Mangroves –A Muddy Affair!
Whenever we think of a tropical vacation, we imagine vibrant and colorful coral reefs, or we dream of magnificent sandy beaches. But there is a different kind of ecosystem —a less popular tourist destination for sure, but a true heaven for naturalists. Can you take a guess?… I am talking about those trees with weird looking roots! These are known as “mangroves”.
Outline: ■ For the Love of Mangroves… ■ What Are Mangroves? ■ Where Are Mangroves Distributed? ■ What Services do Mangroves Provide? ■ The Global Status of Mangroves ■ A World Without Mangroves ■ Awareness & What We Can Do ■ References & Further Reading
My very first visit to a mangrove swamp was almost a decade back (in 2009) and it was love at first sight!
I’ve been a nature lover since childhood. Being born and raised in a small town surrounded by a thick forest belt did nurture my love for the environment. Although one must admit that the sudden yearly visits of a herd of wild elephants can give you quite a fright!
I was thrilled beyond imagination when I finally got that chance to convert my passion into my profession. It was a small project intended to determine the relationship between habitat heterogeneity of mangrove forests and the crab diversity therein. Twice a month, I would visit the mighty Sundarbans mangroves (located in the coastal region of the Bay of Bengal) and its various islands to collect samples necessary for my project, and gather information from the locals.
The more I learned about that unique ecosystem, the more I felt connected to it. I also realized how badly mangroves need to be studied and protected.
Mangroves are intertidal plant communities, mostly found in the tropics or subtropics. There are at least 80 different species of mangrove plants, and together they form a unique landscape. Mangroves constitute some of the most productive and complex ecosystems on Earth.
The term ‘mangrove’ describes both the ecosystem and the plant families that have developed specialized adaptations to survive in an estuarine environment [EA01]. For instance, do you know that mangroves are able to turn their leaves to reduce the surface area of the leaf exposed to the hot sun? (This enables them to reduce water loss through evaporation).
A mangrove forest is an assemblage of trees or shrubs, which extensive root systems form intricate networks. Sometimes mangroves are referred to as “walking trees”. This unique vegetation can live and thrive in saltwater, and when conditions are favorable it creates dense forest patches known as mangrove swamps.
Mangroves are generally located in tropical areas and more specifically closer to the main ocean currents, as well as above a 20°C seawater isotherm (i.e. mangroves require relatively warm temperatures to thrive).
Mangroves occupy less than 1% of the world’s surface [SP02]. They are found in 120 countries and territories and cover more than 15 million ha worldwide [SM10]. The area covered by mangroves and the number of countries where they are distributed have been the major focus of many studies ([SM10], [GC11]). More than 41% of the world’s mangroves are located in South and Southeast Asia -23% of which in Indonesia alone. A further 20% of the total mangrove area lies in Brazil, Australia and Nigeria [SM97].
Mangroves provide many services, benefiting humans’ well-being either directly or indirectly.
Mangroves are considered “ecosystem engineers” providing an array of ecosystem services. They shelter juvenile fish and crabs, serving as nursery grounds; they protect numerous other delicate invertebrates from predation. Three-quarters of all the tropical fish are born here. Even coral reefs depend on mangroves for filtering and recycling waters. They trap nutrients; they host a plethora of biodiversity; they provide habitats for different faunal communities (including fish, crustaceans, amphibians, reptiles, many various mammals, as well as several aquatic and terrestrial insect species); they prevent coastal erosion; they support local livelihoods through provisioning of food, fuel and construction materials. They act as barriers for tropical storms and help to protect against Tsunamis much more efficiently than any man-made structure.
Mangrove forests, like some other coastal ecosystems (salt marsh, seagrass), can “capture and hold” carbon as a sink. Mangroves and associated soils are said to approximately sequester 22.8 million metric tons of carbon each year. The carbon trapped by the world’s ocean and coastal ecosystems is known as “Blue Carbon” [Check out the Blue Carbon Initiative]. Since mangrove forests contain both above and below-ground biomass, they can trap carbon as effectively as the Amazonian rainforests do.
