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THE VALUE OF MANGROVE ECOSYSTEM

1.	Introduction
Mangrove ecosystems represent natural capital capable of producing a wide range of goods and services for coastal environments and communities and society as a whole. Some of these outputs, such as timber, are freely exchanged in formal markets. Value is determined in these markets through exchange and quantified in terms of price. However, many other outputs or services like provision of wildlife and native fish habitat, watershed protection, coastal protection, water quality maintenance carbon storage, scenic beauty, wildlife and native fish, for example are freely provided and contribute to the quality of our coastal environment, our quality of life and support our market economy, but are without formal markets and therefore without prices. These “free” services are known as ecological values or functions of mangroves. The fact that these mangrove benefits or free services are not priced does not mean they lack value, only that market indicators of the value do not exist. By ignoring the value of these free services, development has too often favoured their rapid conversion and loss.

Fig. 1 Mangrove plants
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2.	Ecological Values
The ecological values of mangrove in most tropical countries have been well documented and recognised. However there is surprisingly little scientific data to back this up. Most of the evidence is observational and anecdotal This section describes some ecological values of mangrove that is observed in certain tropical countries.

2.1 Marine Fisheries
Mangroves are nursery habitat for many wildlife species, including commercial fish and crustaceans, and thus contribute to sustaining local abundance of fish and shellfish populations The following data indicate the diversity of tropical mangrove-associated fish species. In Selangor, Malaysia 119 species were recorded, 83 species in Kenya, 133 from Queensland Australia, 59 species in Puerto Rico and 128 from the Philippines (Chong, 1990). While mangroves in the Caribbean have been demonstrated to support juvenile coral reef fish ]] mangroves in Papua New Guinea and the Solomon Islands have been found to be important nurseries for sandy and muddy-bottom demersal and surface feeding species. Seventy-five percent of the game fish and ninety percent of the commercial species in south Florida are dependent on mangrove ecosystems An estimated 75 per cent of the commercially caught prawns and fish in Queensland, Australia, depend on mangroves for part of their lives and on nutrients exported from the mangroves to other ecosystems

2.2 Wildlife habitat.
Mangrove systems support a range of wildlife species including crocodiles, birds, tigers, deer, monkeys and bees for honey production Many animals find shelter either in the roots or branches of mangroves. Mangroves serve as rookeries, or nesting areas, for beautiful coastal birds such as brown pelicans and roseate spoonbills. Many migratory species depend on mangroves for part of their seasonal migrations. For instance, an estimated two million migratory shorebirds of the East Asian-Australasian Flyway, which annually migrate from the Arctic Circle through South-East Asia to Australia and New Zealand and back, stop to forage at numerous wetlands along this Flyway, include the wetlands of Oceania

2.3 Improve coastal water quality.
Mangroves maintain coastal water quality by abiotic and biotic retention, removal, and cycling of nutrients, pollutants, and particulate matter from land-based sources, filtering these materials from water before they reach seaward coral reef and seagrass habitats Mangrove root systems slow water flow, facilitating the deposition of sediment. Toxins and nutrients can be bound to sediment particles or within the molecular lattice of clay particles and are removed during sediment deposition. Compared with the expense of constructing a wastewater treatment plant, mangroves are commonly selected as receiving areas of effluent. Increasingly the notion of specifically constructed mangrove wetlands is being adopted and used for treatment of aquaculture and sewage effluents

Fig. 2 mangrove roots facilitating the deposition of sediment
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2.4 External Support
Mangroves are functionally linked to neighbouring coastal ecosystems ). For instance, terrigenous sediments and nutrients carried by freshwater runoff are first filtered by coastal forests, then by mangrove wetlands, and finally by seagrass beds before reaching coral reefs. The existence and health of coral reefs are dependent on the buffering capacity of these shoreward ecosystems, which support the oligotrophic conditions needed by coral reefs to limit overgrowth by algae Mangroves supply nutrients to adjacent coral reef and seagrass communities, sustaining these habitats’ primary production and general health.

Fig. 3 Relationship between mangrove, seagrass and coral reef
Link titleRelationship between mangroves and coral reefs Source: )

2.5 Protection of coastlines and Development
As a result of their intricately entangled above-the ground root systems, mangrove communities protect shorelines during storm events by absorbing wave energy and reducing the velocity of water passing through the root barrier Wave energy may be reduced by 75 per cent in the wave's passage through 200 metres of mangrove ( but other factors also have an influence, including coastal profile, water depth and bottom configuration. Mangrove covered shorelines are less likely to erode, or will erode significantly more slowly, than unvegetated shorelines during periods of high wave energy Protecting mangroves sustains natural protection, and is less expensive than seawalls and similar erosion control structures, which can increase erosion in front of the structure and at adjacent properties.

Fig.4 Entangled root system traps sediment and absorb wave energy
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The tsunami has provided an opportunity to illustrate that healthy mangroves serve as a natural barrier against massive waves – protecting infrastructure developments and saving lives. The World Conservation Union (IUCN) compared the death toll from two villages in Sri Lanka that were hit by the devastating tsunami giant waves. Two people died in the settlement with dense mangrove and scrub forest, while up to 6,000 people died in the village without similar vegetation

2.6 Why Mangroves are undervalued?
Despite their documented ecological functions or free services mangroves are still under pressure from development, agriculture and industry. Only direct goods and services are included in economic calculations. The “free” services provided by mangroves are more difficult to measure and consequently are often ignored. These free services would require considerable energy, technology and money if they were provided from goverments or other sources.

