Mangroves

Mangroves are a diverse group comprising about 70 species of tropical woody trees. They live primarily in intertidal environments, usually forming thick forests. 

Covering an estimated global area of 167,000 km2 to 181,000 km2, mangrove forests protect the shoreline, store carbon, enhance water quality and promote biodiversity by providing food and shelter for fish, marine invertebrates, and birds.

Mangroves are threatened by mariculture (primarily shrimp farming), timber harvest, deforestation due to development, water diversion and over-exploitation. Rising sea levels and erosion force mangroves to retreat landward, but that is only possible if there is undeveloped land for them to occupy. 

Mangroves are disappearing at a rate of 1 to 2% per year (Duke 2007)

Mangrove Food Cycle
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Which Goals Does This Affect?


How Was It Measured?

Data on the geographical extent of mangrove forests is from a global, raster-based, 30m resolution dataset created by Giri et al. (2011). The majority of the Landsat images used in this analysis were from 2000.  Mangrove extent was measured within a band from 1 km inland to 1 km seaward from the shore. Extent per oceanic region was reported in kmper country.  

Prior to 2015, data from FAO (2007) were used to determine the condition/health of mangrove habitat as a function of extent within each country for the years 1980, 1990, 2000 and 2005 and trend was calculated as the rate of change in habitat extent across those years.

For 2015, a new data source published by Hamilton and Casey (2014) was used. The new data permit direct estimation of changes in mangrove habitat.

Data on the condition of mangrove forests, e.g. presence of disease, percentage cover or other indicator of health are not globally available, so condition is estimated by the change in extent of forests over time.

The reference point for mangroves is the extent of mangrove habitat in km2 in 1980 so the current Status for each region is:
the current extent of mangroves in km2 ÷ the extent (km2) of mangroves in 1980. 

Mangroves are used as a component in the following goals: Carbon Storage, Coastal Protection and Biodiversity (subgoal Habitats). Beginning in 2015, for calculating scores for the Carbon Storage and Coastal Protection goals, the contribution of mangroves and the other habitats included are weighted by their relative contributions to the goal. For Carbon Storage the weights are Salt Marsh 210; Mangroves 139 and Seagrass 83, using information from Laffoley and Grimsditch (2009).



What Are the Impacts?

Carbon Dynamics in Mangroves
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ECOLOGICAL IMPACT
Mangroves remove significant amounts of carbon from the atmosphere and store it above ground in their leaves and stems and below ground in their roots. When dead leaves, stems and roots are covered by sediment and detritus, oxygen cannot reach them and oxidize their carbon. If left undisturbed, their buried carbon can be stored for centuries or longer. When mangrove forests are damaged or destroyed, that carbon is quickly oxidized and emitted back into the atmosphere as CO2.

Mangroves take in about 5000 tons of carbon dioxide per kilometer per year. Only about twenty percent of that is released as CO2 through respiration (Lovelock, 2011)

Mangroves provide shelter and food for numerous fish, birds, crustaceans, and other wildlife.  High nutrient concentrations in the surrounding sediment, as well as protected spaces created by submerged tree roots, make them ideal nurseries and breeding grounds for many species.
HUMAN HEALTH IMPACT
Mangroves protect human lives (and property) in coastal areas by absorbing the impacts of waves, tsunamis, storm surges, and floods.

Mangroves can absorb 70 to 90 percent of the energy of a normal wave (Miththapala, 2008).
ECONOMIC IMPACT
Mangroves play a critical role in supporting commercial fisheries, improving water quality, and providing fuel and raw materials.

Commercial logging of mangroves for timber has reached an unsustainable rate in many areas as mangrove forests are not allowed sufficient time to be renewed as a resource.

Juvenile fish find refuge and food in mangrove forests, resulting in higher survival rates and larger adult fish populations.  The biomass of adult fish populations in healthy mangrove forests along certain coastlines is much higher than those in mangrove-poor systems – sometimes by as much as up to 25 to 50%.

The global value of the goods and services mangroves provide to humans (e.g. food production, raw materials, recreational fishing, art, education, science, climate regulation, and waste treatment) has been estimated to be as much as $9,990 per hectare, or $1.6 trillion in total (Costanza et al., 1997).

In 2002, mangrove-dependent commercial fish comprised an annual catch of almost 30 million tons (Nagelkerken, 2008). This constitutes approximately a third of the global annual wild catch, as recorded at 89.7 million tons in 2008 (FAO, 2008).


What Has Been Done?

In Florida, Legislature Provides State-Level Protection

In 1996, the Mangrove Trimming and Preservation Act was put into effect, providing legislation to protect the 555,000 acres of mangroves in Florida. The Act bans the alteration or trimming of mangroves on uninhabited public land and allows private property owners to trim mangrove trees in accordance with established guidelines.  The Act has effectively protected coastal ecosystems by ensuring continued shoreline protection while simultaneously enhancing coastal economies.                                    

(Environmental Services, 2009)  Florida Department of Environmental Protection
 A thick tangle of mangrove tree roots. © Michael Nichols/National Geographic Stock

A thick tangle of mangrove tree roots.© Michael Nichols/National Geographic Stock



Get More Information

Mangrove Action Project (MAP)
A global network dedicated to the preservation of mangrove ecosystems, MAP promotes the rights of coastal communities who are dependent upon mangrove forests for survival.

Global Mangrove Database & Information System (GLOMIS)
This information system is geared towards decision makers, law-makers, administrators and overall audiences interested in the sustainable use and management of mangrove ecosystems and species.    



References




PHOTO(S): © Keith A. Ellenbogen
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