These are only a few of the many ecosystem services that mangroves offer us, as well as to a rich diversity of species. More scientific work is definitely needed to evaluate what we do not know yet about mangrove ecology. We need to get a better understanding of what mangroves provide that is critical to the biodiversity it sustains (including us humans), so that we protect them as well.
Despite the many contributions of mangroves to sustaining local livelihoods and biodiversity, they are greatly threatened by natural and anthropogenic stresses. The rate of their degradation is twice those of terrestrial rainforests. This global deterioration is primarily due to habitat conversion for the growing aquaculture industry in developing countries.
- Mangroves are being lost globally at a rate of 1-2% per year [FAO07] —amounting roughly to 150,000 hectares (370,050 acres) being lost each year
- 20-35% of mangroves have been lost in last 30 years [Px10]
- Mangroves are critically endangered or approaching extinction in 26 countries (Global Marine Species Assessment).
#DYK — Strong Winds, Storm Surges, and Mangroves
There is ample scientific evidence and literature about how mangroves protect coastal areas from strong winds to storm surges. They not only alleviate the impact of storms in their immediate vicinity but also far away from them. For instance, Florida mangroves helped to reduce the storm surges of hurricanes Irma, Wilma, and Katrina.
Studying coastal water levels during hurricane Katrina and Wilma showed that intact mangrove forests reduced surge heights up to 9.4 cm/km inland. The coastal wetlands in the U.S. are estimated to provide $23.3 billion per year in storm protection. Mother Nature maintains and protects our coasts far more cost-effectively than any man-made structure. It is encouraging to see that we start thinking about using mangroves in urban planning to mitigate future storm surges in South Florida.
These aren’t isolated cases where mangroves have helped. In effect, worldwide mangroves are protecting our coasts from the U.S. to Mexico, the Caribbean all the way to Bangladesh, India, Indonesia, and Australia. With the increase of extreme weather events and their effects on both humans and wildlife, it is becoming critical to protect the world’s mangroves and even to restore them in critically exposed areas.
- The role of mangroves in attenuating storm surges by Zhang, K & al. (2012)
- Mangroves can provide protection against wind damage during storms by Das, S., & Crépin, A., (2013)
Mangroves are disappearing very rapidly from almost every country. The rate of disappearance is much higher in developing countries, where unfortunately more than 90% of the world’s mangroves are located.
When mangroves are cleared due to habitat conversion (aquaculture, urbanization, coastal landfill etc.) or when they are deteriorated due to pollution and upstream land use, their species richness generally ends up declining. And when the decline of a mangrove is advanced, then it is likely to be followed by an acceleration of the loss of species. This, in turn, degrades the functioning of the ecosystem as a whole.
Mangroves have a very high rate of primary productivity (i.e., the synthesis of organic compounds from atmospheric or aqueous carbon dioxide). Then when a mangrove is affected by deforestation, its carbon sequestration capacity is greatly reduced. The support that mangroves provide to terrestrial and marine food webs are also lost in the process.
We don’t know what the world would be without mangroves. However, and according to a report from the SCIENCE magazine [DN07], a further decline in mangrove would affect their dependent fauna with their complex habitat linkages, as well as other would impact the many physical benefits that mangrove provides (e.g., buffering of seagrass beds or coral reefs against siltation or protecting the coastal communities from sea-level rise and tropical tsunamis).
#DYK — Shrimp Farming & Mangrove Loss
Shrimp originated in Southeast Asia some 600 years ago. Mangroves are usually preferred for shrimp farming because of the tidal water exchange, and the naturally abundant supply of shrimp post-larvae that they host.
