The Vulnerabilities & Value of Mangroves: A Deep-Rooted Issue in Shallow Waters
1. Introducing Mangroves
1.1 — An Overview
Along the world’s coastlines within 25° of the equator, groves of water-based trees play a quiet but vital role in the world’s health. These trees, called mangroves, exist in ~70 different species and make up .5% of the world’s coastlines (Alongi). Mangroves are special for a number of reasons — they can uniquely thrive in salty waters, support numerous fish ecosystems, protect coastal villages, and provide highly prized wood. Their biggest role, however, impacts the whole planet. Due to their outstanding ability to sequester 10–15% of annual carbon emissions (Alongi), mangrove forests act as critical defenders against climate change. Unfortunately, 2% of mangrove environments disappear each year (Carugati et al). Conservation efforts thus far have been well-intentioned but often ineffective. In an ecosystem that connects biodiversity, economy, and emissions, it will take a concentrated revitalization movement to sustain mangrove health and push the planet toward a more sustainable future.
Figure 1
Mangroves cover an estimated area of 167,000 km2 to 181,000 km2.
Source: BCI. “Global Distribution of Blue Carbon Ecosystems.” The Blue Carbon Initiative.
1.2 — What are Mangroves?
Mangroves are tropical trees known for their gnarled roots and extensive environmental influence. They cover an estimated global area of 167 to 181 thousand km2 (see fig. 1). 90% of the world’s most active mangrove forests reside in six regions: the West Coral Triangle, Sunda Shelf, Bay of Bengal, Northwest Atlantic, West Myanmar, and Northern Brazil (Adame et al.). Under the strictest definition, 54 true species of mangroves exist within 16 different families, but that number expands to well over 80 species under looser guidelines. Much more tolerant to salt than most plants, mangroves thrive in brines between 3–27 parts per thousand, though they can survive up to 75 ppt — nearly twice the salinity of normal ocean water! Since they inhabit brackish waters, half of mangrove species survive by extracting salt in their waxy leaves, while others filter water through discerning pores in their submerged roots (Erin Spencer et al). As a partially submerged plant, mangrove soils access less oxygen than land-based vegetation. Therefore, mangroves obtain air through cable roots (called pneumatophores) which stick up above the surface (see fig. 2). In some species, like xylocarpus granatum, kinked “knee” roots and S-shaped pneumatophores grant the tree stability despite shifting water levels. Mangroves reproduce through a process called vivipary. Offspring seedlings (called propagules) fall off parent trees then drift with ocean currents. Eventually, the propagules become waterlogged and sink to the muddy floor, where they lodge and continue to grow. Fascinatingly enough, various mangrove species vastly differ in the propagule timeline; rhizophora propagules can still grow after a year of floating, whereas laguncularia racemosa loses its ability to take root after eight untethered days.
Figure 2
Mangroves access oxygen through vertical roots called pneumatophores.
2. Why do Mangroves Matter?
2.1 — A Keystone Species
As a bridge between land and marine ecosystems, mangroves play a key role in global biodiversity. Keystone species (an idea introduced by zoologist Robert T. Paine in 1969) are species that disproportionately affect their natural environment relative to their population. Mangroves are a keystone variant known as an “ecosystem engineer.” From massive to microscopic, thousands of life forms directly depend upon mangroves for survival. Forty percent of the animals restricted to mangrove habitats have been assessed as extinction risks by the IUCN Categories and Criteria (Polidoro et al). As an inhabitant and architect of the tropics, mangroves provide food and shelter to the world’s most diverse food webs.
2.2 — Biodiversity Below the Bayou
Below the surface, mangroves’ dense root systems make a perfect support system for marine creatures. Mangroves serve as nurseries for 90% of the world’s commercial fish species (U.S. Fish and Wildlife Service et. al), in addition to two species on the IUCN’s Red List, the rainbow parrotfish and the goliath grouper. Underwater sponges, snails, worms, anemones, barnacles, and oysters cling to hard mangrove roots for survival. Microbes and fungi use decaying mangrove material as fuel and recycle the nutrients nitrogen, phosphorus, sulfur, and iron. In the calm waters behind mangrove mangles, crocodiles laze in saltwater havens. Mangroves clear the water for coral reefs and seagrass beds, and they are able to absorb pollutants into deep mud without detriment to marine life. Fiddler crabs live in underground burrows, and mud lobsters excavate holes that expose deep soil to air, helping new mangroves take root. Not all crustaceans are beneficial for mangrove life (the Sphaeroma terebrans crab burrows into prop roots, causing them to snap), but all of these animals contribute to a balanced, biodiverse food cycle that — both directly and indirectly — supports our own communities.
