Mangroves

Ecological Importance of Mangroves

Mangroves are made of a rigid network of mangrove trees cemented into the sediment, which provides homes for a plethora of terrestrial and marine species. It is for these reasons that mangroves offer many ecological services. Let’s go over them now.

Coastline Protection

Mangroves offer protection from tides, waves and storms by acting as a physical barrier between the sea and the coastline. Additionally, mangroves’ intricate root system, which is embedded into coastal sediments, helps stabilise these sediments, limiting erosion caused by natural forces such as hurricanes and tsunamis. Mangroves will build up soils over time, further protecting the coast against rising sea levels.

Figure 1: Sundarban mangrove forest in Asia, via Wikimedia commons

Habitat for Wildlife

Mangroves act as nurseries for a plethora of aquatic and terrestrial wildlife, meaning that they provide a habitat for some of the most biodiverse ecosystems on earth. Home to a variety of globally sought-after fish (i.e. catfish, stream fish, pufferfish), they also offer fantastic fishing opportunities for commercial fishing industries around the world. Mangroves provide a shelter from predators and a feeding ground for many marine species, as well as supplying an abundance of nutrients for aquatic and terrestrial wildlife. The many species which reside in mangroves will be covered later on in ‘Mangrove ecosystems’.

Carbon Sinks

Mangroves offer up another ecological service - they are excellent carbon sinks. Mangroves not only store carbon as organic material in their branches (produced through photosynthesis) but also store massive amounts of carbon in their soil. This is because organic matter sinks into the mangrove mud and becomes trapped. This is often in the form of dead leaves and branches, dead organisms from the ecosystem, and detritus. Decomposers struggle to recycle all this organic matter quickly, so much of it becomes trapped in the soil for decades.

Economic Benefit

In addition to mangrove trees being harvested for water-resistant wood, mangrove leaves have proven to be useful for a range of purposes. These include livestock feed, tea and various medicinal products. Inroads have been made into sourcing antibacterial compounds from mangrove leaves too.

Maintaining Water Quality

Mangroves support surrounding communities too, through their ability to maintain water quality. Nutrient runoff, which can potentially cause eutrophication of nearby water, is absorbed by the mangrove soil, reducing the frequency of algal blooms. Their role in preventing coastal erosion also improves water quality because debris that would pass into the sea builds up in the mangrove soil instead. Mangrove’s ability to improve surrounding water quality results in a symbiotic relationship forming between nearby habitats like coral reefs and seagrass.

Mangrove Ecosystems

The complex entanglement of mangrove tree roots provides shelter from predators and so often protect the larvae of nearby marine communities. The extensive producer population stemming from the fertile soil also offers a feeding ground for fish. Both these factors result in a wealth of terrestrial and marine species either living in or regularly visiting the mangrove, making for a complex and diverse ecosystem.

Let’s go over some organisms which inhabit or depend on mangroves.

Mangrove Snapper

The mangrove snapper is a red-coloured snapper native to the Atlantic coastline of America and Brazil, the Caribbean and the Gulf of Mexico. They thrive in brackish water, so are often found feeding at coastal habitats such as coral reefs, mangroves and seagrass. Mangrove snappers feed on small fish and crustaceans, which are found in abundance in these habitats. The health of reefs and seagrass depends on mangroves’ ability to improve water quality, so the mangrove snapper is completely dependent on mangroves for their nutrition.

Fig. 2 - Photograph of a mangrove snapper.

Mangrove Monitor

The mangrove monitor is a venomous lizard which is native to the coastal areas of Australia and the Pacific Islands. The mangrove monitor is perfectly adapted to hunting in mangroves because it has a specific nasal gland which reduces the amount of salt passing into its body. Therefore it can hunt successfully in the high salinities of the mangrove. They are carnivores, so prey on fish, birds, eggs and some crustaceans.

Whale Sharks

As described earlier in the article, Whale sharks are herbivorous, spotted sharks that grow up to 18m and can be found in warm, tropical seas. Whale sharks are the largest fish in the sea and are filter feeders meaning they are the only organisms that are dangerous to are plankton. Mangroves and coral reefs are home to massive amounts of plankton, so whale sharks will often be found feeding around these habitats.

Fiddler Crabs

Fiddler crabs are a group of crustaceans which can be found in coastal areas in the Pacific Ocean and on both sides of the Atlantic Ocean. These crabs inhabit burrows underneath the protective structure of entangled mangrove trees. You can easily determine the sex of fiddler crabs because the male has a huge colourful claw. They use this to fend off predators and fight other males for territory, but also to court female crabs. Amazingly, these males will work in harmony when resisting predators in order to maximise the time they have to attract females.

Fig. 3 - Photograph of a fiddler crab.

Threats to Mangroves

Mangroves are essential for the health of coastal waters with the coastal protection, nurturing habitat and water quality maintenance they provide. In spite of this, human interference and climate change are endangering mangroves around the world.

Some reasons mangroves are under threat include:

• Coastal Development: Many human communities live by the sea and the building of roads, hotels, restaurants and other infrastructures physically break down mangroves, reducing the habitat available to the species which live there. The machinery used to build these infrastructures creates a lot of noise which disturbs the wildlife, and emits pollution, which may damage the mangrove trees.

• Agriculture: mangroves are deforested to allow the land to be used for agricultural purposes. The overuse of fertilisers, herbicides and pesticides can lead to the leaching of nutrient-rich soils, which will runoff into the ocean and potentially cause algal blooms via eutrophication of the water. Algal blooms will restrict sunlight received by producers in the water, decreasing food resources for the community.

• Waste: poor recycling initiatives in coastal communities can lead to a lot of waste and debris being dumped in the sea. Tourism, which is especially prevalent in the tropical beaches where you find mangroves, results in careless people dumping rubbish on the beach, which is swept into the ocean. Waste will contain harmful chemicals which pollute the water and can even become trapped between mangrove trees, ruining the mangrove habitat for the ecosystem.

• Exploitation: mangrove wood is a useful material because of its durability and water resilience so that industrial companies will deforest mangrove communities for resources. Additionally, many commercially caught fish and shrimp reside in and around mangroves, so mangrove habitats are subjected to huge amounts of fishing throughout the year. The decline in fish and shrimp populations negatively impacts interspecies relationships in the ecosystem.