A pair of brown pelicans and their chick have taken up residence in this mangrove tree. Many other kinds of birds also nest, roost, and feed in mangroves. And they’re not alone. Mangroves buzz with insects. Ants, spiders, moths, termites, and scorpions feed and nest in hollowed twigs. Snakes and lizards crawl along tree limbs. Frogs cling to bark and leaves. Crocodiles laze in the salt water. There are about 70 species of mangroves, ranging from sprawling shrubs to trees that stand 60 meters (200 feet) high and provide habitats for organisms large and small. All are adapted to living in shallow seawater. Some can live in places nearly three times saltier than the sea. As a whole, mangroves are among the most productive and biologically complex ecosystems on Earth.
Mangroves are survivors. With their roots submerged in water, mangrove trees thrive in hot, muddy, salty conditions that would quickly kill most plants. How do they do it? Through a series of impressive adaptations—including a filtration system that keeps out much of the salt and a complex root system that holds the mangrove upright in the shifting sediments where land and water meet. Not only do mangroves manage to survive in challenging conditions, the mangrove ecosystem also supports an incredible diversity of creatures—including some species unique to mangrove forests. And, as scientists are discovering, mangrove swamps are extremely important to our own well-being and to the health of the planet. The question is: Will mangroves be able to survive the impact of human activities?
Dive underwater, and a mangrove’s smooth brown roots suddenly take on the textures and hues of the multitude of marine organisms clinging to its bark. Anchored in mud, the roots are literally coated with creatures—barnacles, oysters, crabs, sponges, anemones, and much, much more. The dense, intertwining roots serve as nurseries for many colorful coral reef fishes and for fishes valued by fishermen. Juvenile fish find shelter there during their first vulnerable weeks of life, before swimming off to deeper, more dangerous waters.
How do you tell a male mudflat fiddler crab from a female? It’s easy. Males are the ones flaunting huge claws. They use the claws to woo females—and to pound male rivals on the head. Sometimes the crabs chase male competitors all the way to their burrows. The females don’t even seem to notice. They just wander around eating as much as they can.
The spectacled caiman (Caiman crocodilus) doesn’t wear glasses, of course. But a bony ridge between its eyes gives it that appearance. Even without glasses, females of this species keep a sharp eye out for their young. They raise the young in nurseries, taking turns caring for their own as well as others' offspring…and protecting them fiercely.
Only two species of crabs climb trees. You’re looking at one of them. By digging in with its sharp claws, the mangrove tree crab can cling to tree bark as well as to wooden docks and pilings. The crabs dine on leaves and insect larvae in the trees. When threatened, they flee to the water, where they select from a different menu of food. Smithsonian researchers have even spotted a mangrove tree crab feasting on a seahorse.
Mangrove Research at the Smithsonian
How diverse are mangroves? How do their components work? What threats do they face—and how can we conserve them? Smithsonian scientists and colleagues from around the world are searching for answers to these and other urgent questions. The scientists make use of the extensive collections at the National Museum of Natural History as well as the facilities at several Smithsonian research stations outside of Washington, D.C.—including an environmental research center in Maryland, a tropical research institute in Panama, and field stations along the Atlantic and Caribbean coasts from Florida to Panama. These natural laboratories enable the scientists to conduct long-term studies on mangrove ecosystems from a range of latitudes.
“As a child, I played in a swamp near my grandmother’s house,” says Dr. Candy Feller. “I still do the same thing today.” Dr. Feller spends much of her time perched in mangrove trees or sitting among their gnarled thickets—counting, measuring, weighing, photographing and comparing the leaves and insects she finds. An insect and plant ecologist at the Smithsonian Environmental Research Center, she has collected dozens of insects once unknown to science. For more than 15 years, she has been carefully fertilizing mangroves to understand how excess nutrients—like those from industrial, residential, and agricultural sources—affect mangrove ecosystems.
What happens on tropical islands when mangroves are removed and human development moves in? That’s one question Dr. Karen L. McKee set out to answer on the Pelican Cays and Twin Cays ranges in Belize. Developers there want to clear-cut mangroves and dredge fill to raise the elevation of the land and create sand islands. Dr. McKee found out that when mangroves are removed, the islands begin to sink and erode. Nearby reef flats and seagrass beds are also destroyed because dredge material is taken from those habitats. Dr. McKee is a research ecologist at the National Wetlands Research Center of the U.S. Geological Survey.
Mangroves Destroyed for Human Development
Despite their critical importance, mangroves are disappearing at an alarming rate around the world. Human development, industrial activity, climate change and aquaculture are rapidly replacing these salt-tolerant trees and the ecosystems they support. There are a few places where mangrove cover is increasing. But, in just in the last decade, at least 35 percent of the world's mangroves have been destroyed. That’s a rate of loss that exceeds the disappearance of tropical rainforests.
Mangroves form dense barriers against storms and tsunamis, saving lives and protecting property. They also provide us with many other important benefits—more than many people may realize. For example, mangroves produce seafood, fruits, medicines, fiber, and wood. They stabilize shores by trapping sediments and building land. They improve water quality by filtering runoff and polluted waters. They protect the climate by absorbing carbon dioxide and reducing the amount of greenhouse gas. All in all, researchers estimate, the world's mangrove forests provide human communities with many billions of dollars worth of free services.