2.3 — Life on Land
Mangroves are no less integral above ground. Monkeys, birds, and hundreds of plant varieties live in mangal branches. The critically endangered pygmy-toed sloth lives predominantly among rhizophora mangle trees off the coast of Panama, and a rare hummingbird (Amazilia boucardi) feeds on sweet nectar only found in the mangroves of Nicaragua and Ecuador. Hollowed mangrove twigs are home to thousands of species of ants, spiders, moths, termites, and scorpions, which supports larger animals including lizards, snakes, and frogs. In Malaysia, mangroves house flashing fireflies, and in Borneo, the endangered proboscis monkey rarely leaves mangrove thickets. In southern China, bats are essential mangrove pollinators, particularly for the sonneratia, a flowering plant that opens at dusk. The critically endangered Bengal tiger, which inhabits the Sundarbans of India and Bangladesh, can adeptly navigate the thick undergrowth and sinking mud pits of mangrove forests. Unfortunately, rising sea levels are forcing tigers into human-inhabited zones, which risks both human and tiger lives due to reciprocal hunting.
2.4 — Support for Civilization
In a human-focused lens, mangroves are also a vital pillar of the global marketplace, contributing $1.6 billion each year to local economies (Polidoro et al). As GEO Blue Planet, an international climate network, explains, “Healthy ecosystems are important for society since they provide services including food security, feed for livestock, raw materials for medicines, building materials from coral rock and sand, and natural defenses against hazards such as coastal erosion and inundation” (GEO Blue Planet). Mangrove wood is used to build stilt houses, furniture, fences, and bridges. In communities near the Sundarbans, villagers construct fishing poles, traps, canoes, rafts, and boats from mangrove timber. Japan highly prizes mangrove charcoal, and the trees provide seafood, fruit, medicine, fiber, and wood for coastal villages. In addition to providing food, resources, and safety, mangroves’ ability to clean surrounding waters allows humans to swim and fish in their local seasides. These trees’ most notable effect on civilization, however, is their ability to decrease storm surge. Every 330 feet of mangrove forest can reduce wave height up to 66% (Spalding et al.), which saves lives along cyclone paths. In the 1950s, a five-mile-thick blanket of mangroves cushioned Indo-Pacific villages from storms. Today, forests have been reduced such that these vulnerable communities sit directly at the water’s edge. This was best illustrated in 1991, when a typhoon hit an area of Bangladesh where mangroves had been stripped away. The 20 foot storm surge killed 138,000 people and left 10 million homeless (Rudolph) — a toll equivalent to the population of modern day Los Angeles County. This event spurred many restoration projects, including a three year long planting effort which resulted in an outstanding 70% mangrove survival rate in sheltered areas (SER). Ultimately, mangrove preservation saves human lives.
2.5 — Blue Carbon: Climate Change Defense
Beyond aiding biodiversity and human economies, however, mangrove protection has been prioritized by global environmental agencies for yet another purpose: fighting climate change. Like most plants, mangrove tree leaves absorb carbon dioxide for photosynthesis. When the trees eventually die, they fall to the muddy seafloor, taking stored carbon with them to be buried in the underwater soil. This is called a blue carbon system (as opposed to terrestrial “green” carbon and gaseous “black” carbon). Through this process, mangroves store 28 million tons of carbon annually. Although mangroves make up less than 2% of marine ecosystems, they account for 10–15% of global carbon burial each year (Alongi). One acre of mangroves can store 1,450 lbs of carbon annually, securing a title among the world’s “big three” carbon sequesters (one acre of seagrass and salt marsh respectively stores 740 and 1940 lbs of carbon per year) (Feller). Remarkably, neither mangroves nor their animal ecosystems suffer from densely carbonated soils. However, blue carbon areas become devastatingly dangerous when disturbed. When farmers cut down trees and turn over soils to build crop fields and shrimp ponds, the carbon trapped below gets released to the air. A study in 2012 found that deforested mangrove soils were responsible for 240 million metric tons of CO2 exposure (Pendleton et al.), or 4.5% of that year’s total carbon emissions (EIA). By simply leaving mangrove forests alone, we protect the Earth from further pollution-induced warming.
