
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