Ecosystems

In 1947, the biologist L.H. Holdridge introduced a system of classifying vegetation types or "zones" according to a matrix based on analyzing combinations of temperature, rainfall, and seasonality. Each zone has a distinctive natural vegetation and ecosystem. Costa Rica has 12 such zones, ranging from tidal mangrove swamps to subalpine paramó, with its stunted dwarf plants above the timberline atop the high mountains.

Costa Rican Natural History, edited by Daniel Janzen, provides a description of the vegetation types associated with each life zone. You can also obtain a life zone map from the Tropical Science Center (Calle 1, Avenidas 4/6), a private nonprofit organization which operates the Monteverde Cloud Forest Reserve.

Costa Rica's tropical situation, in combination with both a remarkable diversity of local relief and climates plus generous infusions since Miocene times of plants and animals from the adjoining continents, has resulted in the evolution of a stupendously rich biota. Some habitats, such as the mangrove swamps, are relatively noncomplex. Others, particularly the ecosystem of the tropical rainforests of the Caribbean lowlands and the Osa Peninsula (the only rainforest still extant on the Pacific side of Central America), are among the most complex on the planet.

There is no barrier in Costa Rica to the entry of South American species of flora, and the lowland rainforests have strong affinities with the selva of South America and form a distinctive assemblage of species in which the large number of palms, tree ferns, lianas, and epiphytes emphasize the constant heat and humidity of the region. The impressive tropical rainforest of eastern Costa Rica and the Osa Peninsula gives way on the central Pacific to a dry evergreen forest at lower elevations and dry deciduous forest farther north. These, too, are of essentially South American composition. Above about 1,000 meters, the species are fewer and the affinities with North America are stronger. In the Cordillera Talamanca, conifers of South American provenance are joined by North American oaks. Above the treeline (approximately 3,000 meters), hikers familiar with the midelevation flora of the high Andes of Peru and Ecuador will find many affinities in the shrubby open landscape of Costa Rica's cordillera.

In 1898 the German botanist A.F.W. Schimper coined the phrase "tropical rainforest." Since then botanists have made distinctions between 30 or so different types of rainforest, whose species content is determined by temperature and rainfall. Tropical evergreen rainforest exists in areas of high rainfall (at least 200 cm) and regular high temperatures averaging no less than 77deg. F. In Costa Rica, the lush tropical evergreen rainforest of the Caribbean lowlands gives way on the Pacific side to a seasonally dry evergreen forest in the well-watered south, and tropical deciduous forest--dry forest--in the northwest.

TROPICAL DRY FORESTS

Unlike Costa Rica's rainforests, the rare tropical dry forest is relatively sparsely vegetated, with far fewer tree species and only two strata. Canopy trees have short, stout trunks with large, flat-topped crowns, rarely more than 15 meters above the ground. Beneath is an understory layer of trees with small, open-top crowns, and a layer of shrubs with vicious spines and thorns. Missing are the great profusion of epiphytes and the year-round lush evergreens of the rainforest.

For half the year (Nov.-March), no rain relieves the parching heat. Then, the deciduous dry forests undergo a dramatic seasonal transformation, the purple jacaranda, pink-and-white meadow oak, yellow corteza amarilla, scarlet poró, and the bright orange flame-of-the-forest exploding in Monet colors in the midst of drought.

Before the arrival of the Spanish in the early 16th century, dry forests blanketed the Pacific coastal lowlands from Panama to Mexico. Today, they cling precariously to some two percent of their former range--a mere 200 square miles of Costa Rica in scattered patches centered on the lower Río Tempisque of Guanacaste. Far rarer than rainforests, they are significantly more endangered, especially by fires, which eviscerate whole forest patches, opening holes in which ecological opportunists--weeds and grasses such as African jaragua--rush in. Eventually savannah comes to replace the forest.

Fires set by the Spanish and by generations of farmers and ranchers thereafter spread savannahs across the province, whose flat alluvial plains and rich volcanic soils are perfect for crops and cattle ranchland. Within the last decade, the introduction of electricity and irrigation to southern Guanacaste has exacerbated the problem by attracting increasing numbers of farmers to areas where dry-forest patches still exist. The fate of even the preserved dry-forest parcels hinges on the success of two ambitious conservation projects (see special topic, "Dry Forest Restoration," p. 500).

TROPICAL RAINFOREST

Once upon a time, about 140 million years ago, near the beginning of the Cretaceous period in the age of dinosaurs, before the freezing embraces of the Ice Ages, thick evergreen forests blanketed much of the world's warm, humid surface. Today's tropical rainforests--the densest and richest proliferation of plants ever known--are the survivors of these primeval jungles of ages past.

