Fauna Overview

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My friend Lynn Ferrin wrote: "the birds are like jewels, the animals like creatures in a Rousseau painting." Noisy flocks of oropendolas, with long tails "the color of daffodils," sweep from tree to tree. The scarlet macaws are like rainbows, the toucans and hummingbirds like the green flash of sunset. The tiny poison-dart frogs, red and evil, are bright enough to scare away even the most dim-witted predator. And the electric-blue morphos, the neon narcissi of the butterfly world, make even the most unmoved of viewers gape wide-mouthed in awe.

Then there are all the creatures that mimic other things and are harder to spot: insects that look like rotting leaves, moths that look like wasps, the giant Caligo memnon (cream owl) butterfly whose huge open wings resemble the wide-eyed face of an owl, and the mottled, bark-colored machaca (lantern fly), which is partly to blame for Costa Rica's soaring birthrate. According to local folklore, if a girl is stung by a machaca she must go to bed with her boyfriend within 24 hours or she will die.

Much of the wildlife is glimpsed only as shadows. Some, like the dreaded fer-de-lance, for example, uncurling in the rotten leaves, you'll not want to meet. Well-known animals that you are not likely to see are the cats--pumas, jaguars, and ocelots--and tapirs and white-lipped peccaries. With patience, however, you can usually spot monkeys galore, as well as iguanas, quetzals, and three-toed sloths--"long-armed tree-dwelling Muppets"--that, as one guidebook puts it, get most of their aerobic exercise by scratching their bellies.

The National Institute of Biodiversity, a private, nonprofit organization formed in 1989, has been charged with the formidable task of collecting, identifying, and labeling every plant and animal species in Costa Rica. The task is expected to take at least 10 years to complete. "After 100 years of work by the National Museum we still only know 10 to 20 percent of what we have in the country," says Rodrigo Gómez, director of the NIBR. Over the course of the last 110 years, the National Museum collected some 70,000 specimens. In their first 18 months, the NIBR's hundreds of "parataxonomists" (ordinary citizens trained to gather and preserve specimens) gathered almost two million!

Identifying the species is a prodigious task, which every day turns up something new. Insects, for example, make up about half of the estimated 500,000 to one million plant and animal species in Costa Rica. The country is home seasonally to over 850 bird species--one-tenth of all known bird species (the U.S. and Canada combined have less than half that number). One source reports there are 5,000 different species of grasshoppers (with an estimated 2,000 yet to be discovered), 160 known amphibians, 220 reptiles, and 10% of all known butterflies (Corcovado National Park alone has at least 220 different species). It's like being caught up in a kind of botanical rush hour!

IT ALL BEGAN

About three million years ago, the Central American isthmus began to rise from the sea to form the first tentative link between the two Americas. Going from island to island, birds, insects, reptiles and the first mammals began to move back and forth between the continents. During this period, rodents of North America reached the southern continent, and so did the monkeys, which found the tropical climate to their liking.

In due course, South America connected with North America. Down this corridor came the placental mammals to dispute the possession of South America with the marsupial residents. Creatures poured across the bridge in both directions. The equids used it to enter South America, the opossums to invade North America. A ground sloth the size of an elephant headed north, too, reaching all the way to what is now Texas before it died out. Only a few South American mammals, notably armadillos, ground sloths, and porcupines, managed to establish themselves successfully in the north. The greatest migration was in the other direction.

A procession of North American mammals swarmed south, with disastrous effects on native populations. The mammals soon came to dominate the environment, diversifying into forms more appropriate to the tropics. In the course of this rivalry, many marsupial species disappeared, leaving only the tough, opportunistic opossums.

The isthmus has thus served as a "filter bridge" for the intermingling of species and the evolution of modern distinctive Costa Rican biota, a fairly recent amalgam as the isthmus has been in existence for only some three million years. Costa Rica's unique location and tropical setting, along with a great variety of local relief and microclimates, have meant that refuge areas for ancient species endangered by changes in environmental conditions have been widely available, and species which have died out elsewhere can still be found here. This, together with generous infusions of plants and animals from both continents, has resulted in a proliferation of species that in many important respects is vastly richer than the biota of either North or South America. Costa Rica's biota shares much with both.

