Between Land And Sea: Mangrove Ecosystems And Their Mysterious Inhabitants

Table of Contents

Most of us, probably, have accidentally overwatered a houseplant at some point.

 

Like us, plants that live in soil need moisture to survive, but their roots require oxygen to breathe. If the soil inside the flowerpot gets too waterlogged, the roots begin to drown – and the plant might die.

 

How, then, do some trees survive with their roots underwater for up to twelve hours of the day?

The unique features of mangroves

Mangroves are plants that are neither terrestrial (like oak or coconut trees) nor marine (like seagrasses). Instead, they are found in the salty or brackish  waters of coastal areas. When the tide comes in, it submerges most or all of the plants. Scientists recognize more than 70 species of mangroves and call the submerged forests that they form mangals . They are found on every continent except Europe and Antarctica.

The intertidal zone
The intertidal zone - where land meets sea between high and low tides. Photograph taken in the Sundarbans, West Bengal, by Priyanjana Pramanik

These hardy species have developed various adaptations  that allow them to thrive in this challenging intertidal  environment. They have waxy, water-resistant leaves. Some, like the large-leafed orange mangrove (Bruguiera gymnorrhiza), have roots that look like bent knees. Others, like the blind-your-eye mangrove (Excoecaria agallocha), have winding snake roots that are easy to trip over.

 

Meanwhile, the grey mangrove (Avicennia marina) has pencil roots that stick straight out of the soil. These modified roots  are all aerial – not only do they allow the trees to grip the soil tightly, but the roots can breathe from the air instead of the soil!

Aerial roots
Aerial roots such as these cone roots or pneumatophores allow the mangrove tree to hold tighter to the soil and to breathe. Photograph taken in the Sundarbans, West Bengal, by Priyanjana Pramanik

Most plants produce seeds that are dispersed  into the environment before they germinate and begin to grow. Mangroves, on the other hand, are viviparous  – their seeds germinate while still attached to the parent plant, forming propagules where you can see the young roots sticking out on one end. These propagules, once they are developed, detach from their parents and are carried away by the tide; if the soil where they come ashore is favorable, that’s where they will grow.

 

Mangroves also have to make sure they do not absorb too much salt from their saline surroundings. They have developed different mechanisms either to exclude salt or secrete it; some take the easy way out and accumulate it instead.

 

Since the intertidal zone exists between the land and the sea, the areas closer to the sea are submerged for longer than the areas closer to land. This leads to zonation , where the species that can tolerate more salt and oxygen stress are found closer to the sea. Scientists have noticed that the grey mangrove is found in areas with the highest salinity, while the blind-your-eye mangrove and the large-leafed orange mangrove are found in areas with moderate-to-low salinity.

The inhabitants of mangrove forests

When you are about to enter a mangrove forest, perhaps it is the smell that will hit you first. Microbes break down the organic matter dropped by the trees, including leaves and seeds, releasing hydrogen sulfide (which some people say smells a little like rotten eggs). With each step you take in the forest, your feet squish into the silty mud carried in by the tide and you can feel the aerial roots from the Avicennia.

 

The inhabitants of the forests feel your unfamiliar presence. Some, like the little fiddler crabs , scurry out of your path and into their burrows in their hundreds. The mudskippers , which are fascinating fish that can walk on land and climb trees, inch away with little plopping sounds. If you stand still, they’ll soon be back to investigate.

Mudskippers
Mudskippers - fascinating fish like this Boleophthalmus boddarti that can breathe and move about on land. Photograph taken in the Sundarbans, West Bengal, by Priyanjana Pramanik

Fiddler crabs construct and live in elaborately constructed burrows, which protect them from the tide and hide them from predators. In males, one claw is much larger than the other – they use these in waving displays to challenge other males, defend their territories, and attract females.

 

Fiddlers eat tiny pieces of organic matter that they scoop up with their claws and bring to their mouth parts. They get their name from the way that they ‘fiddle’ with soil particles. Their digging activities are called bioturbation, which has been found to increase the nutrient content of intertidal soil and make it more favourable for mangroves to grow.

 

Mudskippers are equally fascinating. They also create burrows to escape into and also store their eggs in them until they hatch. They will swim if they have to, but some seem to prefer living on land even after the tide comes in! Mudskippers are animals that truly exist between land and sea, and their history can teach us many lessons about how animal life evolved on land.

The importance of mangrove ecosystems

Mangroves only cover about 0.1% of the earth’s surface but are critical for the well-being of human beings and our environment. They are responsible for invaluable ecosystem services, such as providing fish, timber, and other products like mangrove honey. Scientists believe that nearly 80% of fish caught globally depend on mangroves – many species spawn in these areas, as young fish use mangroves as refuges from predators.

Mangrove forests
Mangrove forests are important for the world's fisheries. Photograph taken in the Sundarbans, West Bengal, by Priyanjana Pramanik

They also buffer humans and inland areas from erosion and storms – after many natural disasters, relief workers and researchers have noticed that the areas with more mangrove cover faced less damage and destruction. Mangrove ecosystems also sequester  an estimated 25.5 million tons of carbon every year. These roles will become even more vital as climate change causes more extreme weather events.

