The Fascinating World of the Mangrove Swamp.
I first heard about mangroves in the early 1970s when I was still in the cattle business. Another cattleman told me, "They're worthless swamps, no good for anything." He also told me that he had discovered a type of pasture called "German Grass" that would invade the mangroves and dry them up. My second impression of mangrove swamps came from a lady with a similar opinion. "The mangrove is so ugly," she exclaimed, "the government should just cut down the trees, drain the swamp and fill it all in with earth. Then there would be enough room so everyone could have a nice weekend cabin near the beach." Based on these two opinions, I was prepared for something horrible when I first visited Hacienda Barú in February of 1972. At that time I had never seen a mangrove swamp and my mental image of one was vague. Reality, of course, is often quite different than our preconceived ideas. In the case of mangroves, what I saw and experienced was so diametrically opposed to what I had been told that I wondered if I might be looking at something else. The experience turned out to be a catalyst that sparked a life long fascination with one of the most complex ecosystems on our planet.
The mangrove estuary at Hacienda Barú is a shallow channel that flows inland from the Barú River mouth, parallel to the beach for more than a kilometer. Since the entrance to the channel begins, near the sea, it is affected by tides. As the tide rises, salt water flows into the Barú River where it collides and mixes with the fresh water coming down stream. This, of course, causes the water to rise even further and, when it reaches a certain level, this mixture called "brackish" water flows into the estuary. The amount depends both on the height of the tide and volume of flow in the Barú River. As the tide recedes, the water level drops, and the brackish water in the estuary reverses its flow, returning to the river mouth and out to sea. Some of the brackish water remains permanently trapped in low spots in the estuary. This is the habitat where the salt tolerant tree called "mangrove" thrives.
In 1972, when I first began raising cattle on Hacienda Barú, I went to see the mangrove. Armed with the knowledge that German grass would turn this "useless swamp" into a thriving pasture, I wanted to see it first hand. The possibility of converting 20 hectares of "unproductive wetland" into profit generating cattle pasture was foremost in my mind. Arriving at the estuary on horseback, we were greeted by the joyous chatter of a troop of white-faced capuchin monkeys feeding in a strangler fig tree, the first wild monkeys I had ever seen. In less than an hour we saw a spectacled caiman, boat-billed herons, a coati, a tree boa and several green iguanas. I was hooked. Never again would I entertain the idea of converting this bubbling caldron of life into a cattle pasture. The thought simply disappeared from my mind. That day I began a journey of discovery that, over the next 20 years, transformed me from a cattleman into a naturalist and environmentalist. That journey has yet to end, and I doubt if it ever will.
Five species of mangrove are found in the at the Barú River estuary, but curiously, none of the five are related to each other. Common to all is a tolerance for salt and an affinity for wet, silty conditions. The five species differ in form, color, root structure and leaves. Some are very high in a chemical called tannin and have been traditionally exploited by the leather tanning industry. Others have, in the past, been used for lumber and charcoal. Today all are protected because of the vital role they play in erosion control and habitat for countless species marine life. Some of the species most important to commercial fishermen, require the mangrove swamp environment, directly or indirectly, during part of their life cycle.
Red mangrove (Rhizophora mangle,) the most common species, seldom exceeds ten meters in height, grows in several meters of water most of the year and has as much branching in its underwater roots as in the crown. At Hacienda Barú National Wildlife Refuge many species of birds roost and nest there including boat-billed herons, cattle egrets, green-backed herons, neotropical cormorants, anhingas and white ibis. Others, such as the snowy egrets, great blue herons, chestnut-bellied herons, roseate spoonbills and wood storks merely visit at certain times of year. Barnacles, sponges, coral and oysters colonize the roots and are fed upon by the fish, clams, shrimp, crabs and lobsters that thrive in the water amongst the tangled mass of mangrove roots. The bottom of the estuary is an organic soup, a thick mixture of silt, decaying vegetable matter and bird guano that supports a wide spectrum of microorganisms which play an important and little understood role in the complex realm of mangrove ecology.
Both the spectacled caiman (Caiman crocodilus) and the American crocodile (Crocodylus acutus) lurk in the dark waters beneath the trees. The larger and more agressive crocodile generally controls the lucrative territory around the nesting site of the water birds and quickly gobbles up any unfortunate nestling or fledgling that falls from its nest. Iguanas that occasionally fall from the overhanging branches of the mangrove trees, may also become supper for the crocodilians. Both the caiman and crocodile find the thick, inhospitable vegetation in and around the mangrove to be an ideally protected location for their own nests and reproduction.