3. Why Are Mangroves Disappearing?
3.1 — Shrimp Farming
Among the numerous reasons for mangrove forest disappearance, the most egregious and most easily-remedied culprit takes the form of shrimp farming. Because shrimp farms require flat expanses of shallow, floodable land, 35% of the world’s mangrove forests were cut down for aquaculture in the 1980s and 1990s (Friess et al). This is not a sustainable rate of loss. When shrimps reach maturity, which takes about 3–6 months, ponds are drained for harvest. This releases shrimp waste, pesticides, chemicals, and antibiotics to infect the surrounding environment. After a number of harvest cycles, the pond’s sludgy floor develops into acid sulfate soil. This releases lethal amounts of acid, iron, and aluminum into shrimp pools upon refilling (Stevenson) and forces farmers to abandon their plots. Thailand has enforced restrictions on aquaculture in response, but the issue exists far outside of just one country. Between 1951 and 1988, the Philippines lost about half of their mangrove forests to shrimp farms (Friess et al). Since 1970, an analysis of eight countries in South America and Asia indicated that 52% of mangroves had been depleted. In Indonesia, aquaculture production is expected to grow by 7% yearly between 2012 and 2030, and the government has considered expanding aquaculture fields by another 26 million hectares to meet such demand. If Indonesian farmers instead used more efficient, sustainable shrimp farming techniques, they could amplify their crop results without further destruction of mangrove forests.
3.2 — Rising Seas
Mangroves are susceptible to rapid changes in sea level. In 2013, a study concluded that 71% of mangrove forests experience about 656 feet of coastline retreat per year (Feller). These trees used to be able to move inland when sea levels rose. One particular mangrove-climbing frog, the aratus pisonii, was never seen north of Miami before 1918. In just 100 years, they’ve crept northward 450 miles to Georgia in tandem with rising sea levels and expanding mangrove territory. However, human development and lack of mud often impede mangroves from moving with the oscillating ground level.
Since the 1970s, global warming has shattered normal temperature fluctuations. Our onslaught of fossil fuel-based energy has started a vicious cycle of trapped heat that’s only gaining momentum. Over the past few decades, the ocean has absorbed the heat we’ve released when burning gas and oil. Now, the sea is starting to lose its balance, and the polar ice caps are warming. This arctic melting produces a whole host of new problems. Much like how mangroves bury carbon, ice buries methane, a gas 84% more potent than CO2 (UNECE). Methane exponentially speeds global warming when added to the atmosphere. Simultaneously, melting icecaps have decreased the planet’s white reflective surface and increased our dark ocean area, which absorbs the sun’s energy. To make matters worse, water expands as it warms, and the oceans are gaining unprecedented amounts of new water. Thermal expansion and ice melt used to play equal roles in sea level rise, but in the past two decades, ice melt has doubled in power (see fig. 3) (Lindsey). Since 2002, Antarctica’s ice melt alone has annually added 149 billion metric tons of water to the sea (see fig. 4) (NASA). Between 1994 and 2014, the rate of ice loss from the Greenland Ice Sheet increased from 34 to 247 billion tons per year (Lindsey).
Figure 3
In the past two decades, ice melt has doubled in power.
Source: Lindsey, Rebecca. “Climate Change: Global Sea Level.” Climate Watch Magazine, National Oceanic and Atmospheric Administration, 25 Jan. 2021.
Figure 4
Since 2002, Antarctica’s ice melt alone has annually added 149 billion metric tons of water to the sea.
Source: NASA. “Ice Sheets.” Global Climate Change: Facts, NASA, 26 Jan. 2021.
