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Mangrove Forests

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Mangrove forests are a habitat in tropical coastal areas consisting of mangrove trees and shrubs.

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Mangrove trees grow in tropical areas, where the temperatures are always warm. Mangroves survive in an environment that would kill most trees. They sit in salty, muddy water. Mangrove trees have long tangled roots (seen in the image below) that prop the tree above the water. They stand tall like stilts, which keeps the trees from going underwater when the tides come in.

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Some species of mangrove trees are so well adapted to salty conditions that they can survive in water twice as salty as ocean water. To think of how extraordinary this is, if you fed a houseplant with salt water, it would dry up and die. Similarly, you could not drink salt water. Salt pulls the moisture out of most living things. Mangroves have come up with two adaptations to deal with salt:

  • Some species have a barrier they use to prevent salt from getting to the inside of the tree. This allows them to "drink" in the water they need while keeping the salt, from the saltwater, out.

  • In other species, the saltwater is pulled into the plant, but then it is excreted (or pushed back out) through special pores. You can see in the photo below, as the salty water dries, salt crystals remain on the surface of leaves.

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(Ulf Mehlig, Wikimedia Commons)

Mangrove forests are one of the most productive ecosystems in the world. They support the health of the surrounding habitats. 

Mangrove forests protect the shoreline from waves. They keep the water healthy for nearby coral reefs. They also provide a habitat for thousands of species.

Mangroves provide very important services to the ecosystem:

 

Mangroves physically stabilize the landscape

  • Mangroves are important for coastal defense. They reduce erosion along the coastline by absorbing the energy from waves and tides (see video below). They also create a barrier to protect the coastline from flooding and hurricanes. When storms impact regions bordered by mangroves, the trees can protect the land, and the towns along it, from being destroyed.

  • Mangroves maintain water quality. Roots hold onto sand and dirt in the water, making the water clear and healthy. They also filter out pollutants that run off the land. Coral reefs and seagrass beds rely on mangroves to purify water for them.

  • Mangrove forests are very efficient at absorbing and storing carbon. This helps reduce greenhouse gases, which helps with climate change.

Mangroves are the basis of the mangrove forest ecosystem

  • Mangroves act as the foundation of the food chain. Crabs and insects eat the live mangrove leaves. Microbes and fungi live on the roots and eat decaying leaves, recycling the nutrients back to the mangroves.

  • The roots of the mangroves create complex habitats underwater, which serve as hiding places for fish. The tree tops also create habitats for birds and monkeys.

  • Mangroves serve as a nursery for many crabs, shrimp, and fish. Because the roots provide safety from predators and ample food, these animals live the early stages of their life in the mangroves before moving out to the sea.

 

Scale model showing how mangroves protect shorelines from waves:

Animals that live in mangrove forests:
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River otter:

Some river otters, such as the Asian small-clawed otter, can live in mangroves. Otters have dense fur that keeps them dry when they're in the water. They live in family groups of up to 15 individuals. Asian small-clawed otters are very social, and spend much of their time playing with others in its group. They can also attack as a group, scaring off even large predators. Asian small-clawed otters use their hands to hunt, rather than catching food with their mouths. While in the water, they dig in sand and under rocks for shellfish and crabs. Asian small-clawed otters are very sensitive to habitat pollution and destruction, and their numbers have been decreasing.

American crocodile:

They live in rivers but prefer salt water, and can be found in mangroves. American crocodiles eat a wide range of food - insects, crabs, fish, turtles, and birds. Large crocodiles even eat cattle occasionally. They will also eat stones to aid in digestion. Crocodiles are very intelligent and provide advanced parental care. Females build a nest on the water's edge, lay their eggs in it, and guard the nest until the eggs hatch. The baby crocodiles call out as they are hatching, and the mother will help them hatch, and then scoop them in her mouth and carry them to the water. She will care for them for a few weeks until they go on their own way.

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Upside down jellyfish:

These jellyfish get their energy through a symbiotic relationship with zooxanthellae. This is the same microscopic algae that lives in coral. By flipping upside down, they expose the zooxanthellae, living in the tissue of their arms, to more sunlight, and the zooxanthellae provide the jelly with nutrients. However, upside down jellyfish do a little hunting of their own. They hover near the ground around the mangroves, and pump a cloud of mucus into the water around them. This mucus also contains the stinging cells that jellyfish arms normally have, and this allows the upside down jellyfish to capture small shrimp.

Fiddler crab:

Fiddler crabs are easy to recognize because the males have one claw much larger than the other. These crabs are detritivores- meaning they eat dead plant and animal matter. The small claw grabs a ball of sediment and brings it to the mouth. Before swallowing, the crab sifts through the sediment to find anything edible- fungus, algae, or decaying bits (detritus). The remaining sediment gets spit out as a ball. Fiddler crabs are very important to the ecosystem. As they sift through mud, they aerate it, or mix it up and add air. They also help clean up and decompose decaying matter, which stops the spread of disease. The detritus, or decaying matter, stores energy and nutrients. Once the fiddler crab breaks down the detritus by eating it, some of those nutrients are deposited back into the sediment, which helps other things grow.

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Mangrove snakes:

The black and yellow snakes are nocturnal, meaning they hunt at night. During the day they are usually basking in the sun, 100 feet up in tree branches. They hunt for bats and birds, but also eat lizards and frogs. Their venom is powerful enough to kill birds, but not humans.

Mangrove Forests are one of the most threatened habitats in the world.

By the year 2000, half the world's mangrove forests were gone. 

  • Shrimp farming is the largest threat to mangroves. Wetlands are cleared and water is diverted to create the farms. The trees die by having freshwater supplies cut off, and the surrounding areas become contaminated with chemicals and the waste created by the farms.

  • Unsustainable tourism brings trash and pollution. Tourists that walk off trail, light fires, or collect shells and plants harm the ecosystem as well.

  • Mangrove forests are removed to create palm oil plantations and rice paddies. 

  • Mangroves are cleared for coastal development - to build things like hotels, golf courses, and marinas

  • Mangroves are also cut down to be used as timber and charcoal.

How you can help
  • Look for sustainable options for shrimp. Avoid eating shrimp that is farmed in mangrove areas.

  • If you live in an area that offers goods made of mangrove wood, buy an alternative instead.

  • Be mindful of your impact when visiting a mangrove forest. Stay on trails, don't pick up anything to take home (unless it's trash!). When swimming around mangroves, keep your legs as still as possible to avoid turning up the sediment in the water.  

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