These forests, the largest of which is Brazil's Amazonian jungle, are found in a narrow belt that girdles the earth at the equator. In the tropics, constant sunlight, endless rains and high temperatures year-round spell life. The steamy atmosphere and fast nutrient turnover have promoted favorable growth conditions and intense competition that have allowed the forest flora to evolve into an extraordinary multitude of different species, exploiting to the full every conceivable niche. Nowhere else on earth is biological productivity and diversity so evident: tropical rainforests contain over half of all living things known to man. Costa Rica alone has as many plant species as the whole of Europe, and the number of insect species in a hectare of rainforest is so great that no successful count has been made. Entomologists have collected from just one species of tree over 950 different species of beetles alone!

Only superficially does the rainforest resemble the fictional jungles of Tarzan. Yes, the foliage can indeed be so dense that you cannot move without a machete. But since only about 10% of the total sunlight manages to penetrate through the forest canopy, the undergrowth is generally correspondingly sparse, and the forest floor surprisingly open and relatively easy to move about in. Within the shadowed jungle the dark subaqueous greens are lit here and there by beams of sunlight pouring down from above.

The stagnant air, however, is loaded with moisture. Even the briefest trail walk leaves clothing saturated with sweat, and molds and fungus seem to appear virtually overnight. There is supposedly even a fungus that flourishes inside binoculars and cameras and eats away the protective coating of lenses. To a visitor, the tropical rainforest seems always the same: uniform heat and stifling 90% humidity which scarcely varies. But this is true only near the ground. High in the tops of the trees, where the sun comes and goes, breezes blow, and moisture has a chance to be carried away, the swings in temperature between day and night are as much as 15 degrees, whereas the humidity may drop from 95%, its fairly constant nighttime level, to as low as 60% as the sun rises and warms the forest. Thus, within 100 vertical feet, two distinctly different climates prevail.

The Attraction

Costa Rica's tropical rainforests have an allure that abates discomfort. They are places of peace and renewal, like a vast vaulted cathedral, mysterious, strangely silent, and of majestic proportions. As one writer says: "a fourteenth century stonemason would have felt at home [in the rainforest], with its buttressed, moss-columned, towering trees and dark recesses."

The rich rainforest green backdrops the jewel colors of its many inhabitants. Sit still awhile and the unseen beasts and birds will get used to your presence and emerge from the shadows. Enormous morpho butterflies float by, flashing like bright neon signs. Is that vine really moving? More likely it's a brilliantly costumed tree python, so green it is almost iridescent, draped in sensuous coils on a branch.

Plunging deep into the forest, you are soon struck by how much variety there is. While in temperate forests distinct species of flora congregate neatly into distinctive plant "neighborhoods" with few other species interspersed, in the rainforest you may pass one example of a particular tree species, then not see another for half a mile. In between, however, are hundreds of other species. In the rainforest, too, you'll notice that life is piled upon life--literally. The firm and unyielding forest floor is a "dark factory of decomposition," where bacteria, mold, and insects work unceasingly, degrading the constant rain of leaf litter and dislodged fruits into nutrient molecules.

Flora

Strange-shaped umbrellas, curtains, and globes of fungi proliferate, too. They are a key to providing the nourishment vital to the jungle's life cycle. While a fallen leaf from a North American oak may take a year to decompose, a leaf in the tropical rainforest will fully decay within a month. If these precious nutrients and minerals thus released are not to be washed away by the daily drenching of rain, they must be reclaimed quickly and returned to the canopy to restart the cycle of life. The trees suck up the minerals and nutrients through a thick mat of rootlets that grow close to the surface of the soil. To counteract their inherent instability, many species grow side buttresses: wafer-thin flanges which radiate in a ring around the base of the tree like the tail fins of rockets.

The dark nave of the rainforest cathedral is rich with ferns, saplings, and herbaceous plants, seeping in moisture. For every tree in the jungle, there is a clinging vine fighting for a glimpse of the sun. Instead of using up valuable time and energy in building their own supports, these clutching vines and lianas rely on the straight, limbless trunks typical of rainforest tree species to provide a support in their quest for sunlight. They ride piggyback to the canopy, where they continue to snake through the treetops, sometimes reaching lengths of 300 meters (their weight can often cause their hosts to come crashing down). One species spirals around its host like a corkscrew; another cements itself to a tree with three-pronged tendrils.

The bully of the forest, however, is the strangler fig, which isn't content to merely coexist. While most lianas and vines take root in the ground and grow upward, the strangler figs do the opposite. After sprouting in the forest canopy from seeds dropped by birds and bats, the strangler fig sends roots to the ground, where they dig into the soil and provide a boost of sustenance. Slowly but surely--it may take a full century--the roots grow and envelop the host tree, choking it till it dies and rots away, leaving a hollow, trellised, freestanding cylinder.

