“Mangroves are among the most productive and biologically complex ecosystems on Earth.”
You might recognize mangroves as those bushy trees that grow in the saltwater tidal zones of warmer climes, standing on root systems that form a complex, intertwining network that appears as if no living thing could navigate its way through. But rather, mangrove ecosystems are responsible for supporting a glorious abundance of life, much more than is apparent at first, second, even third glance!
In fact, mangroves provide an enormous multitude of environmental contributions and ecosystem services which benefit a plethora of species (including us!) as well as contributing to the health of the environment itself.
The Salt Life: How Do They Survive?
Mangroves occur worldwide within the salty and brackish waters of tropical & subtropical latitudes and withstand the twice-daily rise and fall of the tides. Rather than denoting one particular species, the word “mangrove” in fact makes up more than 80 tree or shrub species known as “halophytes” meaning able to survive in saltwater conditions. Red mangroves achieve this by directing salt to specific leaves which turn yellow and die. When the dead leaf drops, the tree has rid itself of that excess salt. Red mangroves also have salt-filtering taproots to filter out freshwater from the salty environment in which they exist. Other species, such as our white, black and tea mangroves, excrete salt through glands on their leaves, leaving a surface of dried salt crystals.
The Submerged Life: How Do They Breathe?
Mangroves truly live in conditions that are nearly intolerable. Not only do they have to constantly extract or exude salt from their system, but also there’s that pesky universal dependence on oxygen that all life shares, leaving these trees with the complicated job of obtaining enough with which to grow and thrive despite twice-daily inundation and roots sunk into oxygen-deprived mud. But mangroves have evolved unique adaptations to survive against all these odds and colonize an otherwise unoccupied and ultimately harsh environment. Special arial roots in some mangroves reach slowly downward from taller branches and take in air, as do specialized underground roots in other species that send up “pneumatophores”, or upward facing roots, which gather oxygen at low tide. The prop roots of the red mangrove have tiny holes called “lenticels” which close when submerged at high tide and open as the waters recede to gather the essential oxygen.
A Forest of Roots & How it all Begins:
Some species, like the red mangrove, grow upon prop roots, meaning the base of the tree is supported aerially by a multitude of bowed roots that plant into the mud and provide a wide support system allowing the tree to withstand constant tidal and wave action including storms, hurricanes and even tsunamis by dissipating wave energy. This provides essential protection to coastal communities and can mean devastation in strong storm surges for regions where mangrove forests have been removed.
Mangroves are actually able to grow their own, unique ecosystems, practically from nothing more than a bit of sand! Near us here at Saladero, Balto (husband of Paulina, our lovely and talented cook), remembers as a boy a shallow sandbar near the mouth of nearby Rio Esquinas that now, sixty years later, is a completely established mangrove island.
Mangroves seeds are known as propagules, meaning they are actually living seedlings before they even fall from the parent tree. Red mangrove seeds are elongated and as they float in the shallows they’ll slowly turn vertical when ready to root so as to more easily lodge into the mud. If unable to root, the seedling will alter its density to float horizontally again until it senses more favorable conditions. In effect, the seed is actually “looking” for calm, shallow waters appropriate for a young mangrove to begin to grow and thrive..and more are always sure to soon follow. As soon as a root network is formed, fine silt and sediments floating through the slow moving water collect and the resulting substrate is better able to support even more mangrove seedlings, eventually forming a forest.
A Thriving Ecosystem Results:
And so begins the construction of an ecosystem that will not only support a fantastic diversity of species, including some that are endemic (found nowhere else) to mangrove forests, but the intricate tangle of roots also provides a nursery for young fish that will grow into many of our reef and commercially harvested species. But not just fish benefit from the shelter and protection from larger prey and food offered by a healthy functioning mangrove system and its thick network of prop roots. In fact, mangrove roots themselves are literally coated with life—crabs, snails, barnacles, oysters (which are commercially harvested), sponges, algae, anemones, shrimp and a great deal more.
And all of this life provides a massive food supply to support even more life. Wading birds nest and feed in mangrove forests, various mammals (even monkeys!) hunt among the prop roots, taking fish and crustaceans, green sea turtles pick algae off the underwater roots during high tide, even snakes, lizards and frogs can use the mangroves as their hunting grounds. And then there’s the managerie of ants, spiders, moths, termites, and scorpions that feed among the branches and nest in hollowed twigs above the water. Here in the Golfo Dulce, we have the critically endangered Mangrove Hummingbird that relies on the nectar of the flowering Tea Mangrove. This is a species that originally evolved on the Osa Peninsula which originally was an island and over time evolved this completely unique, endemic species. Hundreds of thousands of years ago, a geological uplift created a land bridge connecting the Osa to mainland Costa Rica and the hummingbird was able to widen its range and is now found here as well, but is still very rare and endemic only to the Osa area.
Life Has Leaf Litter to Thank:
So among all of these species, how is all this life supported by a “simple” collection of salt-loving trees? Where does this food web begin, you might wonder? Well, the growing mangroves drop leaves throughout the year, directly adding nutrients to the water and sediment below. When you see yellow leaves on the trees, they’re not just dead leaves, they’re a special means of extracting the salt that they are taking in, actually directing it all into specific individual leaves (known as the “sacrificial leaf”) which turn yellow and die, falling into the water beneath. All this “detritis” (dead organic matter) creates a rich leaf litter layer that is full of nutrients that supply food to microorganisms below the water, including bacteria and fungi. These organisms are key species that assist in the decomposition process. Microbes and aquatic invertebrates feed on the decay and the young nursery fish and crustaceans in turn feed upon them. And on up the food web a multitude of species is sated, resulting in a plethora of thriving, well-fed life.
When Mangroves Are Around, Everyone Benefits:
And as if all this weren’t enough, mangroves are also crucially important in their role in cycling and storing carbon, even more so than primary rainforests, throughout tropical ecosystem, helping to reduce this greenhouse gas in our atmosphere. Mangroves also prevent coastal erosion and filter rainwater runoff.
Yet despite how essential mangrove ecosystems are to so many species, it is sadly true that, despite protection and restoration efforts, over half of the world’s mangroves have been removed for development (including for tourism, agriculture expansion, shrimp farming, marinas and roadways) in recent times. According to the Mangrove Action Project, “We have already lost over half of the world’s original mangrove forest area, estimated at 32 million hectares (app. 80 million acres). In 2007, less than 15 million hectares (37 million acres) of mangroves remain. The current rate of mangrove loss is approximately 1% per annum (according to the Food and Agriculture Organization), or roughly 150,000 hectares (370,050 acres) of mangrove wetlands lost each year.”
If we keep in mind the importance of these beautiful, rich, diverse ecosystems, that is a step toward ensuring their survival into the future.