Although there are many causes for the loss of mangroves other than the conversion of mangroves to shrimp farms, still such conversions have drawn considerable attention and raised controversy. Studies estimate losses ranging from 5% to 10% and up to 38%. No matter what that loss really is, it remains clear that the rise of shrimp farming has an impact on mangroves (as well as that the activity affects some countries more than others). Vietnam, Ecuador, Philippines, and Thailand showed a direct correlation between the increase of shrimp farms and the decrease of mangrove cover.
Global awareness campaigns informing about the impacts of shrimp farming, and focused on re-establishing a sustainable use of mangroves have had some success. The international scientific communities, NGOs, and local agencies have proposed guidelines for the sustainable management of mangroves.
Recommendations and guidelines are a start, following them is the next step. Unfortunately, inconsistencies between policies, management, and enforcement often contribute to the set of causes for the loss of mangroves in shrimp farming areas.
Now you know: my story and love for mangroves deepened when I became more intimate with the Sundarbans mangroves when studying for my Ph.D. Knowledge is powerful this way: it can transform an emotion into wanting to work at protecting a whole ecosystem, and raise awareness about it…
Sadly enough, most people are not aware of the dreaded situation of the mangroves. Although there have been some continuous initiatives by researchers, nonprofits, and governments throughout the world, there are some critical knowledge gaps. That is, we don’t know enough yet about the current status of mangroves; we lack information about the different types of mangroves and the drivers of mangrove losses in different regions.
From a scientific perspective, we could greatly improve the situation by implementing conservation management strategies focusing on comprehensive and comparable data on mangroves from all the countries…
At an individual level, I invite you to read about mangroves, and if you live close by one, visit it and discover its many wonders. A step further would be to join local efforts to protect mangroves, especially against harmful urban development.
Learn More About Mangroves…
What Is A “Mangrove” Forest? (NOAA)
Mangroves (The Ocean Portal Team)
Forest of the Tide (Nat Geo)
[FAO17] Status And Trends In Mangrove Area Extent Worldwide, by M.L. Wilkie and S. Fortuna. Forest Resources Assessment Working Paper No. 63. Rome. FAO. (available at www.fao.org/forestry/mangroves/statistics). Rome; 2003.
[FAO07] The World’s Mangroves 1980-2005. FAO. Food and Agriculture Organization of the United Nations. Rome; 2007.
[GC11] Giri C., Ochieng E., Tieszen LL., Zhu Z., Singh A., Loveland T., et al. Status And Distribution of Mangrove Forests Of The World Using Earth Observation Satellite Data. Global Ecology and Biogeography. 2011; 20 (1):154–159. doi.org/10.1111/j.1466-8238.2010.00584.x
[SM97] Spalding M, Blasco F, Field C. World Mangrove Atlas. Okinawa Japan International Society for Mangrove Ecosystems 1997. 1997.
[SM10] Spalding M. World Atlas of Mangroves. Routledge; 2010.
[PB10] Polidoro BA, Carpenter KE, Collins L, Duke NC, Ellison AM, Ellison JC, et al. The Loss of Species: Mangrove Extinction Risk and Geographic Areas of Global Concern. PLoS ONE. 2010; 5(4):e10095. doi.org/10.1371/journal.pone.0010095
[EA01] Ellison AM, Farnsworth, EJ. Mangrove Communities. In: Bertness, M.D., Gaines, S.D., Hay, M.E. (Eds.), Marine Community Ecology. Sinauer, Sunderland, MA. 2001. pp. 423–442.
[SP02] Saenger P. Mangrove Ecology, Silviculture and Conservation. Kluwer Academic Publishers, Dordrecht, The Netherlands. 2002. pp. 11-18.
[DN07] Duke NC, Meynecke JO, Ellison AM, Anger K, Berger U, Cannicci S. et al. A World Without Mangroves? In: SCIENCE, 06 JUL 2007: 41-42.
Feb 2nd 2018 | by Shilpa Sen
The photos in this article are the property of the author, the following pictures excepted: the distribution map and the images featured in two #DYK boxes (all linked to their source), and the article banner (by Anton Bielousov – Wikimedia Commons CC).
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