Black mangrove (Avicena bicolor,) thrives in the mud at water's edge and is known for the pencil-like air breathing roots that sprout up like an extremely coarse shag rug, covering the surface around the base of the tree. The thick trunks and branches are often hollow and provide shelter and nesting nooks for wildlife. At Hacienda Barú National Wildlife Refuge we often find black-headed vultures incubating their eggs there. Both green and black iguanas hide in the hollows, frequent the branches of Avicena and may often be seen sunning themselves in the crown.
Three other species of mangrove are present in the Hacienda Barú estuary including the rare buttonwood mangrove (Conocarpus erectus,) tea mangrove (Pelliciera rhizophorae) and white mangrove (Laguncularia racemosa,) the latter of which used to be exploited for its tall straight trunk which is ideal for pole construction.
One denizen of this captivating environment, the mangrove tree crab (Aratus pisonii,) is of particular interest. It lives in the tops of mangrove trees, especially red mangrove and eats mangrove leaves and large insects.
Flocks of several types of egrets and herons sometimes descend into the tops of trees to prey on these small crustaceans, thus creating general panic. Some of the fleeing crabs run down the trunk, but others leap from tree to tree and many fall into the water, only to be gobbled up by the waiting jaws of hungry fish. A general feeding frenzy ensues for both birds and fish. Fishermen often take advantage of these situations, knowing that if they throw a baited hook into the water in the midst of this confusion, it will be taken almost instantly by a crazed fish with a momentarily lowered threshold of wariness. Local fishermen refer to this phenomenon as a "garzeria," derived from the word "garza," the local name for the egrets and herons that start the commotion. Groups of feeding white-faced capuchin monkeys also create panic in the mangrove tree crab and can precipitate a similar chaotic feeding situation. I don't know if the monkeys actually eat the crabs or simply dislodge them.
Along the edge of the Hacienda Barú National Wildlife Refuge estuary we have erected a stilted bird blind over the water. It is located near a roosting and nesting site for wetland birds of all kinds. At this year's Christmas bird count, Audubon Club ornithologists watching from the bird blind, counted over a thousand water birds representing 12 different species, as they flew in to roost. The event begins as the sun sets. The boat-billed heron, a nocturnal feeder, flies out to forage while the many diurnal feeders come in for the night. Flocks of fifty or more individuals drop out of the sky, flare their wings and settle into the mangrove trees. For protection from predators the birds prefer islands of trees that are completely surrounded by water, and competition for space at the safest sites within the rookery can erupt into a cacophony of squawks and squabbles. The experience is truly unforgettable.
I once decided to get up before daylight and go to the bird blind for the morning activity when the majority of the birds fly away to feed. The event wasn't nearly as riotous as the dusk activity, but, as is often the case when observing tropical nature, I was rewarded with something different. As the first rays of the rising sun diluted the blackness of night I became aware of a shadow moving through the air from one side of the open water to another. After a few minutes I caught a glimpse of the reflection from the ripples where the shadow lightly broke the glassy surface. As darkness steadily retreated, the shadow transformed into a winged hunter snatching small surface feeding fish from the water. At some point the lightness became too strong for its sensitive eyes and the greater bulldog bat (Noctilio leporinus) made its final swoop across the swamp and flew away to the darkness of its roost.
As daylight spilled into the protected hollow of mangrove, the familiar shapes of the place emerged. A southern river otter swimming near the surface of the inky black water dove beneath the surface for a minute and emerged on the bank with a crab, which it promptly dismembered, picking out the juicy flesh from within. On top of a log, three large dome-shaped profiles of river turtles came into focus at the same time as the first of the boat-billed herons flew in to roost. Over the next half hour the nighttime inhabitants of the rookery flew off to feed, mostly in silence, and the spectacle was over. But the never ending interactions of life in the mangrove continued. I pondered the vastness of life on our planet and marveled at the wonder of this incredibly diverse habitat which supports untold thousands of species and millions of individuals. I thanked the powers that be for giving me the insight that kept me from destroying it and replacing it with a single species of plant for consumption by a single species of hoofed mammal, which in turn, would be slaughtered for the exclusive consumption of humans and their pets.
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