3.	The role of mangroves in New Zealand
Comparisons of the productivity of mangroves from different latitudes worldwide suggest that productivity and plant biomass decreases with increasing latitude. From this global pattern it is expected that mangroves in New Zealand, near their southern geographical limit would have relatively low productivity compared to their tropical equivalents.

[3.1 Intrinsic and unique values]

Many aspects of New Zealand mangrove systems have not yet been sufficiently studied; therefore their importance in relation to marine and estuarine species and their role in terms of ecosystem structure and function is inadequate. The role played by mangroves in New Zealand estuarine foodwebs is, however, probably significant.

3.2 Benthic fauna of mangroves
Relatively few studies have been undertaken on the benthic assemblages and species of mangrove forests in New Zealand. The benthic invertebrate fauna of New Zealand’s mangroves forests appear to be modest in both abundances and species diversity compared to other estuarine habitats

3.3 Fish fauna of mangroves
Recent studies have shown that the temperate mangrove forests of northern New Zealand support high abundances of small fishes, but that New Zealand support high abundances of small fishes compared to other estuarine habitats, with most of the small fish assemblage dominated by juveniles of the ubiquitous yellow-eyed mullet (Aldrichetta forsteri), as well as juvenile grey mullet (Mugil cephalus) in the west coast estuaries. Nineteen fish species are ‘confirmed’ to be associated with mangroves, of which three species are probably partially reliant on them as juvenile nurseries It seems unlikely that New Zealand mangroves are important as spawning grounds for coastal fish or as habitat for their larvae.

3.4 Use of mangroves by birds
While many species make extensive use of mangroves for roosting, feeding and breeding, no bird species is totally dependant on mangroves in New Zealand. The range of bird species that are found regularly in New Zealand mangroves includes several native species, such as banded rail, white-faced heron, harriers, kingfishers, welcome swallow and pükeko.

3.5 Sediment trapping and erosion protection
This question has not been fully addressed in relation to New Zealand mangroves. However, previous and ongoing research, is extending the understanding of the role of mangrove contribution to habitat change (Morrisey, Beard, Morrison, Craggs, Lowe, 2007). Remains of rows of mangroves planted to stabilize the coast by early generations of Maoris can still be seen in New Zealand (

4.	MANGROVE MANAGEMENT

The next section briefly discusses how mangrove has been managed today at the international and national level. Mangrove biodiversity, management and conservation have received considerable attention in recent years since research has increased the understanding of the values, functions and attributes of mangrove ecosystems.

4.1	International level
At the International Level, the common approach to major environmental policy issues has been to formulate conventions,[ treaties]] and agreements, which all concerned countries become signatories to. Mangroves are today a global issue because more than 100 countries worldwide have mangrove resources Of the approximately 100 countries that have mangrove vegetation, around 20 have undertaken rehabilitation initiatives, establishing nurseries and attempting afforestation and re-planting in degraded areas. More than half a dozen international agreements and various regional agreements are directly relevant to the conservation of mangrove biodiversity. Two of the most important international agreements are described below;

4.2 Ramsar
In 1971, a convention to protect Wetlands of International Importance" was adopted in the Iranian city of Ramsar. To become a signatory to the Ramsar Convention, a country had to designate at least one such site and guarantee its protection. Around 110 countries have become signatories to the treaty, which is proving to become a major salvation to the world's mangroves. Some 850 "Ramsar sites" have been designated by these countries covering over 53 million hectares. About a third of these contain mangroves. (Mangrove Action Project

4.3 Marine Protected Areas (MPA)
Spalding (1997) reviewed the global status for mangrove conservation: “There are 685 protected areas containing mangroves globally, distributed between 73 countries and territories. Countries with very large areas of mangroves have a significant number of protected areas notably Australia (180), Indonesia (64) and Brazil (63). Examples of marine reserves in New Zealand where mangrove form an important component of the protected foreshore vegetation are Motu Manawa (Pollen Island) marine reserve in the Waitemata Harbour, and Te Matuku marine reserve, Waiheke Island; both managed by the Department of Conservation.

4.4 National level
Historically the responsibility of mangrove management at the national level in many tropical mangrove countries have been assigned on a sectoral basis to executing agencies of the government, institutions for example Forestry, Fishery or Agriculture Departments. The agencies responsible for administering mangroves differ between each country and even between states and districts within Countries. Sectoral management has inevitably resulted in prejudices regarding their objectives, leading to conflicts of interest, to unsustainable resource use, and to poor and less powerful groups becoming more disadvantaged and disenfranchised (Brown, 1997). These limitations are now recognised as a major constraint to achieving sustainable development of mangrove resources.

4.5.1 Improve and enhance the scientific knowledge base
Lack of knowledge of mangrove ecosystems, their extent, status and linkages to other ecosystemshampers efforts to conserve and manage mangroves, leading to the unsustainable exploitation of this productive coastal resources. According to a comprehensive information database of mangrove biodiversity in each country is necessary to monitor the status of mangrove biological diversity, realise its economic potential and areas of application. This is critical in planning an effective management of mangroves.

Economic arguments carry the greatest weight in conservation and management of mangroves. However, the true economic value of mangrove diversity and natural resources is difficult to measure and important ecological processes and functions undervalued. All development plans and policies should include economic valuations that fully reflect the sociological, ecological and environmental costs of resource use, physical developments and pollution.

In New Zealand for example much of the basic information required to address concerns and manage mangrove is lacking. Research has established that, regardless of which approach is decided upon, sustainable management can only be achieved if evaluation of mangrove areas is undertaken on a site-by-site basis ). There is, therefore a strong need for local studies to provide information that will allow understanding and management of mangroves in most tropical countries as well as New Zealand