3.3 — Invasive Species
As we learn how to protect these vital trees, it becomes invaluable to understand and recognize which flora and fauna can diminish mangrove populations, and where mangroves can act as an invasive species themselves. In China 1979, Spartina alterniflora (a seagrass) was introduced to maintain banks and tidal flats. It spread uncontrollably and took over 66% of mangrove biomass in human-disturbed mesohaline (3–10 ppt) and polyhaline sites (18–30 ppt) (Zhang, Yihui, et al). In a reverse invasion, the Asian mangrove species Lumnitzera racemosa infested Florida coastlines after its 2008 induction to a Miami botanical garden. This new mangrove species transformed many mudflats into salt marshes, drastically reducing the populations of mud-flat dependent, commercially valuable shorebirds and shellfish (Teal 2490–2495). Similarly, when the American Sugar Company brought Floridan rhizophora mangle to Hawaii to counteract eroding coastlines in 1902, the species hurt native birds who could not roost or hide in its branches. In the 1930s, Texans released Asian elk Nilgai as an exotic hunting game. Today, these 600 pound antelopes infect cattle ranches (Brezosky) and feast on endangered mangrove leaves. For better or for worse, mangroves weave themselves into the complex interdependencies of their environments. It’s up to us to thoroughly investigate these ecosystems before introducing a potentially disruptive foreign species.
4. Mangrove Protection
4.1 — Past Efforts Which Fell Short
Mangroves are now internationally recognized as a major defender against biodiversity loss and temperature rise, but few restoration projects have achieved desired results. Most efforts have been approached the same way one would try to replant a forest: by growing saplings in greenhouses and transplanting them to mudflats. However, this method hasn’t worked well. Between 1984 and 1992, World Bank spent $35 million to plant nearly 3 million mangrove seedlings along the Phillippine coast. By 1996, fewer than 20% of these trees had survived (Feller). Similarly, when restoration groups were flooded with donations to revitalize mangroves after a 2004 tsunami demolished Sri Lanka, they rushed into action without proper research. “Mangrove restoration projects in Sri Lanka have been generally unsuccessful, despite the good intentions which fueled them in the first place,” reported Dr. Kodikara, a botanist at the University of Ruhuna, Sri Lanka. “Our study shows a frequent mismatch between the aims of restoration initiatives and the realities on the ground.” Despite investing $13 million, many mangroves were planted in waters too deep, in abnormal zones, and too close to grazing animals like cows, goats, and donkeys (SER). The 2010 Convention on Biological Diversity developed the twenty Aichi Targets, which aimed to improve global biodiversity by 2020. Although 44% of biodiverse regions are now protected (a 15% increase from 2000) and roughly 200 invasive species on islands have been eradicated, the convention unfortunately fell short of every target (Earth.org).
4.2 — A Triple-Threat Solution
Facing the future, we must address the world’s disintegrating mangrove populations with a solution as interconnected as the complex nature of our warming ecosystem. The best path forward will protect mangroves through a three-pronged approach: halting deforestation through improved farming technology, replenishing mangrove populations through coastline construction, and mitigating sea level rise by switching to renewable energy.
4.2.1 — Innovative Farming
A new form of shrimp farming, called Intensive 2.0, shows great promise in parts of Southeast Asia. In Intensive 2.0, farmers use probiotics to break down waste (ammonia, nitrite) into harmless dissolved nitrogen. They use advanced filtration systems, like a central pump, to recirculate each pond’s water 3–4 times per day. By using more wheel aerators and micro bubblers, farmers can better oxygenate their ponds, which helps shrimp breathe, grow, and avoid disease. Furthermore, these innovative farmers raise juvenile shrimps in external nurseries, where they can catch diseases before spreading contagions to the rest of the pond (Rubicon Resources), without using antibiotics. A professor of agricultural systems at the University of Missouri, David Blune, has proven that aerator paddle wheels can also produce high amounts of algae, which processes all the shrimp waste internally, leaving behind zero waste (Blank).
To address abandoned shrimp ponds filled with toxic acid sulfate soil, multiple techniques have been successfully implemented. In Malaysia, scientists have added lime, basalt, and organic fertilizer to alleviate soil infertility before repurposing shrimp ponds as palm oil, rice, and cocoa fields. Even better, others have flushed out acidic drain water with tidal flows, which helps replenish mangrove populations. By neutralizing acidic soil with alkaline seawater, the land can slowly return to its former, invaluable biodiversity. Of course, high-tech farming isn’t the only way to help shrimpers sustainably meet market standards. By educating wealthy consumers about the industry’s environmental detriments, we can reduce global demand. In a less intensive environment, shrimp farmers can focus on quality over quantity.