The threats facing mangroves

Unfortunately, mangroves are disappearing worldwide and nearly one-third have been wiped out in the last fifty years alone. In India, the Sundarban mangrove forests, which are among the largest in the world, have been designated an endangered ecosystem . As the number of humans living near shorelines increases, mangroves are cleared; they are also removed for other coastal developments, farming, and fisheries.

 

Many animals that are dependent on mangroves are vulnerable to extinction as their habitats disappear. These include the fishing cat and the Ganges river dolphin in Asia, and the critically endangered mangrove finch in the Galapagos islands. A mangrove tree called Sundari (from which the Sundarban mangroves get their name) is also endangered.

Protecting existing mangrove forests

However, there is hope that mangroves can recover and continue to be havens for biodiversity , protective forces against extreme weather, and fascinating places to visit. Scientists believe that the best way to conserve them is to protect the mangrove patches that currently remain, and carefully sustainably increase mangrove cover by creating plantations and encouraging natural regrowth.

 

Encouragingly, 42% of the surviving mangrove forests are now protected, and many local communities have led the charge to safeguard and nurture them. In India, interest in the Bengal tiger drove efforts to protect the mangrove forests of the Sundarbans, and this project continues today.

 

There is a great deal of work yet to be done there is cause for optimism, especially as the United Nations has declared 2021 to 2030 to be the Decade on Ecosystem Restoration, to “prevent, halt, and reverse the degradation of ecosystems on every continent and in every ocean.” The first step is to learn more about them, which you have done here!

Flesch Kincaid Grade Level: 10.9

 

Flesch Kincaid Reading Ease: 51.6

Glossary

Adaptation: The biological mechanism through which an organism adjusts to changes in their current environment, or to a new environment.

 

Biodiversity: Biological diversity – the wide variety of life that we see on earth, from genes to entire ecosystems.

 

Bioturbation: Moving of soil particles by plant roots or animals so that they can access resources like nutrients, food, and water.

 

Brackish: Water that is a mixture of freshwater (like from rivers) and saline water from the sea. Brackish water is found in estuaries, where rivers flow into the sea.

 

Dispersed: Dispersal is the mechanism through which seeds are transported from their parent plants to new sites where they can germinate and grow.

 

Endangered ecosystem: An ecosystem that faces a high risk of collapse according to criteria defined by the International Union for Conservation of Nature (IUCN).

 

Fiddler crabs: Marine crabs of the family Ocypodidae in which males have one claw that is much larger than the other.

 

Intertidal: The area in which the ocean meets land between low and high tides.

 

Mangals: A forest made up of mangrove trees, often in swampy areas.

 

Mangrove: Trees that can survive in low-oxygen, high-salinity, and intertidal areas because of biological adaptations such as modified root systems.

 

Mudskippers: Fish of the sub-family Oxudercinae in the goby family which spend a significant portion of time outside water and can breathe through their skin in addition to their gills.

 

Modified roots: Plant roots that have changed their shape and structure so that they can absorb and transport water and minerals, or for support, respiration, and food storage.

 

Propagule: A vegetative structure that detaches from the parent plant and can grow into a new, independent plant.

 

Sequestration: Capture and storage of atmospheric carbon dioxide.

 

Viviparous: Plants that can reproduce from buds that grow into plantlets while they are still attached to the parent plant, or from seeds that germinate while still inside the fruit.

 

Zonation: Bands or groupings in a plant or animal community in environments such as rocky shorelines or mangrove forests.

Barik, J., Mukhopadhyay, A., Ghosh, T., Mukhopadhyay, S. K., Chowdhury, S. M., & Hazra, S. (2018). Mangrove species distribution and water salinity: an indicator species approach to Sundarban. Journal of Coastal Conservation, 22, 361-368.

 

Hogarth, P. J. (2015). The biology of mangroves and seagrasses. Oxford University Press.

 

Jaafar, Z., & Murdy, E. O. (Eds.). (2017). Fishes out of water: biology and ecology of mudskippers. CRC Press.

 

McKee, K. L. (1993). Soil physicochemical patterns and mangrove species distribution–reciprocal effects? Journal of Ecology, 477-487.

 

Valiela, I., Bowen, J. L., & York, J. K. (2001). Mangrove Forests: One of the World’s Threatened Major Tropical Environments: At least 35% of the area of mangrove forests has been lost in the past two decades, losses that exceed those for tropical rain forests and coral reefs, two other well-known threatened environments. Bioscience, 51(10), 807-815.

 

Vázquez-Lule, A., Colditz, R., Herrera-Silveira, J., Guevara, M., Rodríguez-Zúñiga, M. T., Cruz, I., … & Vargas, R. (2019). Greenness trends and carbon stocks of mangroves across Mexico. Environmental Research Letters, 14(7), 075010.

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