The vigorous competition for light and space has promoted the evolution of long, slender, branchless trunks, many well over 35 meters tall, and flat-topped crowns with foliage so dense that rainwater from driving tropical downpours often may not reach the ground for 10 minutes. Above this dense carpet of greenery rise a few scattered giants towering to heights of 70 meters or more. This great vaulted canopy--the clerestory of the rainforest cathedral--is the jungle's powerhouse, where more than 90% of photosynthesis takes place.

The scaffolding of massive boughs is colonized at all levels by a riot of bromeliads, ferns, and other epiphytes (plants that take root on plants but that are not parasitic). Tiny spores sprout on the bark, gain a foothold, and spread like luxuriant carpets. As they die and decay, they form a compost on the branch capable of supporting larger plants which feed on the leaf mold and draw moisture by dangling their roots into the humid air. Soon every available surface is a great hanging gallery of giant elkhorns and ferns, often reaching such weights that whole tree limbs are torn away and crash down to join the decaying litter on the forest floor.

Wildlife

The Babylonian gardens of the jungle ceiling--naturalist William Beebe called it an "undiscovered continent"--host a staggeringly complex, unseen world of wildlife that runs into millions of species. In the canopy, forest trees flower and fruit in the steaming tropical sunshine. Scarlet macaws and lesser parrots plunge and sway in the high branches, announcing their playacting with an outburst of shrieks. Arboreal rodents leap and run along the branches, searching for nectar and insects, while insectivorous birds watch from their vantage points for any movement that will betray a stick insect or leaf-green tree frog to scoop up for lunch. Legions of monkeys, sloths, and fruit- and leaf-eating mammals also live in the green world of the canopy, browsing and hunting, thieving and scavenging, breeding and dying.

Larger hunters are up there too. In addition to the great eagles plunging through the canopy to grab monkeys, there are also tree-dwelling cats. These superbly athletic climbers are quite capable of catching monkeys and squirrels as they leap from branch to branch and race up trunks. There are also snakes here. Not the great monsters so common in romantic fiction, which dangle, says David Attenborough, "optimistically from a branch, waiting to pick up a human passer-by," but much smaller creatures, some twig-thin, like the chunk-headed snake with catlike eyes, which feasts on frogs and lizards and nestling birds.

Come twilight, the forest soaks in a brief moment of silence. Slowly, the lisping of insects begins. There is a faint rustle as nocturnal rodents come out to forage in the ground litter. And the squabbling of fruit bats replaces that of the birds. All around, myriad beetles and moths take wing in the moist velvet blanket of the tropical night.

MANGROVES

Costa Rica's shorelines are home to five species of mangroves. These pioneer land builders thrive at the interface of land and sea, forming a stabilizing tangle that fights tidal erosion and reclaims land from the water. The irrepressible, reddish-barked, shrubby mangroves rise from the dark water on interlocking stilt roots. Small brackish streams and labyrinthine creeks wind among them like snakes, sometimes interconnecting, sometimes petering out in narrow culs-de-sac, sometimes opening suddenly into broad lagoons. A few clear channels may run through the rich and redolent world of the mangroves, but the trees grow so thickly over much of it that you cannot force even a small boat between them.

Mangroves are what botanists call halophytes, plants that thrive in salty conditions. Although they do not require salt (they in fact grow better in fresh water), they thrive where no other tree can. Costa Rica's young rivers have short and violent courses which keep silt and volcanic ash churned up and suspended, so that a great deal of it is carried out of the mountains onto the coastal alluvial plains. The nutrient-rich mud generates algae and other small organisms that form the base of the marine food chain. Food is delivered to the estuaries every day from both the sea and the land so those few plants--and creatures--that can survive here flourish in immense numbers. And their sustained health is vital to the health of other marine ecosystems.

The nutrients the mangrove seeks lie not deep in the acid mud but on its surface, where they have been deposited by the tides. There is no oxygen to be had in the mud either: estuarine mud is so fine-grained that air cannot diffuse through it, and the gases produced by the decomposition of the organic debris within it stay trapped until your footsteps release them, producing a strong whiff of rotten eggs (the mud also clings so tenaciously it can suck the boots from your feet). Hence, there is no point in the mangroves sending down deep roots. Instead, the mangroves send out peculiar aerial roots, like spider's legs, to form a horizontal platform that sits like a raft, maintaining a hold on the glutinous mud and giving the mangroves the appearance of walking on water. The mangroves draw oxygen from the air through small patches of spongy tissue on their bark.

Mangrove swamps are esteemed as nurseries of marinelife and as havens for water birds--cormorants, frigate birds, pelicans, herons, and egrets--which feed and nest here by the thousands, producing guano that makes the mangroves grow faster. The big birds roost on the top canopy, while smaller ones settle for the underbrush. Frigate birds are particularly fond of mangrove bushes and congregate in vast numbers along the swampy shorelines of the Gulf of Nicoya (see "Birds," below). The bushes in which they build their nests rise some six to ten feet above the mudflats--just right to serve as launching pads.