4.2.2 — Sculpting Shorelines
Erosion has weakened forest foundations, but specialized coastline sloping methods can help mangroves naturally replenish. In 1998, American biologist Robin Lewis engineered a technique to stabilize shorelines. Using helicopters, his crews dropped over 1000 cubic yards of oyster shells onto Florida’s Atlantic Pelican Island shoreline, which provided a nutrient-rich environment to hand plant mangroves and a “nurse” species, smooth cordgrass. This project preserved 16 different bird species, including endangered Wood Storks. On the gulf coast, the Coastal Resources Group used this methodology to remove tidal flow obstacles and install culverts. Mangrove propagules best survive when submerged 30% of the time and dry for the other 70%. By aiding tidal flow and gradually sloping the coastline toward the sea, they restored 225 acres of Marco Island’s black mangrove basin forest (Flynn). Since then, Lewis has helped restore coastlines in 22 different countries, including Vietnam, Cuba, Thailand, Nigeria, and the Phillippines (Biohabits). In each location, he stresses the importance of understanding an area’s historical ecology: What mangrove species live here? How do they interact with marsh plants? Where do they exist along the coastline? By researching biological interactions, aiding tidal flows, sloping shorelines, and enlisting the help of government agencies such as the National Oceanic and Atmospheric Administration, we can begin restoring mangrove populations around the world.
4.2.3 — Switch to Renewables
Lewis’s technique defends mangroves against rising seas, but a more proactive approach could address this problem at its root. The most powerful way to decelerate sea level ascent is to enthusiastically invest in renewable energy sources. In December 2015, 189 countries made history by signing the world’s first international, legally binding treaty which holds every nation accountable to limit global warming under 2°C. Adherence to the Paris Agreement is absolutely critical to mitigate the sixth mass extinction.
Particularly dedicated and innovative countries have led the march toward clean energy. By late 2019, Morocco reached a 35% reliance on solar, thanks to their investment in the world’s largest and densest solar farm, the Noor Ouarzazate complex. By 2018, eco-conscious Costa Rica had already reached 98% renewable energy from hydropower and extended its moratorium on oil extraction from 2021 until 2051 (Mulvaney). India, the second-most populous country in the world, approached its goal to generate 40% of power from wind and solar a decade earlier than expected (Ministry). Gambia has unrolled plans to construct one of the largest photovoltaic plants in West Africa and restore 10,000 hectares of forests, mangroves, and savannas (Mulvaney).
These countries serve as models for the world’s worst polluters — China, Saudi Arabia, Russia, and the United States — to emulate. Although China has heavily invested in electric vehicles and solar power, the magnitude of its population burdens the country with a greater responsibility to set an example, and its current course is inadequate to decrease global temperature by 2°C. Saudi Arabia, the world capital of oil, set its insufficient goal at double the acceptable emissions rate, and is projected to overreach even that (Mulvaney). If all countries were emitting carbon per capita at Russia’s rate, we’d easily reach a 4°C increase in global temperature (CAT). In an utterly backwards and embarrassing move, President Trump revoked America’s participation in the Paris Agreement and rolled back over 100 environmental regulations over the course of his four-year presidency. Under Biden’s leadership, the US recently rejoined the Paris Agreement and indefinitely paused new fossil fuel ventures on public lands and offshore waters “to the extent possible” (Mena). These are steps in the right direction, but as the world’s second-worst polluter (UCSUSA), America must drastically readjust its targets and tenaciously tackle independence from gas.
4.3 — Forests of the Future
Even in the best-case scenario, our planet’s mangroves will suffer from high waters and low biodiversity. As the Oregon State University climate scientist Peter Clark aptly put it, “If we were to meet these initial goals of the Paris Agreement, the sea-level commitment from global warming will still be significant” (Oceanographic). However, renewable energies will mitigate the catastrophe. Educated, wealthy, and empowered nations must choose short-term hardship over long-term disaster. The future looks like a bleak, inevitable series of one-way doors toward mass extinction, but bidding a final and swift farewell to fossil fuels and deforestation provides the best fate we can hope for. In many ways, mangroves act as a microcosm of the planet. If we can restore these foreshore forests’ health, we can protect our most vulnerable communities, and ultimately, ourselves.
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