NUTRIENT RECYCLING
AND SPECIES RICHNESS

One of the most important differences between tropical and temperate environments is what biologists call "species richness." A natural forest patch of a few hundred acres in Michigan, for example, might contain 25-30 species of trees; an equivalent tract of Costa Rican rainforest might contain over 400. Ohio has only about 10 species of bats; Costa Rica has more than 100, although it is less than half Ohio's size. That's species richness.

Why such complexity, such stupefying abundance of species in the neotropics--the tropics of the New World? It all has to do with the rapid pace of nutrient recycling and the way the natural world competes most effectively for nourishment.

In the temperate world, with the warm days and sunlight of spring, plants burst forth with protein-rich buds, shoots, and young leaves which appear simultaneously in a protein "pulse." Animals bring forth their young during this period of protein abundance: birds return from the south to lay eggs and raise their broods, insect eggs hatch, frogs and toads crawl out of hibernation to reproduce. Come autumn, the same plants produce a second protein glut as tender berries, seeds, and nuts which critters pack in to sustain themselves through the hardships of impending winter. The synchronized budding and fruiting of foliage is so great that all the hungry mouths gobbling protein hardly threaten a plant species's survival.

In the tropics, by contrast, the seasonal cycle is far less pronounced: sunlight, rain, and warm temperatures are constant, and plants germinate, grow, flower, and seed year-round. Hence, there is no distinct protein surplus. Leaf fall, too, occurs continuously and slowly in the tropical rainforest, unlike the autumnal drops of temperate deciduous forests, and the same tropical conditions of heat and moisture which fuel year-round growth also sponsor fast decomposition of dead leaves. The humus that enriches the soils of more temperate latitudes doesn't have a chance to accumulate in the tropics. Thus, soils are thin and, after millions of years of daily rainfall and constant heat, leached of their nutrient content.

Result? The ecosystem of a tropical rainforest is upside down when compared with forests in the temperate zone, where nutrients are stored in the soil. In the tropics it's stored overhead: in the densely leafed canopy. Leaves and young shoots represent a major investment of scarce nutrients which plants cannot afford to have gobbled up by hungry multitudes of animals, insects, and birds. Hence, says one biologist, "It might be said that the plants `want' to be different from one another in order to avoid being devoured." Intense competition has pressured tropical plants to diversify greatly, to disperse, and to develop defense mechanisms, such as thorns or sickening toxins. Other species stagger their production of shoots and new leaves throughout the year so that they never expose too much new growth to predation at any one time.

Because plant protein is scarce at any given time, and because plants have evolved stratagems to guard it, animals have been forced to compete fiercely. They've diversified like the plants, and competition has resulted in notable examples of specialization, with individual species staking claims to a narrow ecological niche in which other creatures can't compete. One bird species eats only insects driven up from the ground by army ants, while its droppings provide food for a certain species of butterfly.

Through these intricate associations, specific plants and predators become totally dependent on one another. A perfect example is the ant acacia, a tree common along the Pacific coast. The plant is weak and defenseless, a poor competitor in the upward race for sunlight, and easily overshadowed by faster-growing neighbors. Its tiny nectaries (glands that exude sugar) and leaf-tip swellings filled with proteins and vitamins are tempting morsels for hungry insects and birds. None, however, dare steal a nibble, for a species of tiny yet aggressive ants act as the acacia's praetorian guard. In exchange for the honeylike food which they love, the ants defend their plant fiercely. Any predator foolish enough to touch the acacia is attacked; if a vine threatens to envelop the tree, the ants cut the vine down. If the branches of a neighboring plant threaten to steal the acacia's sunlight, the ants will prune back the interloper; if a neighbor's seeds fall to the ground beneath the acacia, the insects will cart them off before they can germinate. If the ants were to become extinct, the plant would never survive. If the plant disappeared, the ants would starve. Each is inextricably in the debt of the other. Nature's a wonderful thing.