What comes to mind when I say rainforests? What about coral reefs? What links the two habitats? They may be totally different, one being marine and the other terrestrial, but they are still the two environments that come into your head when speaking about the abundance and diversity of life.
What would happen if these two were to combine?
A fantastical thought for any naturalist, and one that seems as unreachable as the 95% of the ocean we have not explored. But to me, there is a habitat in Bermuda that is, in essence, a coral reef rainforest; the mangroves.
Mangroves are only one plant species, but we get two types here in Bermuda. It seems almost inevitable that they would grow here due to their adaptations which suit Bermuda’s coast, and our island’s shores have attracted thousands of floating mangrove embryos. The combination of the magnificent and easily adaptable trees with the diverse wildlife off the coast of Bermuda creates one of the richest environments we possess.
To understand this relationship, we must first understand the trees which sustain it; mangroves. We have two species of mangrove tree which are native and are in no way closely related. The black mangrove and the red mangrove are amazing structures which have found different ways to adapt to the oxygen-starved life they lead.
To understand this environment, we must first take a look at it’s structure. There are three types of mangrove swamp in Bermuda; the coastal, fringing, and pond, but today we will only be talking about the pond type of mangrove swamp.
In other examples, Red Mangroves usually form a band towards the water and have Black Mangroves in a zone behind. Pond mangrove swamps are different from the other two types in that they rarely contain both species of mangrove; red and black. Where ponds contain both species, the ponds are usually either very close to the sea, or the second species had been introduced by man.
Mangroves are very adaptable and competitive, and it would seem that if one species was able to colonize a pond, it can occupy the entire habitat, not needing the support of a second species of mangrove. Indeed, when these two species do come together, in coastal situations for instance, there is heavy competitive influence. When a pond habitat is fully formed, the environment does not encourage seedling growth for the other species.
Red Mangrove trees get there name from the brick red wood underneath layers of surface bark. The feature that sets them apart from Black Mangrove trees, or any other terrestrial tree for that matter, is their unique root structure. Red Mangrove roots consist of a system of prop roots and adventitious roots which all serve a distinct purpose.
Prop roots are large roots which branch out from the trunk and whose tops remain well above the water line. Their tips however, delve deep into the muddy sediment below the tree where they separate into fine feeding roots. The tops of prop roots make excellent stepping stones where inquisitive humans as well as hungry animals can find transport above water. As well as mobility to others, the parts of prop roots above water also have pale bumps which act as lungs to the tree, transporting oxygen to roots below water.
Adventitious roots drop down from the branches high up on the tree and descend vertically into the water. Some have blunt tips and provide oxygen for the tree while others go deeper and divide into fine feeding roots, much the same as prop roots.
In summer, after the fragrant yellow flowers are formed and pollinated, mangrove seeds begin to form. The large seed, which can be up to 8 inches long, stays on the tree while it develops its primary root. The completed structure is called an embryo, and when it is fully developed it drops off the tree and into water, where it can float long distances, or into mud, where it can anchor itself and become a new tree.
Black Mangroves, being in no way closely related to red ones, have developed a completely different system for breathing. Instead of a system of prop and adventitious roots, Black Mangroves use pneumatophores which are small projections that ascend vertically out of the water below the tree. The roots can reach up to one foot high and provide excellent bases for seaweeds.
The feeding roots of the Black Mangrove spread out just underneath a layer of sediment, and are unseen above water, unlike the prop roots of the red. The problem of low oxygen content is solved by pneumatophores which grow up from the roots below the surface and are exposed to air at low tide.
The Black Mangrove produces a large seed which also float on the water. Nut like in appearance, the seed of the black mangrove will only germinate at a low tide spot, as it is unable to anchor at a place where water is permanently present.
Mangrove trees have a very important role in inland saltwater ponds. Ponds with overhanging mangrove trees are graced with detrital food from leaf fall, giving them an endless food supply which travels up the food chain. In addition, the roots of both species of mangrove provide anchorage spots for algae and seaweeds as well as sessile marine animals, such as periwinkles. Mangrove trees shelter the shoreline and are an excellent nursery fro juvenile fish whose predators cannot come through the tangle roots. The center of the pond however, is home to a rich population of healthy large fish such as Snapper, Mullet and Grunt.
The large root systems and trees themselves extend terrestrial habitat over the ocean, making them a great place for birds like the Yellow-Crowned Night Heron to nest. They also support populations of Jamaican Anole lizard, Giant Toad, the only amphibian able to survive in salt water, and Land Hermit Crab.
The life under the water in marine mangrove swamps is very hard to access. Since mangrove trees are so readily providing detritus, the water is sludgy and easily stirred up, making the visibility largely poor. If one does get into a pond, many amazing animals make themselves present. The Upside-Down Jellyfish is a unique invertebrate which spends most of it’s time upside-down on the bottom of the pond, extending its tentacles to the light. It is in a symbiotic relationship with algae that gain shelter and nutrients from their host, while the jellyfish gets food from the algae.
Fish are present in the water too. Great populations of Bonefish, Mosquito Fish, Mullet, Crested Goby, Bermuda Chub, Bermuda Bream, endemic Killifish and even American Eel lurk beneath the surface. Factors such as being cut off from the sea, and since the pond is a no fishing zone, enable these fish to grow to considerable size. Green Turtles are temporary residents of the pond, as they must breed and lay eggs on beaches, and the much rarer Diamond-Back Terrapin is also fleetingly sighted.
Sessile filter feeders are attracted by the thousands since they thrive on the habitat mangroves create. Sponges and other invertebrates feed on the abundant supply of detritus and plankton, causing them to cover every inch of submerged mangrove root. Seaweeds, sea grasses and anemones are also present.
Above the water are a vast number of animals. Mollusks, crustaceans, and spiders reside there, along with birds that thrive off of the rich environment. Many animals are unique to only the mangroves, such as the Mangrove Periwinkle, Flat Mangrove Oyster, Coffee Bean Snail, and Mangrove Crabs. Each is beautifully adapted to mangrove life with camouflage and the ability to take advantage of both above an below water feeding. The Flat Mangrove Oyster was common around all of Bermuda, but is now confined to mangroves because of their decreasing numbers. The Mangrove Crab and Periwinkle both spend time underwater but ascend into the trees to feed. Periwinkles are more difficult to spot than crabs, whose amazingly bright coloring and inquisitive nature give them away.
Animals that stay above water include Whistling Frogs, who love to feed on the many types of insects present, and two types of spider. The Hurricane Spider, or Golden Orb-Weaver spider is about the size of a human hand and spins webs even bigger high up in the trees. The Crab Spider, about half the size of a fingernail in adulthood, competes with its larger counterpart for the best web spots. Warblers, the introduced Great Kiskadee, and many species from the heron family reside in the trees, dependent on smaller life forms to feed themselves and their young.
The most striking and characteristic inhabitants of the mangroves are those which are rarely seen today; the Giant Land Crab and the Land Hermit Crab. Both crustaceans, the Giant Land Crab uses an exoskeleton for protection while the Hermit Crab must find a shell of its own to call home; most commonly using those of the West Indian Top Shell after death. Both are fantastically adapted to mangrove life, but sadly greatly declining in number.
The only thing that allows these amazing animals to survive is the existence of mangrove trees. While pollution is a factor contributing to the decline of some of the species contained within the mangrove, many other’s lack of abundance goes unexplained. What I know for sure is that it’s not the deprivation of mangrove habitat. If one thing is certain, it’s that these highly adaptable trees will be around for a long time.