Mangrove swamps, especially those fed by freshwater streams, are marine nurseries of astonishing fertility. A look down into the water reveals luxuriant life: oysters and sponges attached to the roots, small stingrays flapping slowly over the bottom, and tiny fish in schools of tens of thousands. Baby black-tipped sharks and other juvenile fish, too, spend much of their early lives among mangrove roots, out of the heavy surf, shielded by the root maze that keeps out large predators.

High tide brings larger diners--big mangrove snappers and young barracudas hang motionless in the water. Raccoons, snakes, and, as everywhere, insects and other arboreal creatures also inhabit the mangroves. There is even an arboreal mangrove tree crab (Aratus pisonii) which eats mangrove leaves and is restricted to the very crowns of the trees by the predatory activities of another arboreal crab, Goniopsis pulcra.

Mangroves are aggressive colonizers, thanks to one of nature's most remarkable seedlings. The heavy, fleshy mangrove seeds, shaped like plumb bobs, germinate while still on the tree. The flowers bloom for a few weeks in the spring and then fall off, making way for a fruit. A seedling shoot soon sprouts from each fruit and grows to a length of 6-12 inches before dropping from the tree. Falling like darts, at low tide they will land in the mud and put down roots immediately. Otherwise, the seedlings--great travelers--become floating scouts and outriders ahead of the advancing roots.

The seaborne seedling can remain alive for as long as a year, during which time it may drift for hundreds of miles. Eventually, it touches the muddy floor and anchors itself, growing as much as two feet in its first year. By its third year a young tree starts to sprout its own forest of arching prop roots; in about 10 years it has fostered a thriving colony of mangroves, which edge ever out to sea, forming a great swampy forest. As silt builds up among the roots, land is gradually reclaimed from the sea. They build up the soil until they strand themselves high and dry. In the end they die on the land they have created.

ORCHIDS

It's appropriate that the orchid is the national flower of Costa Rica: the country has more than 1,200 identified species, the richest orchid flora in Central America. Countless others await discovery. At any time of year you're sure to find dozens of species in bloom, from sea level to the highest, subfreezing reaches of Chirripó. There is no best time for viewing orchids, although the beginning of both the dry season (especially in the wettest rainforest regions) and wet season are said to be particularly favorable times.

Not only are orchids the largest family of flowering plants, they're also the most diverse: poke around with magnifying glass in hand and you'll come across species with flowers less than one millimeter across. Others, like the native Phragmipedium caudatum, have pendulant petals that can reach more than half a meter. Some flower for only one day. Others will last several weeks. Orchid lovers should head for the cloud forests, for the greatest diversity exists in humid--not wet--midelevation environments where they are abundant as tropical epiphytes (constituting 88% of orchid species). One biologist found 47 different orchid species growing on a single tree.

While not all orchids lead epiphytic lives--the Spanish called them parasitos--those that do are the most exotic of epiphytes, classics of their kind, so heartachingly beautiful that collectors can't resist their siren call and threaten their existence.

Orchids have evolved a remarkable array of ingenious pollination techniques. Some species self-pollinate. Others attract insects by sexual impersonation. One species, for example, produces a flower that closely resembles the form of a female wasp complete with eyes, antennae, and wings. It even gives off the odor of a female wasp in mating condition. Male wasps, deceived, attempt to copulate with it. In their vigor, they deposit pollen within the orchid flower and immediately afterward receive a fresh batch to carry to the next false female.

Guile seems to be the forte of orchids. Another species drugs its visitors. Bees clamber into its throat and sip a nectar so intoxicating that after the merest taste they become so inebriated they lose their footing and slip into a small bucket of liquid. Escape is offered up a spout . . . the proverbial light at the end of the tunnel. As the drunken insect totters up, it has to wriggle beneath an overhanging rod which showers its back with pollen. Pollination techniques have become so species-specific that hybridization of different orchid species is avoided by each having developed its own morphological configuration to attach its pollen, and receive it in return, to a specific part of the insect's body.

Lankester Gardens, part of the University of Costa Rica, features over 800 orchid species, including the collection of Charles Lankester, who once ran Lankester as a private garden. It's located about seven km east of Cartago on the road to Paraíso. Orchid Alley, at La Garita in the Central Highlands, offers a stunning array of orchids for sale, suitably packed for export.

An annual orchid show is held each March in San José. Costa Rica Connections (958 Higuera Street, San Luis Obispo, CA 93401; tel. 800-345-7422, fax 805-543-3626) offers a week-long "Costa Rica National Orchid Show and Tour," with visits to the show and to private orchid collections, Poás Volcano National Park (known for its abundance of epiphytes), Lankester Gardens, and other places of interest to orchid lovers.